[RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

[RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

[Peng Zhang]

From: ZhangPeng <zhangpeng362@huawei.com>

Since commit f1a7941243c1 ("mm: convert mm's rss stats into
percpu_counter"), the rss_stats have converted into percpu_counter,
which convert the error margin from (nr_threads * 64) to approximately
(nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
performance regression on fork/exec/shell. Even after commit 14ef95be6f55
("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
the performance of fork/exec/shell is still poor compared to previous
kernel versions.

To mitigate performance regression, we delay the allocation of percpu
memory for rss_stats. Therefore, we convert mm's rss stats to use
percpu_counter atomic mode. For single-thread processes, rss_stat is in
atomic mode, which reduces the memory consumption and performance
regression caused by using percpu. For multiple-thread processes,
rss_stat is switched to the percpu mode to reduce the error margin.
We convert rss_stats from atomic mode to percpu mode only when the
second thread is created.

After lmbench test, we can get 2% ~ 4% performance improvement
for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
improvement for lmbench page_fault (before batch mode[1]).

The test results are as follows:
             base           base+revert        base+this patch

fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)

[1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/

ChangeLog:
v2->v1:
- Convert rss_stats from atomic mode to percpu mode only when
  the second thread is created per Jan Kara.
- Compared with v1, the performance data may be different due to
  different test machines.

ZhangPeng (2):
  percpu_counter: introduce atomic mode for percpu_counter
  mm: convert mm's rss stats to use atomic mode

 include/linux/mm.h             | 50 +++++++++++++++++++++++++++++-----
 include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++--
 include/trace/events/kmem.h    |  4 +--
 kernel/fork.c                  | 18 +++++++-----
 lib/percpu_counter.c           | 31 +++++++++++++++++++--
 5 files changed, 125 insertions(+), 21 deletions(-)

-- 
2.25.1

[RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[Peng Zhang]

From: ZhangPeng <zhangpeng362@huawei.com>

Depending on whether counters is NULL, we can support two modes:
atomic mode and perpcu mode. We implement both modes by grouping
the s64 count and atomic64_t count_atomic in a union. At the same time,
we create the interface for adding and reading in atomic mode and for
switching atomic mode to percpu mode.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++++++---
 lib/percpu_counter.c           | 31 ++++++++++++++++++++++--
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 3a44dd1e33d2..160f9734c0bb 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -21,7 +21,13 @@
 
 struct percpu_counter {
 	raw_spinlock_t lock;
-	s64 count;
+	/* Depending on whether counters is NULL, we can support two modes,
+	 * atomic mode using count_atomic and perpcu mode using count.
+	 */
+	union {
+		s64 count;
+		atomic64_t count_atomic;
+	};
 #ifdef CONFIG_HOTPLUG_CPU
 	struct list_head list;	/* All percpu_counters are on a list */
 #endif
@@ -32,14 +38,14 @@ extern int percpu_counter_batch;
 
 int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 			       gfp_t gfp, u32 nr_counters,
-			       struct lock_class_key *key);
+			       struct lock_class_key *key, bool switch_mode);
 
 #define percpu_counter_init_many(fbc, value, gfp, nr_counters)		\
 	({								\
 		static struct lock_class_key __key;			\
 									\
 		__percpu_counter_init_many(fbc, value, gfp, nr_counters,\
-					   &__key);			\
+					   &__key, false);		\
 	})
 
 
@@ -130,6 +136,20 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
 	return (fbc->counters != NULL);
 }
 
+static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
+{
+	return atomic64_read(&fbc->count_atomic);
+}
+
+static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
+					     s64 amount)
+{
+	atomic64_add(amount, &fbc->count_atomic);
+}
+
+int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+				       u32 nr_counters);
+
 #else /* !CONFIG_SMP */
 
 struct percpu_counter {
@@ -260,6 +280,23 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
 static inline void percpu_counter_sync(struct percpu_counter *fbc)
 {
 }
+
+static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
+{
+	return fbc->count;
+}
+
+static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
+					     s64 amount)
+{
+	percpu_counter_add(fbc, amount);
+}
+
+static inline int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+						     u32 nr_counters)
+{
+	return 0;
+}
 #endif	/* CONFIG_SMP */
 
 static inline void percpu_counter_inc(struct percpu_counter *fbc)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 44dd133594d4..95c4e038051a 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
 
 int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 			       gfp_t gfp, u32 nr_counters,
-			       struct lock_class_key *key)
+			       struct lock_class_key *key, bool switch_mode)
 {
 	unsigned long flags __maybe_unused;
 	size_t counter_size;
@@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
 #ifdef CONFIG_HOTPLUG_CPU
 		INIT_LIST_HEAD(&fbc[i].list);
 #endif
-		fbc[i].count = amount;
+		if (likely(!switch_mode))
+			fbc[i].count = amount;
 		fbc[i].counters = (void *)counters + (i * counter_size);
 
 		debug_percpu_counter_activate(&fbc[i]);
@@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
 	return good;
 }
 
+/*
+ * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
+ * atomic mode to percpu mode.
+ */
+int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
+				       u32 nr_counters)
+{
+	static struct lock_class_key __key;
+	unsigned long flags;
+	bool ret = 0;
+
+	if (percpu_counter_initialized(fbc))
+		return 0;
+
+	preempt_disable();
+	local_irq_save(flags);
+	if (likely(!percpu_counter_initialized(fbc)))
+		ret = __percpu_counter_init_many(fbc, 0,
+					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
+					nr_counters, &__key, true);
+	local_irq_restore(flags);
+	preempt_enable();
+
+	return ret;
+}
+
 static int __init percpu_counter_startup(void)
 {
 	int ret;
-- 
2.25.1

[RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Peng Zhang]

From: ZhangPeng <zhangpeng362@huawei.com>

Since commit f1a7941243c1 ("mm: convert mm's rss stats into
percpu_counter"), the rss_stats have converted into percpu_counter,
which convert the error margin from (nr_threads * 64) to approximately
(nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
performance regression on fork/exec/shell. Even after commit 14ef95be6f55
("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
the performance of fork/exec/shell is still poor compared to previous
kernel versions.

To mitigate performance regression, we delay the allocation of percpu
memory for rss_stats. Therefore, we convert mm's rss stats to use
percpu_counter atomic mode. For single-thread processes, rss_stat is in
atomic mode, which reduces the memory consumption and performance
regression caused by using percpu. For multiple-thread processes,
rss_stat is switched to the percpu mode to reduce the error margin.
We convert rss_stats from atomic mode to percpu mode only when the
second thread is created.

After lmbench test, we can get 2% ~ 4% performance improvement
for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
improvement for lmbench page_fault (before batch mode[1]).

The test results are as follows:

             base           base+revert        base+this patch

fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)

[1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 include/linux/mm.h          | 50 +++++++++++++++++++++++++++++++------
 include/trace/events/kmem.h |  4 +--
 kernel/fork.c               | 18 +++++++------
 3 files changed, 56 insertions(+), 16 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index d261e45bb29b..8f1bfbd54697 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2631,30 +2631,66 @@ static inline bool get_user_page_fast_only(unsigned long addr,
  */
 static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
 {
-	return percpu_counter_read_positive(&mm->rss_stat[member]);
+	struct percpu_counter *fbc = &mm->rss_stat[member];
+
+	if (percpu_counter_initialized(fbc))
+		return percpu_counter_read_positive(fbc);
+
+	return percpu_counter_atomic_read(fbc);
 }
 
 void mm_trace_rss_stat(struct mm_struct *mm, int member);
 
 static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
 {
-	percpu_counter_add(&mm->rss_stat[member], value);
+	struct percpu_counter *fbc = &mm->rss_stat[member];
+
+	if (percpu_counter_initialized(fbc))
+		percpu_counter_add(fbc, value);
+	else
+		percpu_counter_atomic_add(fbc, value);
 
 	mm_trace_rss_stat(mm, member);
 }
 
 static inline void inc_mm_counter(struct mm_struct *mm, int member)
 {
-	percpu_counter_inc(&mm->rss_stat[member]);
-
-	mm_trace_rss_stat(mm, member);
+	add_mm_counter(mm, member, 1);
 }
 
 static inline void dec_mm_counter(struct mm_struct *mm, int member)
 {
-	percpu_counter_dec(&mm->rss_stat[member]);
+	add_mm_counter(mm, member, -1);
+}
 
-	mm_trace_rss_stat(mm, member);
+static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
+{
+	struct percpu_counter *fbc = &mm->rss_stat[member];
+
+	if (percpu_counter_initialized(fbc))
+		return percpu_counter_sum(fbc);
+
+	return percpu_counter_atomic_read(fbc);
+}
+
+static inline s64 mm_counter_sum_positive(struct mm_struct *mm, int member)
+{
+	struct percpu_counter *fbc = &mm->rss_stat[member];
+
+	if (percpu_counter_initialized(fbc))
+		return percpu_counter_sum_positive(fbc);
+
+	return percpu_counter_atomic_read(fbc);
+}
+
+static inline int mm_counter_switch_to_pcpu_many(struct mm_struct *mm)
+{
+	return percpu_counter_switch_to_pcpu_many(mm->rss_stat, NR_MM_COUNTERS);
+}
+
+static inline void mm_counter_destroy_many(struct mm_struct *mm)
+{
+	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
 }
 
 /* Optimized variant when folio is already known not to be anon */
diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 6e62cc64cd92..a4e40ae6a8c8 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
 		__entry->mm_id = mm_ptr_to_hash(mm);
 		__entry->curr = !!(current->mm == mm);
 		__entry->member = member;
-		__entry->size = (percpu_counter_sum_positive(&mm->rss_stat[member])
-							    << PAGE_SHIFT);
+		__entry->size = (mm_counter_sum_positive(mm, member)
+							<< PAGE_SHIFT);
 	),
 
 	TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
diff --git a/kernel/fork.c b/kernel/fork.c
index 99076dbe27d8..0214273798c5 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -823,7 +823,7 @@ static void check_mm(struct mm_struct *mm)
 			 "Please make sure 'struct resident_page_types[]' is updated as well");
 
 	for (i = 0; i < NR_MM_COUNTERS; i++) {
-		long x = percpu_counter_sum(&mm->rss_stat[i]);
+		long x = mm_counter_sum(mm, i);
 
 		if (unlikely(x))
 			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
@@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 	if (mm_alloc_cid(mm))
 		goto fail_cid;
 
-	if (percpu_counter_init_many(mm->rss_stat, 0, GFP_KERNEL_ACCOUNT,
-				     NR_MM_COUNTERS))
-		goto fail_pcpu;
-
 	mm->user_ns = get_user_ns(user_ns);
 	lru_gen_init_mm(mm);
 	return mm;
 
-fail_pcpu:
-	mm_destroy_cid(mm);
 fail_cid:
 	destroy_context(mm);
 fail_nocontext:
@@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
 	if (!oldmm)
 		return 0;
 
+	/*
+	 * For single-thread processes, rss_stat is in atomic mode, which
+	 * reduces the memory consumption and performance regression caused by
+	 * using percpu. For multiple-thread processes, rss_stat is switched to
+	 * the percpu mode to reduce the error margin.
+	 */
+	if (clone_flags & CLONE_THREAD)
+		if (mm_counter_switch_to_pcpu_many(oldmm))
+			return -ENOMEM;
+
 	if (clone_flags & CLONE_VM) {
 		mmget(oldmm);
 		mm = oldmm;
-- 
2.25.1

Re: [RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[Andrew Morton]

On Thu, 18 Apr 2024 22:20:07 +0800 Peng Zhang <zhangpeng362@huawei.com> wrote:

> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> Depending on whether counters is NULL, we can support two modes:
> atomic mode and perpcu mode. We implement both modes by grouping
> the s64 count and atomic64_t count_atomic in a union. At the same time,
> we create the interface for adding and reading in atomic mode and for
> switching atomic mode to percpu mode.
> 

I think it would be better if we had a detailed code comment in an
appropriate place which fully describes the tradeoffs here.  Tell
people when they would benefit from using one mode versus the other.


> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
>  
>  int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  			       gfp_t gfp, u32 nr_counters,
> -			       struct lock_class_key *key)
> +			       struct lock_class_key *key, bool switch_mode)
>  {
>  	unsigned long flags __maybe_unused;
>  	size_t counter_size;
> @@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  #ifdef CONFIG_HOTPLUG_CPU
>  		INIT_LIST_HEAD(&fbc[i].list);
>  #endif
> -		fbc[i].count = amount;
> +		if (likely(!switch_mode))
> +			fbc[i].count = amount;
>  		fbc[i].counters = (void *)counters + (i * counter_size);
>  
>  		debug_percpu_counter_activate(&fbc[i]);
> @@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
>  	return good;
>  }
>  
> +/*
> + * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
> + * atomic mode to percpu mode.

Describe what happens if that GFP_ATOMIC allocation fails.  We remain
in atomic mode, yes?

> + */
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters)
> +{
> +	static struct lock_class_key __key;
> +	unsigned long flags;
> +	bool ret = 0;
> +
> +	if (percpu_counter_initialized(fbc))
> +		return 0;
> +
> +	preempt_disable();
> +	local_irq_save(flags);

Do we need both?  Does local_irq_save() always disable preemption? 
This might not be the case for RT kernels, I always forget.

> +	if (likely(!percpu_counter_initialized(fbc)))
> +		ret = __percpu_counter_init_many(fbc, 0,
> +					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
> +					nr_counters, &__key, true);
> +	local_irq_restore(flags);
> +	preempt_enable();
> +
> +	return ret;
> +}
> +

Why is there no API for switching back to atomic mode?

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Rongwei Wang]

On 2024/4/18 22:20, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@huawei.com>
>
> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> percpu_counter"), the rss_stats have converted into percpu_counter,
> which convert the error margin from (nr_threads * 64) to approximately
> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> performance regression on fork/exec/shell. Even after commit 14ef95be6f55
> ("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
> the performance of fork/exec/shell is still poor compared to previous
> kernel versions.
>
> To mitigate performance regression, we delay the allocation of percpu
> memory for rss_stats. Therefore, we convert mm's rss stats to use
> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> atomic mode, which reduces the memory consumption and performance
> regression caused by using percpu. For multiple-thread processes,
> rss_stat is switched to the percpu mode to reduce the error margin.
> We convert rss_stats from atomic mode to percpu mode only when the
> second thread is created.
Hi, Zhang Peng

This regression we also found it in lmbench these days. I have not test 
your patch, but it seems will solve a lot for it.
And I see this patch not fix the regression in multi-threads, that's 
because of the rss_stat switched to percpu mode?
(If I'm wrong, please correct me.) And It seems percpu_counter also has 
a bad effect in exit_mmap().

If so, I'm wondering if we can further improving it on the exit_mmap() 
path in multi-threads scenario, e.g. to determine which CPUs the process 
has run on (mm_cpumask()? I'm not sure).

>
> After lmbench test, we can get 2% ~ 4% performance improvement
> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
> improvement for lmbench page_fault (before batch mode[1]).
>
> The test results are as follows:
>
>               base           base+revert        base+this patch
>
> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
I think the regression will becomes more obvious if more cores. How 
about your test machine?

Thanks,
-wrw
>
> [1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>   include/linux/mm.h          | 50 +++++++++++++++++++++++++++++++------
>   include/trace/events/kmem.h |  4 +--
>   kernel/fork.c               | 18 +++++++------
>   3 files changed, 56 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d261e45bb29b..8f1bfbd54697 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2631,30 +2631,66 @@ static inline bool get_user_page_fast_only(unsigned long addr,
>    */
>   static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
>   {
> -	return percpu_counter_read_positive(&mm->rss_stat[member]);
> +	struct percpu_counter *fbc = &mm->rss_stat[member];
> +
> +	if (percpu_counter_initialized(fbc))
> +		return percpu_counter_read_positive(fbc);
> +
> +	return percpu_counter_atomic_read(fbc);
>   }
>   
>   void mm_trace_rss_stat(struct mm_struct *mm, int member);
>   
>   static inline void add_mm_counter(struct mm_struct *mm, int member, long value)
>   {
> -	percpu_counter_add(&mm->rss_stat[member], value);
> +	struct percpu_counter *fbc = &mm->rss_stat[member];
> +
> +	if (percpu_counter_initialized(fbc))
> +		percpu_counter_add(fbc, value);
> +	else
> +		percpu_counter_atomic_add(fbc, value);
>   
>   	mm_trace_rss_stat(mm, member);
>   }
>   
>   static inline void inc_mm_counter(struct mm_struct *mm, int member)
>   {
> -	percpu_counter_inc(&mm->rss_stat[member]);
> -
> -	mm_trace_rss_stat(mm, member);
> +	add_mm_counter(mm, member, 1);
>   }
>   
>   static inline void dec_mm_counter(struct mm_struct *mm, int member)
>   {
> -	percpu_counter_dec(&mm->rss_stat[member]);
> +	add_mm_counter(mm, member, -1);
> +}
>   
> -	mm_trace_rss_stat(mm, member);
> +static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
> +{
> +	struct percpu_counter *fbc = &mm->rss_stat[member];
> +
> +	if (percpu_counter_initialized(fbc))
> +		return percpu_counter_sum(fbc);
> +
> +	return percpu_counter_atomic_read(fbc);
> +}
> +
> +static inline s64 mm_counter_sum_positive(struct mm_struct *mm, int member)
> +{
> +	struct percpu_counter *fbc = &mm->rss_stat[member];
> +
> +	if (percpu_counter_initialized(fbc))
> +		return percpu_counter_sum_positive(fbc);
> +
> +	return percpu_counter_atomic_read(fbc);
> +}
> +
> +static inline int mm_counter_switch_to_pcpu_many(struct mm_struct *mm)
> +{
> +	return percpu_counter_switch_to_pcpu_many(mm->rss_stat, NR_MM_COUNTERS);
> +}
> +
> +static inline void mm_counter_destroy_many(struct mm_struct *mm)
> +{
> +	percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>   }
>   
>   /* Optimized variant when folio is already known not to be anon */
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index 6e62cc64cd92..a4e40ae6a8c8 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
>   		__entry->mm_id = mm_ptr_to_hash(mm);
>   		__entry->curr = !!(current->mm == mm);
>   		__entry->member = member;
> -		__entry->size = (percpu_counter_sum_positive(&mm->rss_stat[member])
> -							    << PAGE_SHIFT);
> +		__entry->size = (mm_counter_sum_positive(mm, member)
> +							<< PAGE_SHIFT);
>   	),
>   
>   	TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 99076dbe27d8..0214273798c5 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -823,7 +823,7 @@ static void check_mm(struct mm_struct *mm)
>   			 "Please make sure 'struct resident_page_types[]' is updated as well");
>   
>   	for (i = 0; i < NR_MM_COUNTERS; i++) {
> -		long x = percpu_counter_sum(&mm->rss_stat[i]);
> +		long x = mm_counter_sum(mm, i);
>   
>   		if (unlikely(x))
>   			pr_alert("BUG: Bad rss-counter state mm:%p type:%s val:%ld\n",
> @@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>   	if (mm_alloc_cid(mm))
>   		goto fail_cid;
>   
> -	if (percpu_counter_init_many(mm->rss_stat, 0, GFP_KERNEL_ACCOUNT,
> -				     NR_MM_COUNTERS))
> -		goto fail_pcpu;
> -
>   	mm->user_ns = get_user_ns(user_ns);
>   	lru_gen_init_mm(mm);
>   	return mm;
>   
> -fail_pcpu:
> -	mm_destroy_cid(mm);
>   fail_cid:
>   	destroy_context(mm);
>   fail_nocontext:
> @@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
>   	if (!oldmm)
>   		return 0;
>   
> +	/*
> +	 * For single-thread processes, rss_stat is in atomic mode, which
> +	 * reduces the memory consumption and performance regression caused by
> +	 * using percpu. For multiple-thread processes, rss_stat is switched to
> +	 * the percpu mode to reduce the error margin.
> +	 */
> +	if (clone_flags & CLONE_THREAD)
> +		if (mm_counter_switch_to_pcpu_many(oldmm))
> +			return -ENOMEM;
> +
>   	if (clone_flags & CLONE_VM) {
>   		mmget(oldmm);
>   		mm = oldmm;

Re: [RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[zhangpeng (AS)]

On 2024/4/19 3:40, Andrew Morton wrote:

> On Thu, 18 Apr 2024 22:20:07 +0800 Peng Zhang <zhangpeng362@huawei.com> wrote:
>
>> From: ZhangPeng <zhangpeng362@huawei.com>
>>
>> Depending on whether counters is NULL, we can support two modes:
>> atomic mode and perpcu mode. We implement both modes by grouping
>> the s64 count and atomic64_t count_atomic in a union. At the same time,
>> we create the interface for adding and reading in atomic mode and for
>> switching atomic mode to percpu mode.
>>
> I think it would be better if we had a detailed code comment in an
> appropriate place which fully describes the tradeoffs here.  Tell
> people when they would benefit from using one mode versus the other.
>
Thanks for your reply!

Yes, of course, I would add before the union:

	/*
	 * Depending on whether counters is NULL, we can support two modes,
	 * atomic mode using count_atomic and perpcu mode using count.
	 * The single-thread processes should use atomic mode to reduce the
	 * memory consumption and performance regression.
	 * The multiple-thread processes should use percpu mode to reduce the
	 * error margin.
	 */
	union {
		s64 count;
		atomic64_t count_atomic;
	};

>> --- a/lib/percpu_counter.c
>> +++ b/lib/percpu_counter.c
>> @@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
>>   
>>   int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>>   			       gfp_t gfp, u32 nr_counters,
>> -			       struct lock_class_key *key)
>> +			       struct lock_class_key *key, bool switch_mode)
>>   {
>>   	unsigned long flags __maybe_unused;
>>   	size_t counter_size;
>> @@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>>   #ifdef CONFIG_HOTPLUG_CPU
>>   		INIT_LIST_HEAD(&fbc[i].list);
>>   #endif
>> -		fbc[i].count = amount;
>> +		if (likely(!switch_mode))
>> +			fbc[i].count = amount;
>>   		fbc[i].counters = (void *)counters + (i * counter_size);
>>   
>>   		debug_percpu_counter_activate(&fbc[i]);
>> @@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
>>   	return good;
>>   }
>>   
>> +/*
>> + * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
>> + * atomic mode to percpu mode.
> Describe what happens if that GFP_ATOMIC allocation fails.  We remain
> in atomic mode, yes?

