[RFC PATCH 00/10] timer: Reduce timers softirq (and other optimizations)

[RFC PATCH 00/10] timer: Reduce timers softirq (and other optimizations)

[Frederic Weisbecker]

Juri Lelli has reported that threaded IRQs don't go along well with
full dynticks.

The tick never manages to stop due to the following endless loop:

* Timer tick raises timer softirq
* Ksoftirqd wakes up, restarts the tick (2 tasks in the runqueue)
* Timer tick raises timer softirq
* Ksoftirqd wakes up, etc...

The main issue here is that the tick fires the timer tick unconditionally,
whether timers have expired or not. This set is a proposal to lower that.

The 1st patch is actually a bugfix for a theoretical issue that I haven't
observed in practice. But who knows?

Patch 2 is a consolidation.

Patch 3 and 4 are optimizations.

The rest is about timer softirqs.

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	timers/softirq

HEAD: d0567dd5546d1f32eca3772a431488f8b0ac26a1

Thanks,
	Frederic
---

Frederic Weisbecker (10):
      timer: Prevent base->clk from moving backward
      timer: Move trigger_dyntick_cpu() to enqueue_timer()
      timer: Simplify LVL_START() and calc_index()
      timer: Optimize _next_timer_interrupt() level iteration
      timers: Always keep track of next expiry
      timer: Reuse next expiry cache after nohz exit
      timer: Expand clk forward logic beyond nohz
      timer: Spare timer softirq until next expiry
      timer: Remove must_forward_clk
      timer: Lower base clock forwarding threshold


 kernel/time/timer.c | 179 +++++++++++++++++++---------------------------------
 1 file changed, 64 insertions(+), 115 deletions(-)

[RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Frederic Weisbecker]

When a timer is enqueued with a negative delta (ie: expiry is below
base->clk), it gets added to the wheel as expiring now (base->clk).

Yet the value that gets stored in base->next_expiry, while calling
trigger_dyntick_cpu(), is the initial timer->expires value. The
resulting state becomes:

	base->next_expiry < base->clk

On the next timer enqueue, forward_timer_base() may accidentally
rewind base->clk. As a possible outcome, timers may expire way too
early, the worst case being that the highest wheel levels get spuriously
processed again.

To prevent from that, make sure that base->next_expiry doesn't get below
base->clk.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 398e6eadb861..9a838d38dbe6 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -584,7 +584,15 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 	 * Set the next expiry time and kick the CPU so it can reevaluate the
 	 * wheel:
 	 */
-	base->next_expiry = timer->expires;
+	if (time_before(timer->expires, base->clk)) {
+		/*
+		 * Prevent from forward_timer_base() moving the base->clk
+		 * backward
+		 */
+		base->next_expiry = base->clk;
+	} else {
+		base->next_expiry = timer->expires;
+	}
 	wake_up_nohz_cpu(base->cpu);
 }
 
@@ -896,10 +904,13 @@ static inline void forward_timer_base(struct timer_base *base)
 	 * If the next expiry value is > jiffies, then we fast forward to
 	 * jiffies otherwise we forward to the next expiry value.
 	 */
-	if (time_after(base->next_expiry, jnow))
+	if (time_after(base->next_expiry, jnow)) {
 		base->clk = jnow;
-	else
+	} else {
+		if (WARN_ON_ONCE(time_before(base->next_expiry, base->clk)))
+			return;
 		base->clk = base->next_expiry;
+	}
 #endif
 }
 
-- 
2.26.2

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Juri Lelli]

Hi,

Thanks for the set, first of all. Reviewing and testing it.

On 01/07/20 03:10, Frederic Weisbecker wrote:
> When a timer is enqueued with a negative delta (ie: expiry is below
> base->clk), it gets added to the wheel as expiring now (base->clk).
> 
> Yet the value that gets stored in base->next_expiry, while calling
> trigger_dyntick_cpu(), is the initial timer->expires value. The
> resulting state becomes:
> 
> 	base->next_expiry < base->clk
> 
> On the next timer enqueue, forward_timer_base() may accidentally
> rewind base->clk. As a possible outcome, timers may expire way too
> early, the worst case being that the highest wheel levels get spuriously
> processed again.
> 
> To prevent from that, make sure that base->next_expiry doesn't get below
> base->clk.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> ---
>  kernel/time/timer.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 398e6eadb861..9a838d38dbe6 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -584,7 +584,15 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
>  	 * Set the next expiry time and kick the CPU so it can reevaluate the
>  	 * wheel:
>  	 */
> -	base->next_expiry = timer->expires;
> +	if (time_before(timer->expires, base->clk)) {
> +		/*
> +		 * Prevent from forward_timer_base() moving the base->clk
> +		 * backward
> +		 */
> +		base->next_expiry = base->clk;
> +	} else {
> +		base->next_expiry = timer->expires;
> +	}
>  	wake_up_nohz_cpu(base->cpu);
>  }
>  
> @@ -896,10 +904,13 @@ static inline void forward_timer_base(struct timer_base *base)
>  	 * If the next expiry value is > jiffies, then we fast forward to
>  	 * jiffies otherwise we forward to the next expiry value.
>  	 */
> -	if (time_after(base->next_expiry, jnow))
> +	if (time_after(base->next_expiry, jnow)) {
>  		base->clk = jnow;
> -	else
> +	} else {
> +		if (WARN_ON_ONCE(time_before(base->next_expiry, base->clk)))
> +			return;

I actually ported it to latest RT tree (v5.6.17-rt10) w/o conflicts,
but hit this one above:

 ...
 000: Kernel command line: BOOT_IMAGE=(hd0,msdos1)/vmlinuz-5.6.17-rt10 root=/dev/mapper/rhel_rt--qe--04-root ro crashkernel=auto resume=/dev/mapper/rhel_rt--qe--04-swap rd.lvm.lv=rhel_rt-qe-04/root rd.lvm.lv=rhel_rt-qe-04/swap console=ttyS0,115200
 000: mem auto-init: stack:off, heap alloc:off, heap free:off
 000: Memory: 2102240K/134089036K available (12292K kernel code, 2449K rwdata, 4332K rodata, 2248K init, 15392K bss, 2278548K reserved, 0K cma-reserved)
 000: random: get_random_u64 called from cache_random_seq_create+0x7c/0x140 with crng_init=0
 000: SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=24, Nodes=2
 000: Kernel/User page tables isolation: enabled
 000: ftrace: allocating 37680 entries in 148 pages
 000: ftrace: allocated 148 pages with 3 groups
 000: rcu: Preemptible hierarchical RCU implementation.
 000: rcu:        RCU restricting CPUs from NR_CPUS=8192 to nr_cpu_ids=24.
 000: rcu:        RCU priority boosting: priority 1 delay 500 ms.
 000: rcu:        RCU_SOFTIRQ processing moved to rcuc kthreads.
 000:     No expedited grace period (rcu_normal_after_boot).
 000:     Tasks RCU enabled.
 000: rcu: RCU calculated value of scheduler-enlistment delay is 100 jiffies.
 000: rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=24
 000: NR_IRQS: 524544, nr_irqs: 1432, preallocated irqs: 16
 000: random: crng done (trusting CPU's manufacturer)
 000: Console: colour VGA+ 80x25
 000: printk: console [ttyS0] enabled
 000: ACPI: Core revision 20200110
 000: clocksource: hpet: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 133484882848 ns
 000: APIC: Switch to symmetric I/O mode setup
 000: DMAR: Host address width 46
 000: DMAR: DRHD base: 0x000000fbffc000 flags: 0x0
 000: DMAR: dmar0: reg_base_addr fbffc000 ver 1:0 cap d2078c106f0466 ecap f020df
 000: DMAR: DRHD base: 0x000000c7ffc000 flags: 0x1
 000: DMAR: dmar1: reg_base_addr c7ffc000 ver 1:0 cap d2078c106f0466 ecap f020df
 000: DMAR: RMRR base: 0x00000079190000 end: 0x00000079192fff
 000: DMAR: RMRR base: 0x000000791f4000 end: 0x000000791f7fff
 000: DMAR: RMRR base: 0x000000791de000 end: 0x000000791f3fff
 000: DMAR: RMRR base: 0x000000791cb000 end: 0x000000791dbfff
 000: DMAR: RMRR base: 0x000000791dc000 end: 0x000000791ddfff
 000: DMAR: RMRR base: 0x0000005a661000 end: 0x0000005a6a0fff
 000: DMAR-IR: IOAPIC id 10 under DRHD base  0xfbffc000 IOMMU 0
 000: DMAR-IR: IOAPIC id 8 under DRHD base  0xc7ffc000 IOMMU 1
 000: DMAR-IR: IOAPIC id 9 under DRHD base  0xc7ffc000 IOMMU 1
 000: DMAR-IR: HPET id 0 under DRHD base 0xc7ffc000
 000: DMAR-IR: Queued invalidation will be enabled to support x2apic and Intr-remapping.
 000: DMAR-IR: Enabled IRQ remapping in x2apic mode
 000: x2apic enabled
 000: Switched APIC routing to cluster x2apic.
 000: ..TIMER: vector=0x30 apic1=0 pin1=2 apic2=-1 pin2=-1
 000: clocksource: tsc-early: mask: 0xffffffffffffffff max_cycles: 0x228d095820e, max_idle_ns: 440795295198 ns
 000: ------------[ cut here ]------------
 000: WARNING: CPU: 0 PID: 0 at kernel/time/timer.c:897 add_timer_on+0x129/0x140
 000: Modules linked in:
 000: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.6.17-rt10 #1
 000: Hardware name: HP ProLiant DL380 Gen9/ProLiant DL380 Gen9, BIOS P89 10/25/2017
 000: RIP: 0010:add_timer_on+0x129/0x140
 000: Code: 24 48 89 ef e8 e8 ca 7d 00 48 8b 44 24 08 65 48 33 04 25 28 00 00 00 75 1f 48 83 c4 10 5b 5d 41 5c c3 48 89 45 48 eb ca 0f 0b <0f> 0b eb c4 e8 53 94 e9 ff e9 7b ff ff ff e8 64 c4 f6 ff 0f 1f 40
 000: RSP: 0000:ffffffffb2c03e80 EFLAGS: 00010083
 000: RAX: 00000000fffb6c25 RBX: ffffffffb3886540 RCX: 0000000000000000
 000: RDX: 00000000fffb6c20 RSI: ffffffffb2c03e80 RDI: 0000000000000001
 000: RBP: ffff92687fa19300 R08: 0000000000000000 R09: 0000000000000001
 000: R10: ffffffffb2c5b4e0 R11: 3235393730343420 R12: 0000000000000000
 000: R13: ffffffffb2cd42c0 R14: 0000000000000000 R15: ffff92687fa27f40
 000: FS:  0000000000000000(0000) GS:ffff92687fa00000(0000) knlGS:0000000000000000
 000: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 000: CR2: ffff92610ac01000 CR3: 00000008c920e001 CR4: 00000000000606b0
 000: Call Trace:
 000:  clocksource_select_watchdog+0x144/0x1a0
 000:  __clocksource_register_scale+0x88/0xf0
 000:  tsc_init+0x1a1/0x268
 000:  start_kernel+0x4ae/0x56e
 000:  secondary_startup_64+0xb6/0xc0
 000: ---[ end trace 0000000000000001 ]---

Guess you might be faster to understand what I'm missing. :-)

Best,

Juri

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Frederic Weisbecker]

On Wed, Jul 01, 2020 at 06:35:04PM +0200, Juri Lelli wrote:
> Guess you might be faster to understand what I'm missing. :-)

So, did you port only this patch or the whole set in order to
trigger this?