Yes. I'll add comments like:
  * Return: 0 if percpu_counter is already in atomic mode or successfully
  * switched to atomic mode; -ENOMEM if perpcu memory allocation fails,
  * perpcu_counter is still in atomic mode.

>> + */
>> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
>> +				       u32 nr_counters)
>> +{
>> +	static struct lock_class_key __key;
>> +	unsigned long flags;
>> +	bool ret = 0;
>> +
>> +	if (percpu_counter_initialized(fbc))
>> +		return 0;
>> +
>> +	preempt_disable();
>> +	local_irq_save(flags);
> Do we need both?  Does local_irq_save() always disable preemption?
> This might not be the case for RT kernels, I always forget.

Yes, we need both. For RT kernels, local_irq_save() doesn't mean
that preemption is disabled.

>> +	if (likely(!percpu_counter_initialized(fbc)))
>> +		ret = __percpu_counter_init_many(fbc, 0,
>> +					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
>> +					nr_counters, &__key, true);
>> +	local_irq_restore(flags);
>> +	preempt_enable();
>> +
>> +	return ret;
>> +}
>> +
> Why is there no API for switching back to atomic mode?

At least for the current test scenario, we don't need to switch back
to atomic mode. The scenario for switching back to atomic mode may be
that the multi-threaded process destroys the last second thread. I could
add this API if needed later.

-- 
Best Regards,
Peng

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[zhangpeng (AS)]

On 2024/4/19 10:30, Rongwei Wang wrote:

> On 2024/4/18 22:20, Peng Zhang wrote:
>> From: ZhangPeng <zhangpeng362@huawei.com>
>>
>> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
>> percpu_counter"), the rss_stats have converted into percpu_counter,
>> which convert the error margin from (nr_threads * 64) to approximately
>> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
>> performance regression on fork/exec/shell. Even after commit 
>> 14ef95be6f55
>> ("kernel/fork: group allocation/free of per-cpu counters for mm 
>> struct"),
>> the performance of fork/exec/shell is still poor compared to previous
>> kernel versions.
>>
>> To mitigate performance regression, we delay the allocation of percpu
>> memory for rss_stats. Therefore, we convert mm's rss stats to use
>> percpu_counter atomic mode. For single-thread processes, rss_stat is in
>> atomic mode, which reduces the memory consumption and performance
>> regression caused by using percpu. For multiple-thread processes,
>> rss_stat is switched to the percpu mode to reduce the error margin.
>> We convert rss_stats from atomic mode to percpu mode only when the
>> second thread is created.
> Hi, Zhang Peng
>
> This regression we also found it in lmbench these days. I have not 
> test your patch, but it seems will solve a lot for it.
> And I see this patch not fix the regression in multi-threads, that's 
> because of the rss_stat switched to percpu mode?
> (If I'm wrong, please correct me.) And It seems percpu_counter also 
> has a bad effect in exit_mmap().
>
> If so, I'm wondering if we can further improving it on the exit_mmap() 
> path in multi-threads scenario, e.g. to determine which CPUs the 
> process has run on (mm_cpumask()? I'm not sure).
>
Hi, Rongwei,

Yes, this patch only fixes the regression in single-thread processes. How
much bad effect does percpu_counter have in exit_mmap()? IMHO, the addition
of mm counter is already in batch mode, maybe I miss something?

>>
>> After lmbench test, we can get 2% ~ 4% performance improvement
>> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
>> improvement for lmbench page_fault (before batch mode[1]).
>>
>> The test results are as follows:
>>
>>               base           base+revert        base+this patch
>>
>> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
>> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
>> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
>> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
> I think the regression will becomes more obvious if more cores. How 
> about your test machine?
>
Maybe multi-core is not a factor in the performance of the lmbench test here.
Both of my test machines have 96 cores.

> Thanks,
> -wrw
>>
>> [1] 
>> https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>>
>> Suggested-by: Jan Kara <jack@suse.cz>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   include/linux/mm.h          | 50 +++++++++++++++++++++++++++++++------
>>   include/trace/events/kmem.h |  4 +--
>>   kernel/fork.c               | 18 +++++++------
>>   3 files changed, 56 insertions(+), 16 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index d261e45bb29b..8f1bfbd54697 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2631,30 +2631,66 @@ static inline bool 
>> get_user_page_fast_only(unsigned long addr,
>>    */
>>   static inline unsigned long get_mm_counter(struct mm_struct *mm, 
>> int member)
>>   {
>> -    return percpu_counter_read_positive(&mm->rss_stat[member]);
>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>> +
>> +    if (percpu_counter_initialized(fbc))
>> +        return percpu_counter_read_positive(fbc);
>> +
>> +    return percpu_counter_atomic_read(fbc);
>>   }
>>     void mm_trace_rss_stat(struct mm_struct *mm, int member);
>>     static inline void add_mm_counter(struct mm_struct *mm, int 
>> member, long value)
>>   {
>> -    percpu_counter_add(&mm->rss_stat[member], value);
>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>> +
>> +    if (percpu_counter_initialized(fbc))
>> +        percpu_counter_add(fbc, value);
>> +    else
>> +        percpu_counter_atomic_add(fbc, value);
>>         mm_trace_rss_stat(mm, member);
>>   }
>>     static inline void inc_mm_counter(struct mm_struct *mm, int member)
>>   {
>> -    percpu_counter_inc(&mm->rss_stat[member]);
>> -
>> -    mm_trace_rss_stat(mm, member);
>> +    add_mm_counter(mm, member, 1);
>>   }
>>     static inline void dec_mm_counter(struct mm_struct *mm, int member)
>>   {
>> -    percpu_counter_dec(&mm->rss_stat[member]);
>> +    add_mm_counter(mm, member, -1);
>> +}
>>   -    mm_trace_rss_stat(mm, member);
>> +static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
>> +{
>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>> +
>> +    if (percpu_counter_initialized(fbc))
>> +        return percpu_counter_sum(fbc);
>> +
>> +    return percpu_counter_atomic_read(fbc);
>> +}
>> +
>> +static inline s64 mm_counter_sum_positive(struct mm_struct *mm, int 
>> member)
>> +{
>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>> +
>> +    if (percpu_counter_initialized(fbc))
>> +        return percpu_counter_sum_positive(fbc);
>> +
>> +    return percpu_counter_atomic_read(fbc);
>> +}
>> +
>> +static inline int mm_counter_switch_to_pcpu_many(struct mm_struct *mm)
>> +{
>> +    return percpu_counter_switch_to_pcpu_many(mm->rss_stat, 
>> NR_MM_COUNTERS);
>> +}
>> +
>> +static inline void mm_counter_destroy_many(struct mm_struct *mm)
>> +{
>> +    percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>>   }
>>     /* Optimized variant when folio is already known not to be anon */
>> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
>> index 6e62cc64cd92..a4e40ae6a8c8 100644
>> --- a/include/trace/events/kmem.h
>> +++ b/include/trace/events/kmem.h
>> @@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
>>           __entry->mm_id = mm_ptr_to_hash(mm);
>>           __entry->curr = !!(current->mm == mm);
>>           __entry->member = member;
>> -        __entry->size = 
>> (percpu_counter_sum_positive(&mm->rss_stat[member])
>> -                                << PAGE_SHIFT);
>> +        __entry->size = (mm_counter_sum_positive(mm, member)
>> +                            << PAGE_SHIFT);
>>       ),
>>         TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 99076dbe27d8..0214273798c5 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -823,7 +823,7 @@ static void check_mm(struct mm_struct *mm)
>>                "Please make sure 'struct resident_page_types[]' is 
>> updated as well");
>>         for (i = 0; i < NR_MM_COUNTERS; i++) {
>> -        long x = percpu_counter_sum(&mm->rss_stat[i]);
>> +        long x = mm_counter_sum(mm, i);
>>             if (unlikely(x))
>>               pr_alert("BUG: Bad rss-counter state mm:%p type:%s 
>> val:%ld\n",
>> @@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct 
>> mm_struct *mm, struct task_struct *p,
>>       if (mm_alloc_cid(mm))
>>           goto fail_cid;
>>   -    if (percpu_counter_init_many(mm->rss_stat, 0, GFP_KERNEL_ACCOUNT,
>> -                     NR_MM_COUNTERS))
>> -        goto fail_pcpu;
>> -
>>       mm->user_ns = get_user_ns(user_ns);
>>       lru_gen_init_mm(mm);
>>       return mm;
>>   -fail_pcpu:
>> -    mm_destroy_cid(mm);
>>   fail_cid:
>>       destroy_context(mm);
>>   fail_nocontext:
>> @@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long clone_flags, 
>> struct task_struct *tsk)
>>       if (!oldmm)
>>           return 0;
>>   +    /*
>> +     * For single-thread processes, rss_stat is in atomic mode, which
>> +     * reduces the memory consumption and performance regression 
>> caused by
>> +     * using percpu. For multiple-thread processes, rss_stat is 
>> switched to
>> +     * the percpu mode to reduce the error margin.
>> +     */
>> +    if (clone_flags & CLONE_THREAD)
>> +        if (mm_counter_switch_to_pcpu_many(oldmm))
>> +            return -ENOMEM;
>> +
>>       if (clone_flags & CLONE_VM) {
>>           mmget(oldmm);
>>           mm = oldmm;
>
>
-- 
Best Regards,
Peng

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Rongwei Wang]

On 2024/4/19 11:32, zhangpeng (AS) wrote:
> On 2024/4/19 10:30, Rongwei Wang wrote:
>
>> On 2024/4/18 22:20, Peng Zhang wrote:
>>> From: ZhangPeng <zhangpeng362@huawei.com>
>>>
>>> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
>>> percpu_counter"), the rss_stats have converted into percpu_counter,
>>> which convert the error margin from (nr_threads * 64) to approximately
>>> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
>>> performance regression on fork/exec/shell. Even after commit 
>>> 14ef95be6f55
>>> ("kernel/fork: group allocation/free of per-cpu counters for mm 
>>> struct"),
>>> the performance of fork/exec/shell is still poor compared to previous
>>> kernel versions.
>>>
>>> To mitigate performance regression, we delay the allocation of percpu
>>> memory for rss_stats. Therefore, we convert mm's rss stats to use
>>> percpu_counter atomic mode. For single-thread processes, rss_stat is in
>>> atomic mode, which reduces the memory consumption and performance
>>> regression caused by using percpu. For multiple-thread processes,
>>> rss_stat is switched to the percpu mode to reduce the error margin.
>>> We convert rss_stats from atomic mode to percpu mode only when the
>>> second thread is created.
>> Hi, Zhang Peng
>>
>> This regression we also found it in lmbench these days. I have not 
>> test your patch, but it seems will solve a lot for it.
>> And I see this patch not fix the regression in multi-threads, that's 
>> because of the rss_stat switched to percpu mode?
>> (If I'm wrong, please correct me.) And It seems percpu_counter also 
>> has a bad effect in exit_mmap().
>>
>> If so, I'm wondering if we can further improving it on the 
>> exit_mmap() path in multi-threads scenario, e.g. to determine which 
>> CPUs the process has run on (mm_cpumask()? I'm not sure).
>>
> Hi, Rongwei,
>
> Yes, this patch only fixes the regression in single-thread processes. How
> much bad effect does percpu_counter have in exit_mmap()? IMHO, the 
> addition
Actually, I not sure, just found a little free percpu hotspot in 
exit_mmap() path when comparing 4 core vs 32 cores.

I can test more next.
> of mm counter is already in batch mode, maybe I miss something?
>
>>>
>>> After lmbench test, we can get 2% ~ 4% performance improvement
>>> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
>>> improvement for lmbench page_fault (before batch mode[1]).
>>>
>>> The test results are as follows:
>>>
>>>               base           base+revert        base+this patch
>>>
>>> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
>>> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
>>> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
>>> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
>> I think the regression will becomes more obvious if more cores. How 
>> about your test machine?
>>
> Maybe multi-core is not a factor in the performance of the lmbench 
> test here.
> Both of my test machines have 96 cores.
>
>> Thanks,
>> -wrw
>>>
>>> [1] 
>>> https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>>>
>>> Suggested-by: Jan Kara <jack@suse.cz>
>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> ---
>>>   include/linux/mm.h          | 50 
>>> +++++++++++++++++++++++++++++++------
>>>   include/trace/events/kmem.h |  4 +--
>>>   kernel/fork.c               | 18 +++++++------
>>>   3 files changed, 56 insertions(+), 16 deletions(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index d261e45bb29b..8f1bfbd54697 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -2631,30 +2631,66 @@ static inline bool 
>>> get_user_page_fast_only(unsigned long addr,
>>>    */
>>>   static inline unsigned long get_mm_counter(struct mm_struct *mm, 
>>> int member)
>>>   {
>>> -    return percpu_counter_read_positive(&mm->rss_stat[member]);
>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>> +
>>> +    if (percpu_counter_initialized(fbc))
>>> +        return percpu_counter_read_positive(fbc);
>>> +
>>> +    return percpu_counter_atomic_read(fbc);
>>>   }
>>>     void mm_trace_rss_stat(struct mm_struct *mm, int member);
>>>     static inline void add_mm_counter(struct mm_struct *mm, int 
>>> member, long value)
>>>   {
>>> -    percpu_counter_add(&mm->rss_stat[member], value);
>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>> +
>>> +    if (percpu_counter_initialized(fbc))
>>> +        percpu_counter_add(fbc, value);
>>> +    else
>>> +        percpu_counter_atomic_add(fbc, value);
>>>         mm_trace_rss_stat(mm, member);
>>>   }
>>>     static inline void inc_mm_counter(struct mm_struct *mm, int member)
>>>   {
>>> -    percpu_counter_inc(&mm->rss_stat[member]);
>>> -
>>> -    mm_trace_rss_stat(mm, member);
>>> +    add_mm_counter(mm, member, 1);
>>>   }
>>>     static inline void dec_mm_counter(struct mm_struct *mm, int member)
>>>   {
>>> -    percpu_counter_dec(&mm->rss_stat[member]);
>>> +    add_mm_counter(mm, member, -1);
>>> +}
>>>   -    mm_trace_rss_stat(mm, member);
>>> +static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
>>> +{
>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>> +
>>> +    if (percpu_counter_initialized(fbc))
>>> +        return percpu_counter_sum(fbc);
>>> +
>>> +    return percpu_counter_atomic_read(fbc);
>>> +}
>>> +
>>> +static inline s64 mm_counter_sum_positive(struct mm_struct *mm, int 
>>> member)
>>> +{
>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>> +
>>> +    if (percpu_counter_initialized(fbc))
>>> +        return percpu_counter_sum_positive(fbc);
>>> +
>>> +    return percpu_counter_atomic_read(fbc);
>>> +}
>>> +
>>> +static inline int mm_counter_switch_to_pcpu_many(struct mm_struct *mm)
>>> +{
>>> +    return percpu_counter_switch_to_pcpu_many(mm->rss_stat, 
>>> NR_MM_COUNTERS);
>>> +}
>>> +
>>> +static inline void mm_counter_destroy_many(struct mm_struct *mm)
>>> +{
>>> +    percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>>>   }
>>>     /* Optimized variant when folio is already known not to be anon */
>>> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
>>> index 6e62cc64cd92..a4e40ae6a8c8 100644
>>> --- a/include/trace/events/kmem.h
>>> +++ b/include/trace/events/kmem.h
>>> @@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
>>>           __entry->mm_id = mm_ptr_to_hash(mm);
>>>           __entry->curr = !!(current->mm == mm);
>>>           __entry->member = member;
>>> -        __entry->size = 
>>> (percpu_counter_sum_positive(&mm->rss_stat[member])
>>> -                                << PAGE_SHIFT);
>>> +        __entry->size = (mm_counter_sum_positive(mm, member)
>>> +                            << PAGE_SHIFT);
>>>       ),
>>>         TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>> index 99076dbe27d8..0214273798c5 100644
>>> --- a/kernel/fork.c
>>> +++ b/kernel/fork.c
>>> @@ -823,7 +823,7 @@ static void check_mm(struct mm_struct *mm)
>>>                "Please make sure 'struct resident_page_types[]' is 
>>> updated as well");
>>>         for (i = 0; i < NR_MM_COUNTERS; i++) {
>>> -        long x = percpu_counter_sum(&mm->rss_stat[i]);
>>> +        long x = mm_counter_sum(mm, i);
>>>             if (unlikely(x))
>>>               pr_alert("BUG: Bad rss-counter state mm:%p type:%s 
>>> val:%ld\n",
>>> @@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct 
>>> mm_struct *mm, struct task_struct *p,
>>>       if (mm_alloc_cid(mm))
>>>           goto fail_cid;
>>>   -    if (percpu_counter_init_many(mm->rss_stat, 0, 
>>> GFP_KERNEL_ACCOUNT,
>>> -                     NR_MM_COUNTERS))
>>> -        goto fail_pcpu;
>>> -
>>>       mm->user_ns = get_user_ns(user_ns);
>>>       lru_gen_init_mm(mm);
>>>       return mm;
>>>   -fail_pcpu:
>>> -    mm_destroy_cid(mm);
>>>   fail_cid:
>>>       destroy_context(mm);
>>>   fail_nocontext:
>>> @@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long clone_flags, 
>>> struct task_struct *tsk)
>>>       if (!oldmm)
>>>           return 0;
>>>   +    /*
>>> +     * For single-thread processes, rss_stat is in atomic mode, which
>>> +     * reduces the memory consumption and performance regression 
>>> caused by
>>> +     * using percpu. For multiple-thread processes, rss_stat is 
>>> switched to
>>> +     * the percpu mode to reduce the error margin.
>>> +     */
>>> +    if (clone_flags & CLONE_THREAD)
>>> +        if (mm_counter_switch_to_pcpu_many(oldmm))
>>> +            return -ENOMEM;
>>> +
>>>       if (clone_flags & CLONE_VM) {
>>>           mmget(oldmm);
>>>           mm = oldmm;
>>
>>

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[zhangpeng (AS)]

On 2024/4/20 11:13, Rongwei Wang wrote:

> On 2024/4/19 11:32, zhangpeng (AS) wrote:
>> On 2024/4/19 10:30, Rongwei Wang wrote:
>>
>>> On 2024/4/18 22:20, Peng Zhang wrote:
>>>> From: ZhangPeng <zhangpeng362@huawei.com>
>>>>
>>>> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
>>>> percpu_counter"), the rss_stats have converted into percpu_counter,
>>>> which convert the error margin from (nr_threads * 64) to approximately
>>>> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() 
>>>> causes a
>>>> performance regression on fork/exec/shell. Even after commit 
>>>> 14ef95be6f55
>>>> ("kernel/fork: group allocation/free of per-cpu counters for mm 
>>>> struct"),
>>>> the performance of fork/exec/shell is still poor compared to previous
>>>> kernel versions.
>>>>
>>>> To mitigate performance regression, we delay the allocation of percpu
>>>> memory for rss_stats. Therefore, we convert mm's rss stats to use
>>>> percpu_counter atomic mode. For single-thread processes, rss_stat 
>>>> is in
>>>> atomic mode, which reduces the memory consumption and performance
>>>> regression caused by using percpu. For multiple-thread processes,
>>>> rss_stat is switched to the percpu mode to reduce the error margin.
>>>> We convert rss_stats from atomic mode to percpu mode only when the
>>>> second thread is created.
>>> Hi, Zhang Peng
>>>
>>> This regression we also found it in lmbench these days. I have not 
>>> test your patch, but it seems will solve a lot for it.
>>> And I see this patch not fix the regression in multi-threads, that's 
>>> because of the rss_stat switched to percpu mode?
>>> (If I'm wrong, please correct me.) And It seems percpu_counter also 
>>> has a bad effect in exit_mmap().
>>>
>>> If so, I'm wondering if we can further improving it on the 
>>> exit_mmap() path in multi-threads scenario, e.g. to determine which 
>>> CPUs the process has run on (mm_cpumask()? I'm not sure).
>>>
>> Hi, Rongwei,
>>
>> Yes, this patch only fixes the regression in single-thread processes. 
>> How
>> much bad effect does percpu_counter have in exit_mmap()? IMHO, the 
>> addition
> Actually, I not sure, just found a little free percpu hotspot in 
> exit_mmap() path when comparing 4 core vs 32 cores.
>
> I can test more next.

Thanks, it would be better if there is test data.