If it was the whole set, can you try this patch alone?

Thanks!

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Juri Lelli]

On 02/07/20 01:20, Frederic Weisbecker wrote:
> On Wed, Jul 01, 2020 at 06:35:04PM +0200, Juri Lelli wrote:
> > Guess you might be faster to understand what I'm missing. :-)
> 
> So, did you port only this patch or the whole set in order to
> trigger this?
> 
> If it was the whole set, can you try this patch alone?

Whole set. And I can't reproduce if I try with this patch alone.

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Frederic Weisbecker]

On Thu, Jul 02, 2020 at 11:59:59AM +0200, Juri Lelli wrote:
> On 02/07/20 01:20, Frederic Weisbecker wrote:
> > On Wed, Jul 01, 2020 at 06:35:04PM +0200, Juri Lelli wrote:
> > > Guess you might be faster to understand what I'm missing. :-)
> > 
> > So, did you port only this patch or the whole set in order to
> > trigger this?
> > 
> > If it was the whole set, can you try this patch alone?
> 
> Whole set. And I can't reproduce if I try with this patch alone.

Ok I can reproduce with the whole series upstream with threadirqs.
Investigating...

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Frederic Weisbecker]

On Thu, Jul 02, 2020 at 11:59:59AM +0200, Juri Lelli wrote:
> On 02/07/20 01:20, Frederic Weisbecker wrote:
> > On Wed, Jul 01, 2020 at 06:35:04PM +0200, Juri Lelli wrote:
> > > Guess you might be faster to understand what I'm missing. :-)
> > 
> > So, did you port only this patch or the whole set in order to
> > trigger this?
> > 
> > If it was the whole set, can you try this patch alone?
> 
> Whole set. And I can't reproduce if I try with this patch alone.
> 

Missing initialization. This should go better after this:

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 25a55c043297..f36b53219768 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1969,6 +1969,7 @@ static void __init init_timer_cpu(int cpu)
 		base->cpu = cpu;
 		raw_spin_lock_init(&base->lock);
 		base->clk = jiffies;
+		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
 		timer_base_init_expiry_lock(base);
 	}
 }

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Juri Lelli]

On 02/07/20 16:32, Frederic Weisbecker wrote:
> On Thu, Jul 02, 2020 at 11:59:59AM +0200, Juri Lelli wrote:
> > On 02/07/20 01:20, Frederic Weisbecker wrote:
> > > On Wed, Jul 01, 2020 at 06:35:04PM +0200, Juri Lelli wrote:
> > > > Guess you might be faster to understand what I'm missing. :-)
> > > 
> > > So, did you port only this patch or the whole set in order to
> > > trigger this?
> > > 
> > > If it was the whole set, can you try this patch alone?
> > 
> > Whole set. And I can't reproduce if I try with this patch alone.
> > 
> 
> Missing initialization. This should go better after this:
> 
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 25a55c043297..f36b53219768 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -1969,6 +1969,7 @@ static void __init init_timer_cpu(int cpu)
>  		base->cpu = cpu;
>  		raw_spin_lock_init(&base->lock);
>  		base->clk = jiffies;
> +		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
>  		timer_base_init_expiry_lock(base);
>  	}
>  }
> 

Yep. Looks like warning is gone.

Re: [RFC PATCH 01/10] timer: Prevent base->clk from moving backward

[Thomas Gleixner]

Frederic Weisbecker <frederic@kernel.org> writes:
> When a timer is enqueued with a negative delta (ie: expiry is below
> base->clk), it gets added to the wheel as expiring now (base->clk).
>
> Yet the value that gets stored in base->next_expiry, while calling
> trigger_dyntick_cpu(), is the initial timer->expires value. The
> resulting state becomes:
>
> 	base->next_expiry < base->clk
>
> On the next timer enqueue, forward_timer_base() may accidentally
> rewind base->clk. As a possible outcome, timers may expire way too
> early, the worst case being that the highest wheel levels get spuriously
> processed again.

Bah. Nice catch.

> To prevent from that, make sure that base->next_expiry doesn't get below
> base->clk.

That wants a Fixes: tag and a CC stable.

Thanks,

        tglx

[RFC PATCH 02/10] timer: Move trigger_dyntick_cpu() to enqueue_timer()

[Frederic Weisbecker]

Consolidate the code by calling trigger_dyntick_cpu() from
enqueue_timer() instead of calling it from all its callers.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 46 +++++++++++++++++++--------------------------
 1 file changed, 19 insertions(+), 27 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 9a838d38dbe6..4c977df3610b 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -529,29 +529,6 @@ static int calc_wheel_index(unsigned long expires, unsigned long clk)
 	return idx;
 }
 
-/*
- * Enqueue the timer into the hash bucket, mark it pending in
- * the bitmap and store the index in the timer flags.
- */
-static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
-			  unsigned int idx)
-{
-	hlist_add_head(&timer->entry, base->vectors + idx);
-	__set_bit(idx, base->pending_map);
-	timer_set_idx(timer, idx);
-
-	trace_timer_start(timer, timer->expires, timer->flags);
-}
-
-static void
-__internal_add_timer(struct timer_base *base, struct timer_list *timer)
-{
-	unsigned int idx;
-
-	idx = calc_wheel_index(timer->expires, base->clk);
-	enqueue_timer(base, timer, idx);
-}
-
 static void
 trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 {
@@ -596,13 +573,29 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 	wake_up_nohz_cpu(base->cpu);
 }
 
-static void
-internal_add_timer(struct timer_base *base, struct timer_list *timer)
+/*
+ * Enqueue the timer into the hash bucket, mark it pending in
+ * the bitmap and store the index in the timer flags.
+ */
+static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
+			  unsigned int idx)
 {
-	__internal_add_timer(base, timer);
+	hlist_add_head(&timer->entry, base->vectors + idx);
+	__set_bit(idx, base->pending_map);
+	timer_set_idx(timer, idx);
+
+	trace_timer_start(timer, timer->expires, timer->flags);
 	trigger_dyntick_cpu(base, timer);
 }
 
+static void internal_add_timer(struct timer_base *base, struct timer_list *timer)
+{
+	unsigned int idx;
+
+	idx = calc_wheel_index(timer->expires, base->clk);
+	enqueue_timer(base, timer, idx);
+}
+
 #ifdef CONFIG_DEBUG_OBJECTS_TIMERS
 
 static struct debug_obj_descr timer_debug_descr;
@@ -1059,7 +1052,6 @@ __mod_timer(struct timer_list *timer, unsigned long expires, unsigned int option
 	 */
 	if (idx != UINT_MAX && clk == base->clk) {
 		enqueue_timer(base, timer, idx);
-		trigger_dyntick_cpu(base, timer);
 	} else {
 		internal_add_timer(base, timer);
 	}
-- 
2.26.2

[RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index()

[Frederic Weisbecker]

LVL_START() makes the first index of a level to start with what would be
the value of all bits set of the previous level.

For example level 1 starts at 63 instead of 64.

To cope with that, calc_index() always adds one offset for the level
granularity to the expiry passed in parameter.

Yet there is no apparent reason for such fixups so simplify the whole
thing.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4c977df3610b..b4838d63a016 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -158,7 +158,7 @@ EXPORT_SYMBOL(jiffies_64);
  * The time start value for each level to select the bucket at enqueue
  * time.
  */
-#define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
+#define LVL_START(n)	(LVL_SIZE << (((n) - 1) * LVL_CLK_SHIFT))
 
 /* Size of each clock level */
 #define LVL_BITS	6
@@ -489,7 +489,7 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
  */
 static inline unsigned calc_index(unsigned expires, unsigned lvl)
 {
-	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
+	expires >>= LVL_SHIFT(lvl);
 	return LVL_OFFS(lvl) + (expires & LVL_MASK);
 }
 
-- 
2.26.2

Re: [RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index()

[Thomas Gleixner]

Frederic Weisbecker <frederic@kernel.org> writes:
> LVL_START() makes the first index of a level to start with what would be
> the value of all bits set of the previous level.
>
> For example level 1 starts at 63 instead of 64.
>
> To cope with that, calc_index() always adds one offset for the level
> granularity to the expiry passed in parameter.
>
> Yet there is no apparent reason for such fixups so simplify the whole
> thing.

You sure?

> @@ -158,7 +158,7 @@ EXPORT_SYMBOL(jiffies_64);
>   * The time start value for each level to select the bucket at enqueue
>   * time.
>   */
> -#define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
> +#define LVL_START(n)	(LVL_SIZE << (((n) - 1) * LVL_CLK_SHIFT))

>  /* Size of each clock level */
>  #define LVL_BITS	6
> @@ -489,7 +489,7 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
>   */
>  static inline unsigned calc_index(unsigned expires, unsigned lvl)
>  {
> -	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
> +	expires >>= LVL_SHIFT(lvl);
>  	return LVL_OFFS(lvl) + (expires & LVL_MASK);
>  }

So with that you move the expiry of each timer one jiffie ahead vs. the
original code, which violates the guarantee that a timer sleeps at least
for one jiffie for real and not measured in jiffies.

base->clk = 1
jiffies = 0
 local_irq_disable()
                        -> timer interrupt is raised in HW
 timer->expires = 1
 add_timer(timer)
        ---> index == 1
 local_irq_enable()
 timer interrupt
    jiffies++;
    softirq()
        expire(timer);

So the off by one has a reason.

Thanks,

        tglx

Re: [RFC PATCH 03/10] timer: Simplify LVL_START() and calc_index()

[Frederic Weisbecker]

On Thu, Jul 02, 2020 at 01:59:17PM +0200, Thomas Gleixner wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
> > LVL_START() makes the first index of a level to start with what would be
> > the value of all bits set of the previous level.
> >
> > For example level 1 starts at 63 instead of 64.
> >
> > To cope with that, calc_index() always adds one offset for the level
> > granularity to the expiry passed in parameter.
> >
> > Yet there is no apparent reason for such fixups so simplify the whole
> > thing.
> 
> You sure?