>> of mm counter is already in batch mode, maybe I miss something?
>>
>>>>
>>>> After lmbench test, we can get 2% ~ 4% performance improvement
>>>> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
>>>> improvement for lmbench page_fault (before batch mode[1]).
>>>>
>>>> The test results are as follows:
>>>>
>>>>               base           base+revert        base+this patch
>>>>
>>>> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms (4.2%)
>>>> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
>>>> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
>>>> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
>>> I think the regression will becomes more obvious if more cores. How 
>>> about your test machine?
>>>
>> Maybe multi-core is not a factor in the performance of the lmbench 
>> test here.
>> Both of my test machines have 96 cores.
>>
>>> Thanks,
>>> -wrw
>>>>
>>>> [1] 
>>>> https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>>>>
>>>> Suggested-by: Jan Kara <jack@suse.cz>
>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>>   include/linux/mm.h          | 50 
>>>> +++++++++++++++++++++++++++++++------
>>>>   include/trace/events/kmem.h |  4 +--
>>>>   kernel/fork.c               | 18 +++++++------
>>>>   3 files changed, 56 insertions(+), 16 deletions(-)
>>>>
>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>> index d261e45bb29b..8f1bfbd54697 100644
>>>> --- a/include/linux/mm.h
>>>> +++ b/include/linux/mm.h
>>>> @@ -2631,30 +2631,66 @@ static inline bool 
>>>> get_user_page_fast_only(unsigned long addr,
>>>>    */
>>>>   static inline unsigned long get_mm_counter(struct mm_struct *mm, 
>>>> int member)
>>>>   {
>>>> -    return percpu_counter_read_positive(&mm->rss_stat[member]);
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_read_positive(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>>   }
>>>>     void mm_trace_rss_stat(struct mm_struct *mm, int member);
>>>>     static inline void add_mm_counter(struct mm_struct *mm, int 
>>>> member, long value)
>>>>   {
>>>> -    percpu_counter_add(&mm->rss_stat[member], value);
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        percpu_counter_add(fbc, value);
>>>> +    else
>>>> +        percpu_counter_atomic_add(fbc, value);
>>>>         mm_trace_rss_stat(mm, member);
>>>>   }
>>>>     static inline void inc_mm_counter(struct mm_struct *mm, int 
>>>> member)
>>>>   {
>>>> -    percpu_counter_inc(&mm->rss_stat[member]);
>>>> -
>>>> -    mm_trace_rss_stat(mm, member);
>>>> +    add_mm_counter(mm, member, 1);
>>>>   }
>>>>     static inline void dec_mm_counter(struct mm_struct *mm, int 
>>>> member)
>>>>   {
>>>> -    percpu_counter_dec(&mm->rss_stat[member]);
>>>> +    add_mm_counter(mm, member, -1);
>>>> +}
>>>>   -    mm_trace_rss_stat(mm, member);
>>>> +static inline s64 mm_counter_sum(struct mm_struct *mm, int member)
>>>> +{
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_sum(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>> +}
>>>> +
>>>> +static inline s64 mm_counter_sum_positive(struct mm_struct *mm, 
>>>> int member)
>>>> +{
>>>> +    struct percpu_counter *fbc = &mm->rss_stat[member];
>>>> +
>>>> +    if (percpu_counter_initialized(fbc))
>>>> +        return percpu_counter_sum_positive(fbc);
>>>> +
>>>> +    return percpu_counter_atomic_read(fbc);
>>>> +}
>>>> +
>>>> +static inline int mm_counter_switch_to_pcpu_many(struct mm_struct 
>>>> *mm)
>>>> +{
>>>> +    return percpu_counter_switch_to_pcpu_many(mm->rss_stat, 
>>>> NR_MM_COUNTERS);
>>>> +}
>>>> +
>>>> +static inline void mm_counter_destroy_many(struct mm_struct *mm)
>>>> +{
>>>> +    percpu_counter_destroy_many(mm->rss_stat, NR_MM_COUNTERS);
>>>>   }
>>>>     /* Optimized variant when folio is already known not to be anon */
>>>> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
>>>> index 6e62cc64cd92..a4e40ae6a8c8 100644
>>>> --- a/include/trace/events/kmem.h
>>>> +++ b/include/trace/events/kmem.h
>>>> @@ -399,8 +399,8 @@ TRACE_EVENT(rss_stat,
>>>>           __entry->mm_id = mm_ptr_to_hash(mm);
>>>>           __entry->curr = !!(current->mm == mm);
>>>>           __entry->member = member;
>>>> -        __entry->size = 
>>>> (percpu_counter_sum_positive(&mm->rss_stat[member])
>>>> -                                << PAGE_SHIFT);
>>>> +        __entry->size = (mm_counter_sum_positive(mm, member)
>>>> +                            << PAGE_SHIFT);
>>>>       ),
>>>>         TP_printk("mm_id=%u curr=%d type=%s size=%ldB",
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index 99076dbe27d8..0214273798c5 100644
>>>> --- a/kernel/fork.c
>>>> +++ b/kernel/fork.c
>>>> @@ -823,7 +823,7 @@ static void check_mm(struct mm_struct *mm)
>>>>                "Please make sure 'struct resident_page_types[]' is 
>>>> updated as well");
>>>>         for (i = 0; i < NR_MM_COUNTERS; i++) {
>>>> -        long x = percpu_counter_sum(&mm->rss_stat[i]);
>>>> +        long x = mm_counter_sum(mm, i);
>>>>             if (unlikely(x))
>>>>               pr_alert("BUG: Bad rss-counter state mm:%p type:%s 
>>>> val:%ld\n",
>>>> @@ -1301,16 +1301,10 @@ static struct mm_struct *mm_init(struct 
>>>> mm_struct *mm, struct task_struct *p,
>>>>       if (mm_alloc_cid(mm))
>>>>           goto fail_cid;
>>>>   -    if (percpu_counter_init_many(mm->rss_stat, 0, 
>>>> GFP_KERNEL_ACCOUNT,
>>>> -                     NR_MM_COUNTERS))
>>>> -        goto fail_pcpu;
>>>> -
>>>>       mm->user_ns = get_user_ns(user_ns);
>>>>       lru_gen_init_mm(mm);
>>>>       return mm;
>>>>   -fail_pcpu:
>>>> -    mm_destroy_cid(mm);
>>>>   fail_cid:
>>>>       destroy_context(mm);
>>>>   fail_nocontext:
>>>> @@ -1730,6 +1724,16 @@ static int copy_mm(unsigned long 
>>>> clone_flags, struct task_struct *tsk)
>>>>       if (!oldmm)
>>>>           return 0;
>>>>   +    /*
>>>> +     * For single-thread processes, rss_stat is in atomic mode, which
>>>> +     * reduces the memory consumption and performance regression 
>>>> caused by
>>>> +     * using percpu. For multiple-thread processes, rss_stat is 
>>>> switched to
>>>> +     * the percpu mode to reduce the error margin.
>>>> +     */
>>>> +    if (clone_flags & CLONE_THREAD)
>>>> +        if (mm_counter_switch_to_pcpu_many(oldmm))
>>>> +            return -ENOMEM;
>>>> +
>>>>       if (clone_flags & CLONE_VM) {
>>>>           mmget(oldmm);
>>>>           mm = oldmm;
>>>
>>>
>
>
-- 
Best Regards,
Peng

Re: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

[zhangpeng (AS)]

On 2024/4/18 22:20, Peng Zhang wrote:

Any suggestions or opinions are welcome. Could someone please review
this patch series?
Thanks!

> From: ZhangPeng <zhangpeng362@huawei.com>
>
> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> percpu_counter"), the rss_stats have converted into percpu_counter,
> which convert the error margin from (nr_threads * 64) to approximately
> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> performance regression on fork/exec/shell. Even after commit 14ef95be6f55
> ("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
> the performance of fork/exec/shell is still poor compared to previous
> kernel versions.
>
> To mitigate performance regression, we delay the allocation of percpu
> memory for rss_stats. Therefore, we convert mm's rss stats to use
> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> atomic mode, which reduces the memory consumption and performance
> regression caused by using percpu. For multiple-thread processes,
> rss_stat is switched to the percpu mode to reduce the error margin.
> We convert rss_stats from atomic mode to percpu mode only when the
> second thread is created.
>
> After lmbench test, we can get 2% ~ 4% performance improvement
> for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
> improvement for lmbench page_fault (before batch mode[1]).
>
> The test results are as follows:
>               base           base+revert        base+this patch
>
> fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
> exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
> shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
> page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
>
> [1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
>
> ChangeLog:
> v2->v1:
> - Convert rss_stats from atomic mode to percpu mode only when
>    the second thread is created per Jan Kara.
> - Compared with v1, the performance data may be different due to
>    different test machines.
>
> ZhangPeng (2):
>    percpu_counter: introduce atomic mode for percpu_counter
>    mm: convert mm's rss stats to use atomic mode
>
>   include/linux/mm.h             | 50 +++++++++++++++++++++++++++++-----
>   include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++--
>   include/trace/events/kmem.h    |  4 +--
>   kernel/fork.c                  | 18 +++++++-----
>   lib/percpu_counter.c           | 31 +++++++++++++++++++--
>   5 files changed, 125 insertions(+), 21 deletions(-)
>
-- 
Best Regards,
Peng

Re: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

[Dennis Zhou]

Hi Peng,

On Wed, Apr 24, 2024 at 12:29:25PM +0800, zhangpeng (AS) wrote:
> On 2024/4/18 22:20, Peng Zhang wrote:
> 
> Any suggestions or opinions are welcome. Could someone please review
> this patch series?
> Thanks!
> 

Sorry, I haven't been very active lately. This is what I remember
discussing a while back. I'll take a close look tomorrow.

Thanks,
Dennis

> > From: ZhangPeng <zhangpeng362@huawei.com>
> > 
> > Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> > percpu_counter"), the rss_stats have converted into percpu_counter,
> > which convert the error margin from (nr_threads * 64) to approximately
> > (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> > performance regression on fork/exec/shell. Even after commit 14ef95be6f55
> > ("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
> > the performance of fork/exec/shell is still poor compared to previous
> > kernel versions.
> > 
> > To mitigate performance regression, we delay the allocation of percpu
> > memory for rss_stats. Therefore, we convert mm's rss stats to use
> > percpu_counter atomic mode. For single-thread processes, rss_stat is in
> > atomic mode, which reduces the memory consumption and performance
> > regression caused by using percpu. For multiple-thread processes,
> > rss_stat is switched to the percpu mode to reduce the error margin.
> > We convert rss_stats from atomic mode to percpu mode only when the
> > second thread is created.
> > 
> > After lmbench test, we can get 2% ~ 4% performance improvement
> > for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
> > improvement for lmbench page_fault (before batch mode[1]).
> > 
> > The test results are as follows:
> >               base           base+revert        base+this patch
> > 
> > fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
> > exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
> > shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
> > page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
> > 
> > [1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@huawei.com/
> > 
> > ChangeLog:
> > v2->v1:
> > - Convert rss_stats from atomic mode to percpu mode only when
> >    the second thread is created per Jan Kara.
> > - Compared with v1, the performance data may be different due to
> >    different test machines.
> > 
> > ZhangPeng (2):
> >    percpu_counter: introduce atomic mode for percpu_counter
> >    mm: convert mm's rss stats to use atomic mode
> > 
> >   include/linux/mm.h             | 50 +++++++++++++++++++++++++++++-----
> >   include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++--
> >   include/trace/events/kmem.h    |  4 +--
> >   kernel/fork.c                  | 18 +++++++-----
> >   lib/percpu_counter.c           | 31 +++++++++++++++++++--
> >   5 files changed, 125 insertions(+), 21 deletions(-)
> > 
> -- 
> Best Regards,
> Peng
> 

Re: [RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[Dennis Zhou]

On Thu, Apr 18, 2024 at 10:20:07PM +0800, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> Depending on whether counters is NULL, we can support two modes:
> atomic mode and perpcu mode. We implement both modes by grouping
> the s64 count and atomic64_t count_atomic in a union. At the same time,
> we create the interface for adding and reading in atomic mode and for
> switching atomic mode to percpu mode.
> 
> Suggested-by: Jan Kara <jack@suse.cz>
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++++++---
>  lib/percpu_counter.c           | 31 ++++++++++++++++++++++--
>  2 files changed, 69 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 3a44dd1e33d2..160f9734c0bb 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -21,7 +21,13 @@
>  
>  struct percpu_counter {
>  	raw_spinlock_t lock;
> -	s64 count;
> +	/* Depending on whether counters is NULL, we can support two modes,
> +	 * atomic mode using count_atomic and perpcu mode using count.
> +	 */
> +	union {
> +		s64 count;
> +		atomic64_t count_atomic;
> +	};
>  #ifdef CONFIG_HOTPLUG_CPU
>  	struct list_head list;	/* All percpu_counters are on a list */
>  #endif
> @@ -32,14 +38,14 @@ extern int percpu_counter_batch;
>  
>  int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  			       gfp_t gfp, u32 nr_counters,
> -			       struct lock_class_key *key);
> +			       struct lock_class_key *key, bool switch_mode);

Nit: the lock_class_key at the end.
>  
>  #define percpu_counter_init_many(fbc, value, gfp, nr_counters)		\
>  	({								\
>  		static struct lock_class_key __key;			\
>  									\
>  		__percpu_counter_init_many(fbc, value, gfp, nr_counters,\
> -					   &__key);			\
> +					   &__key, false);		\
>  	})
>  
>  
> @@ -130,6 +136,20 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>  	return (fbc->counters != NULL);
>  }
>  
> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
> +{
> +	return atomic64_read(&fbc->count_atomic);
> +}
> +
> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
> +					     s64 amount)
> +{
> +	atomic64_add(amount, &fbc->count_atomic);
> +}
> +
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters);
> +
>  #else /* !CONFIG_SMP */
>  
>  struct percpu_counter {
> @@ -260,6 +280,23 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>  static inline void percpu_counter_sync(struct percpu_counter *fbc)
>  {
>  }
> +
> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
> +{
> +	return fbc->count;
> +}
> +
> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
> +					     s64 amount)
> +{
> +	percpu_counter_add(fbc, amount);
> +}
> +
> +static inline int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +						     u32 nr_counters)
> +{
> +	return 0;
> +}
>  #endif	/* CONFIG_SMP */
>  
>  static inline void percpu_counter_inc(struct percpu_counter *fbc)
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index 44dd133594d4..95c4e038051a 100644
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
>  
>  int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  			       gfp_t gfp, u32 nr_counters,
> -			       struct lock_class_key *key)
> +			       struct lock_class_key *key, bool switch_mode)
>  {
>  	unsigned long flags __maybe_unused;
>  	size_t counter_size;
> @@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>  #ifdef CONFIG_HOTPLUG_CPU
>  		INIT_LIST_HEAD(&fbc[i].list);
>  #endif
> -		fbc[i].count = amount;
> +		if (likely(!switch_mode))
> +			fbc[i].count = amount;
>  		fbc[i].counters = (void *)counters + (i * counter_size);
>  
>  		debug_percpu_counter_activate(&fbc[i]);
> @@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
>  	return good;
>  }
>  
> +/*
> + * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
> + * atomic mode to percpu mode.
> + */
> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
> +				       u32 nr_counters)
> +{
> +	static struct lock_class_key __key;

This is an improper use of lockdep. Now all of the percpu_counters
initialized on this path will key off of this lock_class_key.

> +	unsigned long flags;
> +	bool ret = 0;
> +
> +	if (percpu_counter_initialized(fbc))
> +		return 0;
> +
> +	preempt_disable();
> +	local_irq_save(flags);
> +	if (likely(!percpu_counter_initialized(fbc)))
> +		ret = __percpu_counter_init_many(fbc, 0,
> +					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
> +					nr_counters, &__key, true);
> +	local_irq_restore(flags);
> +	preempt_enable();
> +
> +	return ret;
> +}

I'm staring at this API and I'm not in love with it. I think it hinges 
on that there's one user of mm_stats prior hence it's safe. Generically
though, I can't see why this is safe.

I need to give this a little more thought, but my gut reaction is I'd
rather this look like percpu_refcount where we can init dead minus the
percpu allocation. Maybe that's not quite right, but I'd feel better
about it than requiring external synchronization on a percpu_counter to
ensure that it's correct.

> +
>  static int __init percpu_counter_startup(void)
>  {
>  	int ret;
> -- 
> 2.25.1
> 

Thanks,
Dennis

Re: [RFC PATCH v2 1/2] percpu_counter: introduce atomic mode for percpu_counter

[zhangpeng (AS)]

On 2024/4/26 16:11, Dennis Zhou wrote:

> On Thu, Apr 18, 2024 at 10:20:07PM +0800, Peng Zhang wrote:
>> From: ZhangPeng <zhangpeng362@huawei.com>
>>
>> Depending on whether counters is NULL, we can support two modes:
>> atomic mode and perpcu mode. We implement both modes by grouping
>> the s64 count and atomic64_t count_atomic in a union. At the same time,
>> we create the interface for adding and reading in atomic mode and for
>> switching atomic mode to percpu mode.
>>
>> Suggested-by: Jan Kara <jack@suse.cz>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>>   include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++++++---
>>   lib/percpu_counter.c           | 31 ++++++++++++++++++++++--
>>   2 files changed, 69 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
>> index 3a44dd1e33d2..160f9734c0bb 100644
>> --- a/include/linux/percpu_counter.h
>> +++ b/include/linux/percpu_counter.h
>> @@ -21,7 +21,13 @@
>>   
>>   struct percpu_counter {
>>   	raw_spinlock_t lock;
>> -	s64 count;
>> +	/* Depending on whether counters is NULL, we can support two modes,
>> +	 * atomic mode using count_atomic and perpcu mode using count.
>> +	 */
>> +	union {
>> +		s64 count;
>> +		atomic64_t count_atomic;
>> +	};
>>   #ifdef CONFIG_HOTPLUG_CPU
>>   	struct list_head list;	/* All percpu_counters are on a list */
>>   #endif
>> @@ -32,14 +38,14 @@ extern int percpu_counter_batch;
>>   
>>   int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>>   			       gfp_t gfp, u32 nr_counters,
>> -			       struct lock_class_key *key);
>> +			       struct lock_class_key *key, bool switch_mode);
> Nit: the lock_class_key at the end.

Got it.

>>   
>>   #define percpu_counter_init_many(fbc, value, gfp, nr_counters)		\
>>   	({								\
>>   		static struct lock_class_key __key;			\
>>   									\
>>   		__percpu_counter_init_many(fbc, value, gfp, nr_counters,\
>> -					   &__key);			\
>> +					   &__key, false);		\
>>   	})
>>   
>>   
>> @@ -130,6 +136,20 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>>   	return (fbc->counters != NULL);
>>   }
>>   
>> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
>> +{
>> +	return atomic64_read(&fbc->count_atomic);
>> +}
>> +
>> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
>> +					     s64 amount)
>> +{
>> +	atomic64_add(amount, &fbc->count_atomic);
>> +}
>> +
>> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
>> +				       u32 nr_counters);
>> +
>>   #else /* !CONFIG_SMP */
>>   
>>   struct percpu_counter {
>> @@ -260,6 +280,23 @@ static inline bool percpu_counter_initialized(struct percpu_counter *fbc)
>>   static inline void percpu_counter_sync(struct percpu_counter *fbc)
>>   {
>>   }
>> +
>> +static inline s64 percpu_counter_atomic_read(struct percpu_counter *fbc)
>> +{
>> +	return fbc->count;
>> +}
>> +
>> +static inline void percpu_counter_atomic_add(struct percpu_counter *fbc,
>> +					     s64 amount)
>> +{
>> +	percpu_counter_add(fbc, amount);
>> +}
>> +
>> +static inline int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
>> +						     u32 nr_counters)
>> +{
>> +	return 0;
>> +}
>>   #endif	/* CONFIG_SMP */
>>   
>>   static inline void percpu_counter_inc(struct percpu_counter *fbc)
>> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
>> index 44dd133594d4..95c4e038051a 100644
>> --- a/lib/percpu_counter.c
>> +++ b/lib/percpu_counter.c
>> @@ -153,7 +153,7 @@ EXPORT_SYMBOL(__percpu_counter_sum);
>>   
>>   int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>>   			       gfp_t gfp, u32 nr_counters,
>> -			       struct lock_class_key *key)
>> +			       struct lock_class_key *key, bool switch_mode)
>>   {
>>   	unsigned long flags __maybe_unused;
>>   	size_t counter_size;
>> @@ -174,7 +174,8 @@ int __percpu_counter_init_many(struct percpu_counter *fbc, s64 amount,
>>   #ifdef CONFIG_HOTPLUG_CPU
>>   		INIT_LIST_HEAD(&fbc[i].list);
>>   #endif
>> -		fbc[i].count = amount;
>> +		if (likely(!switch_mode))
>> +			fbc[i].count = amount;
>>   		fbc[i].counters = (void *)counters + (i * counter_size);
>>   
>>   		debug_percpu_counter_activate(&fbc[i]);
>> @@ -357,6 +358,32 @@ bool __percpu_counter_limited_add(struct percpu_counter *fbc,
>>   	return good;
>>   }
>>   
>> +/*
>> + * percpu_counter_switch_to_pcpu_many: Converts struct percpu_counters from
>> + * atomic mode to percpu mode.
>> + */
>> +int percpu_counter_switch_to_pcpu_many(struct percpu_counter *fbc,
>> +				       u32 nr_counters)
>> +{
>> +	static struct lock_class_key __key;
> This is an improper use of lockdep. Now all of the percpu_counters
> initialized on this path will key off of this lock_class_key.

Sorry, I don't know much about lock_class_key. I may not understand the reason
why it's not appropriate here. Could you please help me with the details?

>> +	unsigned long flags;
>> +	bool ret = 0;
>> +
>> +	if (percpu_counter_initialized(fbc))
>> +		return 0;
>> +
>> +	preempt_disable();
>> +	local_irq_save(flags);
>> +	if (likely(!percpu_counter_initialized(fbc)))
>> +		ret = __percpu_counter_init_many(fbc, 0,
>> +					GFP_ATOMIC|__GFP_NOWARN|__GFP_ZERO,
>> +					nr_counters, &__key, true);
>> +	local_irq_restore(flags);
>> +	preempt_enable();
>> +
>> +	return ret;
>> +}
> I'm staring at this API and I'm not in love with it. I think it hinges
> on that there's one user of mm_stats prior hence it's safe. Generically
> though, I can't see why this is safe.
>
> I need to give this a little more thought, but my gut reaction is I'd
> rather this look like percpu_refcount where we can init dead minus the
> percpu allocation. Maybe that's not quite right, but I'd feel better
> about it than requiring external synchronization on a percpu_counter to
> ensure that it's correct.

Maybe it would be better if I change this API to the internal function of
mm counter.