No :o)

> 
> > @@ -158,7 +158,7 @@ EXPORT_SYMBOL(jiffies_64);
> >   * The time start value for each level to select the bucket at enqueue
> >   * time.
> >   */
> > -#define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
> > +#define LVL_START(n)	(LVL_SIZE << (((n) - 1) * LVL_CLK_SHIFT))
> 
> >  /* Size of each clock level */
> >  #define LVL_BITS	6
> > @@ -489,7 +489,7 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx)
> >   */
> >  static inline unsigned calc_index(unsigned expires, unsigned lvl)
> >  {
> > -	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
> > +	expires >>= LVL_SHIFT(lvl);
> >  	return LVL_OFFS(lvl) + (expires & LVL_MASK);
> >  }
> 
> So with that you move the expiry of each timer one jiffie ahead vs. the
> original code, which violates the guarantee that a timer sleeps at least
> for one jiffie for real and not measured in jiffies.
> 
> base->clk = 1
> jiffies = 0
>  local_irq_disable()
>                         -> timer interrupt is raised in HW
>  timer->expires = 1
>  add_timer(timer)
>         ---> index == 1
>  local_irq_enable()
>  timer interrupt
>     jiffies++;
>     softirq()
>         expire(timer);
> 
> So the off by one has a reason.

Fair enough. I didn't know about the one jiffy sleep guarantee.
I'll convert this patch to a comment explaining that off by one,
using your above example.

Thanks!

[RFC PATCH 04/10] timer: Optimize _next_timer_interrupt() level iteration

[Frederic Weisbecker]

If a level has a timer that expires before we reach the next level,
there is no need to iterate further.

The next level is reached when the 3 lower bits of the current level are
cleared. If the next event happens before/during that, the next levels
won't give us an earlier expiration.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index b4838d63a016..4aa74aafdd33 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1515,6 +1515,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 	clk = base->clk;
 	for (lvl = 0; lvl < LVL_DEPTH; lvl++, offset += LVL_SIZE) {
 		int pos = next_pending_bucket(base, offset, clk & LVL_MASK);
+		unsigned long lvl_clk = clk & LVL_CLK_MASK;
 
 		if (pos >= 0) {
 			unsigned long tmp = clk + (unsigned long) pos;
@@ -1522,6 +1523,13 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 			tmp <<= LVL_SHIFT(lvl);
 			if (time_before(tmp, next))
 				next = tmp;
+
+			/*
+			 * If the next expiration happens before we reach
+			 * the next level, no need to check further.
+			 */
+			if (pos <= ((LVL_CLK_DIV - lvl_clk) & LVL_CLK_MASK))
+				break;
 		}
 		/*
 		 * Clock for the next level. If the current level clock lower
@@ -1559,7 +1567,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 		 * So the simple check whether the lower bits of the current
 		 * level are 0 or not is sufficient for all cases.
 		 */
-		adj = clk & LVL_CLK_MASK ? 1 : 0;
+		adj = lvl_clk ? 1 : 0;
 		clk >>= LVL_CLK_SHIFT;
 		clk += adj;
 	}
-- 
2.26.2

[RFC PATCH 05/10] timers: Always keep track of next expiry

[Frederic Weisbecker]

So far next expiry was only tracked while the CPU was in nohz_idle mode
in order to cope with missing ticks that can't increment the base->clk
periodically anymore.

We are going to expand that logic beyond nohz in order to spare timers
softirqs so do it unconditionally.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 35 ++++++++++++++++++-----------------
 1 file changed, 18 insertions(+), 17 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4aa74aafdd33..30f491ec875a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -550,8 +550,22 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 	 * timer is not deferrable. If the other CPU is on the way to idle
 	 * then it can't set base->is_idle as we hold the base lock:
 	 */
-	if (!base->is_idle)
-		return;
+	if (base->is_idle)
+		wake_up_nohz_cpu(base->cpu);
+}
+
+/*
+ * Enqueue the timer into the hash bucket, mark it pending in
+ * the bitmap and store the index in the timer flags.
+ */
+static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
+			  unsigned int idx)
+{
+	hlist_add_head(&timer->entry, base->vectors + idx);
+	__set_bit(idx, base->pending_map);
+	timer_set_idx(timer, idx);
+
+	trace_timer_start(timer, timer->expires, timer->flags);
 
 	/* Check whether this is the new first expiring timer: */
 	if (time_after_eq(timer->expires, base->next_expiry))
@@ -570,21 +584,7 @@ trigger_dyntick_cpu(struct timer_base *base, struct timer_list *timer)
 	} else {
 		base->next_expiry = timer->expires;
 	}
-	wake_up_nohz_cpu(base->cpu);
-}
 
-/*
- * Enqueue the timer into the hash bucket, mark it pending in
- * the bitmap and store the index in the timer flags.
- */
-static void enqueue_timer(struct timer_base *base, struct timer_list *timer,
-			  unsigned int idx)
-{
-	hlist_add_head(&timer->entry, base->vectors + idx);
-	__set_bit(idx, base->pending_map);
-	timer_set_idx(timer, idx);
-
-	trace_timer_start(timer, timer->expires, timer->flags);
 	trigger_dyntick_cpu(base, timer);
 }
 
@@ -1482,7 +1482,6 @@ static int __collect_expired_timers(struct timer_base *base,
 	return levels;
 }
 
-#ifdef CONFIG_NO_HZ_COMMON
 /*
  * Find the next pending bucket of a level. Search from level start (@offset)
  * + @clk upwards and if nothing there, search from start of the level
@@ -1574,6 +1573,7 @@ static unsigned long __next_timer_interrupt(struct timer_base *base)
 	return next;
 }
 
+#ifdef CONFIG_NO_HZ_COMMON
 /*
  * Check, if the next hrtimer event is before the next timer wheel
  * event:
@@ -1779,6 +1779,7 @@ static inline void __run_timers(struct timer_base *base)
 
 		levels = collect_expired_timers(base, heads);
 		base->clk++;
+		base->next_expiry = __next_timer_interrupt(base);
 
 		while (levels--)
 			expire_timers(base, heads + levels);
-- 
2.26.2

[RFC PATCH 06/10] timer: Reuse next expiry cache after nohz exit

[Frederic Weisbecker]

Now that we track the next expiry unconditionally when a timer is added,
we can reuse that information on a tick firing after exiting nohz.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 30f491ec875a..4612d26fd09a 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1697,13 +1697,11 @@ static int collect_expired_timers(struct timer_base *base,
 	 * the next expiring timer.
 	 */
 	if ((long)(now - base->clk) > 2) {
-		unsigned long next = __next_timer_interrupt(base);
-
 		/*
 		 * If the next timer is ahead of time forward to current
 		 * jiffies, otherwise forward to the next expiry time:
 		 */
-		if (time_after(next, now)) {
+		if (time_after(base->next_expiry, now)) {
 			/*
 			 * The call site will increment base->clk and then
 			 * terminate the expiry loop immediately.
@@ -1711,7 +1709,7 @@ static int collect_expired_timers(struct timer_base *base,
 			base->clk = now;
 			return 0;
 		}
-		base->clk = next;
+		base->clk = base->next_expiry;
 	}
 	return __collect_expired_timers(base, heads);
 }
-- 
2.26.2

[RFC PATCH 07/10] timer: Expand clk forward logic beyond nohz

[Frederic Weisbecker]

As for next_expiry, we want to expand base->clk catch up logic beyond
nohz in order to avoid triggering useless softirqs.

If we want to fire softirqs only to expire pending timers, periodic
base->clk increments must be skippable for random amounts of time.
Therefore we must prepare to catch-up with missing updates whenever we
need an up-to-date base clock.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 26 ++++----------------------
 1 file changed, 4 insertions(+), 22 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 4612d26fd09a..6c021be0e76f 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -877,19 +877,12 @@ get_target_base(struct timer_base *base, unsigned tflags)
 
 static inline void forward_timer_base(struct timer_base *base)
 {
-#ifdef CONFIG_NO_HZ_COMMON
 	unsigned long jnow;
 
-	/*
-	 * We only forward the base when we are idle or have just come out of
-	 * idle (must_forward_clk logic), and have a delta between base clock
-	 * and jiffies. In the common case, run_timers will take care of it.
-	 */
-	if (likely(!base->must_forward_clk))
+	if (!base->must_forward_clk)
 		return;
 
 	jnow = READ_ONCE(jiffies);
-	base->must_forward_clk = base->is_idle;
 	if ((long)(jnow - base->clk) < 2)
 		return;
 
@@ -904,7 +897,6 @@ static inline void forward_timer_base(struct timer_base *base)
 			return;
 		base->clk = base->next_expiry;
 	}
-#endif
 }
 
 
@@ -1658,10 +1650,8 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
 		 * logic is only maintained for the BASE_STD base, deferrable
 		 * timers may still see large granularity skew (by design).
 		 */
-		if ((expires - basem) > TICK_NSEC) {
-			base->must_forward_clk = true;
+		if ((expires - basem) > TICK_NSEC)
 			base->is_idle = true;
-		}
 	}
 	raw_spin_unlock(&base->lock);
 
@@ -1760,16 +1750,7 @@ static inline void __run_timers(struct timer_base *base)
 	/*
 	 * timer_base::must_forward_clk must be cleared before running
 	 * timers so that any timer functions that call mod_timer() will
-	 * not try to forward the base. Idle tracking / clock forwarding
-	 * logic is only used with BASE_STD timers.
-	 *
-	 * The must_forward_clk flag is cleared unconditionally also for
-	 * the deferrable base. The deferrable base is not affected by idle
-	 * tracking and never forwarded, so clearing the flag is a NOOP.
-	 *
-	 * The fact that the deferrable base is never forwarded can cause
-	 * large variations in granularity for deferrable timers, but they
-	 * can be deferred for long periods due to idle anyway.
+	 * not try to forward the base.
 	 */
 	base->must_forward_clk = false;
 
@@ -1782,6 +1763,7 @@ static inline void __run_timers(struct timer_base *base)
 		while (levels--)
 			expire_timers(base, heads + levels);
 	}
+	base->must_forward_clk = true;
 	raw_spin_unlock_irq(&base->lock);
 	timer_base_unlock_expiry(base);
 }
-- 
2.26.2

[RFC PATCH 08/10] timer: Spare timer softirq until next expiry

[Frederic Weisbecker]

Now that the core timer infrastructure doesn't depend anymore on
periodic base->clk increments, even when the CPU is not in NO_HZ mode,
we can delay the timer softirqs until we have actual timers to expire.

Some spurious softirqs can still remain since base->next_expiry doesn't
keep track of canceled timers but we are still way ahead of the
unconditional periodic softirqs (~15 times less of them with 1000 Hz
and ~5 times less with 100 Hz).