Sorry again, maybe percpu_refcount is better, but I don't understand this
sentence "we can init dead minus the percpu allocation." Please forgive my
ignorance...

Thank you very much for your review and valuable comments!

>> +
>>   static int __init percpu_counter_startup(void)
>>   {
>>   	int ret;
>> -- 
>> 2.25.1
>>
> Thanks,
> Dennis

-- 
Best Regards,
Peng

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Kairui Song]

On Fri, Apr 19, 2024 at 11:32 AM zhangpeng (AS) <zhangpeng362@huawei.com> wrote:
> On 2024/4/19 10:30, Rongwei Wang wrote:
> > On 2024/4/18 22:20, Peng Zhang wrote:
> >> From: ZhangPeng <zhangpeng362@huawei.com>
> >>
> >> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> >> percpu_counter"), the rss_stats have converted into percpu_counter,
> >> which convert the error margin from (nr_threads * 64) to approximately
> >> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> >> performance regression on fork/exec/shell. Even after commit
> >> 14ef95be6f55
> >> ("kernel/fork: group allocation/free of per-cpu counters for mm
> >> struct"),
> >> the performance of fork/exec/shell is still poor compared to previous
> >> kernel versions.
> >>
> >> To mitigate performance regression, we delay the allocation of percpu
> >> memory for rss_stats. Therefore, we convert mm's rss stats to use
> >> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> >> atomic mode, which reduces the memory consumption and performance
> >> regression caused by using percpu. For multiple-thread processes,
> >> rss_stat is switched to the percpu mode to reduce the error margin.
> >> We convert rss_stats from atomic mode to percpu mode only when the
> >> second thread is created.
> > Hi, Zhang Peng
> >
> > This regression we also found it in lmbench these days. I have not
> > test your patch, but it seems will solve a lot for it.
> > And I see this patch not fix the regression in multi-threads, that's
> > because of the rss_stat switched to percpu mode?
> > (If I'm wrong, please correct me.) And It seems percpu_counter also
> > has a bad effect in exit_mmap().
> >
> > If so, I'm wondering if we can further improving it on the exit_mmap()
> > path in multi-threads scenario, e.g. to determine which CPUs the
> > process has run on (mm_cpumask()? I'm not sure).
> >
> Hi, Rongwei,
>
> Yes, this patch only fixes the regression in single-thread processes. How
> much bad effect does percpu_counter have in exit_mmap()? IMHO, the addition
> of mm counter is already in batch mode, maybe I miss something?
>

Hi, Peng Zhang, Rongwei, and all:

I've a patch series that is earlier than commit f1a7941243c1 ("mm:
convert mm's rss stats into
percpu_counter"):

https://lwn.net/ml/linux-kernel/20220728204511.56348-1-ryncsn@gmail.com/

Instead of a per-mm-per-cpu cache, it used only one global per-cpu
cache, and flush it on schedule. Or, if the arch supports, flush and
fetch it use mm bitmap as an optimization (like tlb shootdown).

Unfortunately it didn't get much attention and I moved to work on other things.
I also noticed the fork regression issue, so I did a local rebase of
my previous patch, and revert f1a7941243c1.

The result is looking good, on my 32 core VM machine, I see similar
improvement as the one you posted (alloc/free on fork/exit is gone), I
also see minor improvement with database tests, memory usage is lower
by a little bit too (no more per-mm cache), and I think the error
margin in my patch should be close to zero.

I hope I can get some attention here for my idea...

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Mateusz Guzik]

On Thu, May 16, 2024 at 07:50:52PM +0800, Kairui Song wrote:
> > > On 2024/4/18 22:20, Peng Zhang wrote:
> > >> From: ZhangPeng <zhangpeng362@huawei.com>
> > >>
> > >> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> > >> percpu_counter"), the rss_stats have converted into percpu_counter,
> > >> which convert the error margin from (nr_threads * 64) to approximately
> > >> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> > >> performance regression on fork/exec/shell. Even after commit
> > >> 14ef95be6f55
> > >> ("kernel/fork: group allocation/free of per-cpu counters for mm
> > >> struct"),
> > >> the performance of fork/exec/shell is still poor compared to previous
> > >> kernel versions.
> > >>
> > >> To mitigate performance regression, we delay the allocation of percpu
> > >> memory for rss_stats. Therefore, we convert mm's rss stats to use
> > >> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> > >> atomic mode, which reduces the memory consumption and performance
> > >> regression caused by using percpu. For multiple-thread processes,
> > >> rss_stat is switched to the percpu mode to reduce the error margin.
> > >> We convert rss_stats from atomic mode to percpu mode only when the
> > >> second thread is created.
> 
> I've a patch series that is earlier than commit f1a7941243c1 ("mm:
> convert mm's rss stats into
> percpu_counter"):
> 
> https://lwn.net/ml/linux-kernel/20220728204511.56348-1-ryncsn@gmail.com/
> 
> Instead of a per-mm-per-cpu cache, it used only one global per-cpu
> cache, and flush it on schedule. Or, if the arch supports, flush and
> fetch it use mm bitmap as an optimization (like tlb shootdown).
> 

I just spotted this thread.

I have a rather long rant to write about the entire ordeal, but don't
have the time at the moment. I do have time to make some remarks though.

Rolling with a centralized counter and only distributing per-cpu upon
creation of a thread is something which was discussed last time and
which I was considering doing. Then life got it in the way and in the
meantime I managed to conclude it's a questionable idea anyway.

The state prior to the counters moving to per-cpu was not that great to
begin with, with quite a few serialization points. As far as allocating
stuff goes one example is mm_alloc_cid, with the following:
	mm->pcpu_cid = alloc_percpu(struct mm_cid);

Converting the code to avoid per-cpu rss counters in the common case or
the above patchset only damage-control the state back to what it was,
don't do anything to push things further.

Another note is that unfortunately userspace is increasingly
multithreaded for no good reason, see the Rust ecosystem as an example.

All that to say is that the multithreaded case is what has to get
faster, as a side effect possibly obsoleting both approaches proposed
above. I concede if there is nobody wiling to commit to doing the work
in the foreseeable future then indeed a damage-controlling solution
should land.

On that note in check_mm there is this loop:
        for (i = 0; i < NR_MM_COUNTERS; i++) {
                long x = percpu_counter_sum(&mm->rss_stat[i]);

This avoidably walks all cpus 4 times with a preemption and lock trip
for each round. Instead one can observe all modifications are supposed
to have already stopped and that this is allocated in a banch. A
routine, say percpu_counter_sum_many_unsafe, could do one iteration
without any locks or interrupt play and return an array. This should be
markedly faster and I perhaps will hack it up.

A part of The Real Solution(tm) would make counter allocations scale
(including mcid, not just rss) or dodge them (while maintaining the
per-cpu distribution, see below for one idea), but that boils down to
balancing scalability versus total memory usage. It is trivial to just
slap together a per-cpu cache of these allocations and have the problem
go away for benchmarking purposes, while being probably being too memory
hungry for actual usage.

I was pondering an allocator with caches per some number of cores (say 4
or 8). Microbenchmarks aside I suspect real workloads would not suffer
from contention at this kind of granularity. This would trivially reduce
memory usage compared to per-cpu caching. I suspect things like
mm_struct, task_struct, task stacks and similar would be fine with it.

Suppose mm_struct is allocated from a more coarse grained allocator than
per-cpu. Total number of cached objects would be lower than it is now.
That would also mean these allocated but not currently used mms could
hold on to other stuff, for example per-cpu rss and mcid counters. Then
should someone fork or exit, alloc/free_percpu would be avoided for most
cases. This would scale better and be faster single-threaded than the
current state.

(believe it or not this is not the actual long rant I have in mind)

I can't commit to work on the Real Solution though.

In the meantime I can submit percpu_counter_sum_many_unsafe as described
above if Denis likes the idea.

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Kairui Song]

Mateusz Guzik <mjguzik@gmail.com> 于 2024年5月16日周四 23:14写道：
>
> On Thu, May 16, 2024 at 07:50:52PM +0800, Kairui Song wrote:
> > > > On 2024/4/18 22:20, Peng Zhang wrote:
> > > >> From: ZhangPeng <zhangpeng362@huawei.com>
> > > >>
> > > >> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> > > >> percpu_counter"), the rss_stats have converted into percpu_counter,
> > > >> which convert the error margin from (nr_threads * 64) to approximately
> > > >> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> > > >> performance regression on fork/exec/shell. Even after commit
> > > >> 14ef95be6f55
> > > >> ("kernel/fork: group allocation/free of per-cpu counters for mm
> > > >> struct"),
> > > >> the performance of fork/exec/shell is still poor compared to previous
> > > >> kernel versions.
> > > >>
> > > >> To mitigate performance regression, we delay the allocation of percpu
> > > >> memory for rss_stats. Therefore, we convert mm's rss stats to use
> > > >> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> > > >> atomic mode, which reduces the memory consumption and performance
> > > >> regression caused by using percpu. For multiple-thread processes,
> > > >> rss_stat is switched to the percpu mode to reduce the error margin.
> > > >> We convert rss_stats from atomic mode to percpu mode only when the
> > > >> second thread is created.
> >
> > I've a patch series that is earlier than commit f1a7941243c1 ("mm:
> > convert mm's rss stats into
> > percpu_counter"):
> >
> > https://lwn.net/ml/linux-kernel/20220728204511.56348-1-ryncsn@gmail.com/
> >
> > Instead of a per-mm-per-cpu cache, it used only one global per-cpu
> > cache, and flush it on schedule. Or, if the arch supports, flush and
> > fetch it use mm bitmap as an optimization (like tlb shootdown).
> >
>
> I just spotted this thread.
>
> I have a rather long rant to write about the entire ordeal, but don't
> have the time at the moment. I do have time to make some remarks though.
>
> Rolling with a centralized counter and only distributing per-cpu upon
> creation of a thread is something which was discussed last time and
> which I was considering doing. Then life got it in the way and in the
> meantime I managed to conclude it's a questionable idea anyway.
>
> The state prior to the counters moving to per-cpu was not that great to
> begin with, with quite a few serialization points. As far as allocating
> stuff goes one example is mm_alloc_cid, with the following:
>         mm->pcpu_cid = alloc_percpu(struct mm_cid);
>
> Converting the code to avoid per-cpu rss counters in the common case or
> the above patchset only damage-control the state back to what it was,
> don't do anything to push things further.
>
> Another note is that unfortunately userspace is increasingly
> multithreaded for no good reason, see the Rust ecosystem as an example.
>
> All that to say is that the multithreaded case is what has to get
> faster, as a side effect possibly obsoleting both approaches proposed
> above. I concede if there is nobody wiling to commit to doing the work
> in the foreseeable future then indeed a damage-controlling solution
> should land.

Hi, Mateusz,

Which patch are you referencing? My series didn't need any allocations
on thread creation or destruction. Also RSS update is extremely
lightweight (pretty much just read GS and do a few ADD/INC, that's
all), performance is better than all even with micro benchmarks. RSS
read only collects info from CPUs that may contain real updates.