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 49 ++++++++-------------------------------------
 1 file changed, 8 insertions(+), 41 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 6c021be0e76f..95f51b646447 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1449,10 +1449,10 @@ static void expire_timers(struct timer_base *base, struct hlist_head *head)
 	}
 }
 
-static int __collect_expired_timers(struct timer_base *base,
-				    struct hlist_head *heads)
+static int collect_expired_timers(struct timer_base *base,
+				  struct hlist_head *heads)
 {
-	unsigned long clk = base->clk;
+	unsigned long clk = base->clk = base->next_expiry;
 	struct hlist_head *vec;
 	int i, levels = 0;
 	unsigned int idx;
@@ -1675,40 +1675,6 @@ void timer_clear_idle(void)
 	 */
 	base->is_idle = false;
 }
-
-static int collect_expired_timers(struct timer_base *base,
-				  struct hlist_head *heads)
-{
-	unsigned long now = READ_ONCE(jiffies);
-
-	/*
-	 * NOHZ optimization. After a long idle sleep we need to forward the
-	 * base to current jiffies. Avoid a loop by searching the bitfield for
-	 * the next expiring timer.
-	 */
-	if ((long)(now - base->clk) > 2) {
-		/*
-		 * If the next timer is ahead of time forward to current
-		 * jiffies, otherwise forward to the next expiry time:
-		 */
-		if (time_after(base->next_expiry, now)) {
-			/*
-			 * The call site will increment base->clk and then
-			 * terminate the expiry loop immediately.
-			 */
-			base->clk = now;
-			return 0;
-		}
-		base->clk = base->next_expiry;
-	}
-	return __collect_expired_timers(base, heads);
-}
-#else
-static inline int collect_expired_timers(struct timer_base *base,
-					 struct hlist_head *heads)
-{
-	return __collect_expired_timers(base, heads);
-}
 #endif
 
 /*
@@ -1741,7 +1707,7 @@ static inline void __run_timers(struct timer_base *base)
 	struct hlist_head heads[LVL_DEPTH];
 	int levels;
 
-	if (!time_after_eq(jiffies, base->clk))
+	if (time_before(jiffies, base->next_expiry))
 		return;
 
 	timer_base_lock_expiry(base);
@@ -1754,7 +1720,8 @@ static inline void __run_timers(struct timer_base *base)
 	 */
 	base->must_forward_clk = false;
 
-	while (time_after_eq(jiffies, base->clk)) {
+	while (time_after_eq(jiffies, base->clk) &&
+	       time_after_eq(jiffies, base->next_expiry)) {
 
 		levels = collect_expired_timers(base, heads);
 		base->clk++;
@@ -1789,12 +1756,12 @@ void run_local_timers(void)
 
 	hrtimer_run_queues();
 	/* Raise the softirq only if required. */
-	if (time_before(jiffies, base->clk)) {
+	if (time_before(jiffies, base->next_expiry)) {
 		if (!IS_ENABLED(CONFIG_NO_HZ_COMMON))
 			return;
 		/* CPU is awake, so check the deferrable base. */
 		base++;
-		if (time_before(jiffies, base->clk))
+		if (time_before(jiffies, base->next_expiry))
 			return;
 	}
 	raise_softirq(TIMER_SOFTIRQ);
-- 
2.26.2

[RFC PATCH 09/10] timer: Remove must_forward_clk

[Frederic Weisbecker]

There is no reason to keep this guard around. The code makes sure that
base->clk remains sane and won't be forwarded beyond jiffies nor set
backward.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 22 ++++++----------------
 1 file changed, 6 insertions(+), 16 deletions(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 95f51b646447..439fee098e76 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -204,7 +204,6 @@ struct timer_base {
 	unsigned long		next_expiry;
 	unsigned int		cpu;
 	bool			is_idle;
-	bool			must_forward_clk;
 	DECLARE_BITMAP(pending_map, WHEEL_SIZE);
 	struct hlist_head	vectors[WHEEL_SIZE];
 } ____cacheline_aligned;
@@ -877,12 +876,13 @@ get_target_base(struct timer_base *base, unsigned tflags)
 
 static inline void forward_timer_base(struct timer_base *base)
 {
-	unsigned long jnow;
+	unsigned long jnow = READ_ONCE(jiffies);
 
-	if (!base->must_forward_clk)
-		return;
-
-	jnow = READ_ONCE(jiffies);
+	/*
+	 * No need to forward if we are close enough below jiffies.
+	 * Also while executing timers, base->clk is 1 offset ahead
+	 * of jiffies to avoid endless requeuing to current jffies.
+	 */
 	if ((long)(jnow - base->clk) < 2)
 		return;
 
@@ -1713,16 +1713,8 @@ static inline void __run_timers(struct timer_base *base)
 	timer_base_lock_expiry(base);
 	raw_spin_lock_irq(&base->lock);
 
-	/*
-	 * timer_base::must_forward_clk must be cleared before running
-	 * timers so that any timer functions that call mod_timer() will
-	 * not try to forward the base.
-	 */
-	base->must_forward_clk = false;
-
 	while (time_after_eq(jiffies, base->clk) &&
 	       time_after_eq(jiffies, base->next_expiry)) {
-
 		levels = collect_expired_timers(base, heads);
 		base->clk++;
 		base->next_expiry = __next_timer_interrupt(base);
@@ -1730,7 +1722,6 @@ static inline void __run_timers(struct timer_base *base)
 		while (levels--)
 			expire_timers(base, heads + levels);
 	}
-	base->must_forward_clk = true;
 	raw_spin_unlock_irq(&base->lock);
 	timer_base_unlock_expiry(base);
 }
@@ -1926,7 +1917,6 @@ int timers_prepare_cpu(unsigned int cpu)
 		base->clk = jiffies;
 		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
 		base->is_idle = false;
-		base->must_forward_clk = true;
 	}
 	return 0;
 }
-- 
2.26.2

[RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Frederic Weisbecker]