I understand you may not have time to go through my series... but I
think I should add some details here.

> On that note in check_mm there is this loop:
>         for (i = 0; i < NR_MM_COUNTERS; i++) {
>                 long x = percpu_counter_sum(&mm->rss_stat[i]);
>
> This avoidably walks all cpus 4 times with a preemption and lock trip
> for each round. Instead one can observe all modifications are supposed
> to have already stopped and that this is allocated in a banch. A
> routine, say percpu_counter_sum_many_unsafe, could do one iteration
> without any locks or interrupt play and return an array. This should be
> markedly faster and I perhaps will hack it up.

Which is similar to the RSS read in my earlier series... It is based
on the assumption that updates are likely stopped so just read the
counter "unsafely" with a double (and fast) check to ensure no race.

And even more, when coupled with mm shootdown
(CONFIG_ARCH_PCP_RSS_USE_CPUMASK), it doesn't need to collect RSS info
on thread exit at all.

>
> A part of The Real Solution(tm) would make counter allocations scale
> (including mcid, not just rss) or dodge them (while maintaining the
> per-cpu distribution, see below for one idea), but that boils down to
> balancing scalability versus total memory usage. It is trivial to just
> slap together a per-cpu cache of these allocations and have the problem
> go away for benchmarking purposes, while being probably being too memory
> hungry for actual usage.
>
> I was pondering an allocator with caches per some number of cores (say 4
> or 8). Microbenchmarks aside I suspect real workloads would not suffer
> from contention at this kind of granularity. This would trivially reduce
> memory usage compared to per-cpu caching. I suspect things like
> mm_struct, task_struct, task stacks and similar would be fine with it.
>
> Suppose mm_struct is allocated from a more coarse grained allocator than
> per-cpu. Total number of cached objects would be lower than it is now.
> That would also mean these allocated but not currently used mms could
> hold on to other stuff, for example per-cpu rss and mcid counters. Then
> should someone fork or exit, alloc/free_percpu would be avoided for most
> cases. This would scale better and be faster single-threaded than the
> current state.

And what is the issue with using only one CPU cache, and flush on mm
switch? No more alloc after boot, and the total (and fixed) memory
usage is just about a few unsigned long per CPU, which should be even
lower that the old RSS cache solution (4 unsigned long per task). And
it scaled very well with many kinds of microbench or workload I've
tested.

Unless the workload keeps doing something like "alloc one page then
switch to another mm", I think the performance will be horrible
already due to cache invalidations and many switch_*()s, RSS isn't
really a concern there.

>
> (believe it or not this is not the actual long rant I have in mind)
>
> I can't commit to work on the Real Solution though.
>
> In the meantime I can submit percpu_counter_sum_many_unsafe as described
> above if Denis likes the idea.

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Mateusz Guzik]

On Fri, May 17, 2024 at 11:29:57AM +0800, Kairui Song wrote:
> Mateusz Guzik <mjguzik@gmail.com> 于 2024年5月16日周四 23:14写道：
> > A part of The Real Solution(tm) would make counter allocations scale
> > (including mcid, not just rss) or dodge them (while maintaining the
> > per-cpu distribution, see below for one idea), but that boils down to
> > balancing scalability versus total memory usage. It is trivial to just
> > slap together a per-cpu cache of these allocations and have the problem
> > go away for benchmarking purposes, while being probably being too memory
> > hungry for actual usage.
> >
> > I was pondering an allocator with caches per some number of cores (say 4
> > or 8). Microbenchmarks aside I suspect real workloads would not suffer
> > from contention at this kind of granularity. This would trivially reduce
> > memory usage compared to per-cpu caching. I suspect things like
> > mm_struct, task_struct, task stacks and similar would be fine with it.
> >
> > Suppose mm_struct is allocated from a more coarse grained allocator than
> > per-cpu. Total number of cached objects would be lower than it is now.
> > That would also mean these allocated but not currently used mms could
> > hold on to other stuff, for example per-cpu rss and mcid counters. Then
> > should someone fork or exit, alloc/free_percpu would be avoided for most
> > cases. This would scale better and be faster single-threaded than the
> > current state.
> 
> And what is the issue with using only one CPU cache, and flush on mm
> switch? No more alloc after boot, and the total (and fixed) memory
> usage is just about a few unsigned long per CPU, which should be even
> lower that the old RSS cache solution (4 unsigned long per task). And
> it scaled very well with many kinds of microbench or workload I've
> tested.
> 
> Unless the workload keeps doing something like "alloc one page then
> switch to another mm", I think the performance will be horrible
> already due to cache invalidations and many switch_*()s, RSS isn't
> really a concern there.
> 

I only skimmed through your patchset. I do think it has a legitimate
approach, but personally I would not do it like that due to the extra
work on context switches. However, I have 0 say about this, so you will
need to prod the mm overlords to get this moving forward.

Maybe I was not clear enough in my opening e-mail, so I'm going to
reiterate some bits: there are scalability problems in execve even with
your patchset or the one which uses atomics.  One of them concerns
another bit which allocates per-cpu memory (the mcid thing).

I note that sorting it out would possibly also take care of the rss
problem, outlining an example approach above.

Re: [RFC PATCH v2 2/2] mm: convert mm's rss stats to use atomic mode

[Dennis Zhou]

Hi Mateusz and Kairui,

On Thu, May 16, 2024 at 05:14:06PM +0200, Mateusz Guzik wrote:
> On Thu, May 16, 2024 at 07:50:52PM +0800, Kairui Song wrote:
> > > > On 2024/4/18 22:20, Peng Zhang wrote:
> > > >> From: ZhangPeng <zhangpeng362@huawei.com>
> > > >>
> > > >> Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> > > >> percpu_counter"), the rss_stats have converted into percpu_counter,
> > > >> which convert the error margin from (nr_threads * 64) to approximately
> > > >> (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> > > >> performance regression on fork/exec/shell. Even after commit
> > > >> 14ef95be6f55
> > > >> ("kernel/fork: group allocation/free of per-cpu counters for mm
> > > >> struct"),
> > > >> the performance of fork/exec/shell is still poor compared to previous
> > > >> kernel versions.
> > > >>
> > > >> To mitigate performance regression, we delay the allocation of percpu
> > > >> memory for rss_stats. Therefore, we convert mm's rss stats to use
> > > >> percpu_counter atomic mode. For single-thread processes, rss_stat is in
> > > >> atomic mode, which reduces the memory consumption and performance
> > > >> regression caused by using percpu. For multiple-thread processes,
> > > >> rss_stat is switched to the percpu mode to reduce the error margin.
> > > >> We convert rss_stats from atomic mode to percpu mode only when the
> > > >> second thread is created.
> > 
> > I've a patch series that is earlier than commit f1a7941243c1 ("mm:
> > convert mm's rss stats into
> > percpu_counter"):
> > 
> > https://lwn.net/ml/linux-kernel/20220728204511.56348-1-ryncsn@gmail.com/
> > 

I hadn't seen this series as my inbox filters on percpu, but not
per-cpu. I can take a closer look this week.

> > Instead of a per-mm-per-cpu cache, it used only one global per-cpu
> > cache, and flush it on schedule. Or, if the arch supports, flush and
> > fetch it use mm bitmap as an optimization (like tlb shootdown).
> > 
> 
> I just spotted this thread.
> 
> I have a rather long rant to write about the entire ordeal, but don't
> have the time at the moment. I do have time to make some remarks though.
> 
> Rolling with a centralized counter and only distributing per-cpu upon
> creation of a thread is something which was discussed last time and
> which I was considering doing. Then life got it in the way and in the
> meantime I managed to conclude it's a questionable idea anyway.
> 

To clarify my stance, I'm not against the API of switching a
percpu_counter to percpu mode. We do it for percpu_refcount. I think the
implementation here was fragile. Secondly, Kent did implement
lazy_percpu_counters. We likely should see how that can be leveraged and
how we can reconcile the 2 APIs.

> The state prior to the counters moving to per-cpu was not that great to
> begin with, with quite a few serialization points. As far as allocating
> stuff goes one example is mm_alloc_cid, with the following:
> 	mm->pcpu_cid = alloc_percpu(struct mm_cid);
> 
> Converting the code to avoid per-cpu rss counters in the common case or
> the above patchset only damage-control the state back to what it was,
> don't do anything to push things further.
> 
> Another note is that unfortunately userspace is increasingly
> multithreaded for no good reason, see the Rust ecosystem as an example.
> 
> All that to say is that the multithreaded case is what has to get
> faster, as a side effect possibly obsoleting both approaches proposed
> above. I concede if there is nobody wiling to commit to doing the work
> in the foreseeable future then indeed a damage-controlling solution
> should land.
> 
> On that note in check_mm there is this loop:
>         for (i = 0; i < NR_MM_COUNTERS; i++) {
>                 long x = percpu_counter_sum(&mm->rss_stat[i]);
> 
> This avoidably walks all cpus 4 times with a preemption and lock trip
> for each round. Instead one can observe all modifications are supposed
> to have already stopped and that this is allocated in a banch. A
> routine, say percpu_counter_sum_many_unsafe, could do one iteration
> without any locks or interrupt play and return an array. This should be
> markedly faster and I perhaps will hack it up.

I'm a little worried about the correctness in the cpu_hotplug case, idk
if it's warranted.

> 
> A part of The Real Solution(tm) would make counter allocations scale
> (including mcid, not just rss) or dodge them (while maintaining the
> per-cpu distribution, see below for one idea), but that boils down to
> balancing scalability versus total memory usage. It is trivial to just
> slap together a per-cpu cache of these allocations and have the problem
> go away for benchmarking purposes, while being probably being too memory
> hungry for actual usage.
> 
> I was pondering an allocator with caches per some number of cores (say 4
> or 8). Microbenchmarks aside I suspect real workloads would not suffer
> from contention at this kind of granularity. This would trivially reduce
> memory usage compared to per-cpu caching. I suspect things like
> mm_struct, task_struct, task stacks and similar would be fine with it.
> 
> Suppose mm_struct is allocated from a more coarse grained allocator than
> per-cpu. Total number of cached objects would be lower than it is now.
> That would also mean these allocated but not currently used mms could
> hold on to other stuff, for example per-cpu rss and mcid counters. Then
> should someone fork or exit, alloc/free_percpu would be avoided for most
> cases. This would scale better and be faster single-threaded than the
> current state.
> 
> (believe it or not this is not the actual long rant I have in mind)
> 
> I can't commit to work on the Real Solution though.
> 
> In the meantime I can submit percpu_counter_sum_many_unsafe as described
> above if Denis likes the idea.

To be honest, I'm a little out of my depth here. I haven't spent a lot
of time in the mm code paths to really know when and how we're
accounting RSS and other stats. Given that, I think we should align on
the approach we want to take because in some way it sounds like percpu
RSS might not be the final evolution here for this and other stats.

I think lets hold off on percpu_counter_sum_many_unsafe() initially and
if that's the way we have to go so be it. I'm going to respin a
cpu_hotplug related series that might change some of the correctness
here and have that ready for the v6.11 merge window.

Thanks,
Dennis