There is no apparent reason for not forwarding base->clk when it's 2
jiffies late, except perhaps for past optimizations. But since forwarding
has to be done at some point now anyway, this doesn't stand anymore.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Juri Lelli <juri.lelli@redhat.com>
---
 kernel/time/timer.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/timer.c b/kernel/time/timer.c
index 439fee098e76..25a55c043297 100644
--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -883,7 +883,7 @@ static inline void forward_timer_base(struct timer_base *base)
 	 * Also while executing timers, base->clk is 1 offset ahead
 	 * of jiffies to avoid endless requeuing to current jffies.
 	 */
-	if ((long)(jnow - base->clk) < 2)
+	if ((long)(jnow - base->clk) < 1)
 		return;
 
 	/*
-- 
2.26.2

Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Thomas Gleixner]

Frederic Weisbecker <frederic@kernel.org> writes:
> There is no apparent reason for not forwarding base->clk when it's 2
> jiffies late, except perhaps for past optimizations. But since forwarding
> has to be done at some point now anyway, this doesn't stand anymore.
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> ---
>  kernel/time/timer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 439fee098e76..25a55c043297 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -883,7 +883,7 @@ static inline void forward_timer_base(struct timer_base *base)
>  	 * Also while executing timers, base->clk is 1 offset ahead
>  	 * of jiffies to avoid endless requeuing to current jffies.
>  	 */
> -	if ((long)(jnow - base->clk) < 2)
> +	if ((long)(jnow - base->clk) < 1)
>  		return;

The apparent reason is in the comment right above the condition ...

Thanks,

        tglx

Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Frederic Weisbecker]

On Thu, Jul 02, 2020 at 03:21:35PM +0200, Thomas Gleixner wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
> > There is no apparent reason for not forwarding base->clk when it's 2
> > jiffies late, except perhaps for past optimizations. But since forwarding
> > has to be done at some point now anyway, this doesn't stand anymore.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
> > Cc: Juri Lelli <juri.lelli@redhat.com>
> > ---
> >  kernel/time/timer.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> > index 439fee098e76..25a55c043297 100644
> > --- a/kernel/time/timer.c
> > +++ b/kernel/time/timer.c
> > @@ -883,7 +883,7 @@ static inline void forward_timer_base(struct timer_base *base)
> >  	 * Also while executing timers, base->clk is 1 offset ahead
> >  	 * of jiffies to avoid endless requeuing to current jffies.
> >  	 */
> > -	if ((long)(jnow - base->clk) < 2)
> > +	if ((long)(jnow - base->clk) < 1)
> >  		return;
> 
> The apparent reason is in the comment right above the condition ...

Hmm, that's a comment I added myself in the patch before.

The following part:

> >      * Also while executing timers, base->clk is 1 offset ahead
> >      * of jiffies to avoid endless requeuing to current jffies.
> >      */

relates to situation when (long)(jnow - base->clk) < 0

Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Thomas Gleixner]

Frederic Weisbecker <frederic@kernel.org> writes:
> On Thu, Jul 02, 2020 at 03:21:35PM +0200, Thomas Gleixner wrote:
>> Frederic Weisbecker <frederic@kernel.org> writes:
>> > @@ -883,7 +883,7 @@ static inline void forward_timer_base(struct timer_base *base)
>> >  	 * Also while executing timers, base->clk is 1 offset ahead
>> >  	 * of jiffies to avoid endless requeuing to current jffies.
>> >  	 */
>> > -	if ((long)(jnow - base->clk) < 2)
>> > +	if ((long)(jnow - base->clk) < 1)
>> >  		return;
>> 
>> The apparent reason is in the comment right above the condition ...
>
> Hmm, that's a comment I added myself in the patch before.

:)

> The following part:
>
>> >      * Also while executing timers, base->clk is 1 offset ahead
>> >      * of jiffies to avoid endless requeuing to current jffies.
>> >      */
>
> relates to situation when (long)(jnow - base->clk) < 0

This still is inconsistent with your changelog:

> There is no apparent reason for not forwarding base->clk when it's 2
> jiffies late

Let's do the math:

 jiffies = 4
 base->clk = 2

 4 - 2 = 2

which means it is forwarded when it's 2 jiffies late with the original
code, because 2 < 2.

The reason for this < 2 is historical and goes back to the oddities of
the original timer wheel before the big rewrite.

Thanks,

        tglx

Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Frederic Weisbecker]

On Thu, Jul 02, 2020 at 05:14:23PM +0200, Thomas Gleixner wrote:
> Frederic Weisbecker <frederic@kernel.org> writes:
> > On Thu, Jul 02, 2020 at 03:21:35PM +0200, Thomas Gleixner wrote:
> > The following part:
> >
> >> >      * Also while executing timers, base->clk is 1 offset ahead
> >> >      * of jiffies to avoid endless requeuing to current jffies.
> >> >      */
> >
> > relates to situation when (long)(jnow - base->clk) < 0
> 
> This still is inconsistent with your changelog:

Right.

> 
> > There is no apparent reason for not forwarding base->clk when it's 2
> > jiffies late
> 
> Let's do the math:
> 
>  jiffies = 4
>  base->clk = 2
> 
>  4 - 2 = 2
> 
> which means it is forwarded when it's 2 jiffies late with the original
> code, because 2 < 2.
> 
> The reason for this < 2 is historical and goes back to the oddities of
> the original timer wheel before the big rewrite.

Ok. And is it still needed today or can we now forward even with a 1 delta?

Re: [RFC PATCH 10/10] timer: Lower base clock forwarding threshold

[Thomas Gleixner]

Frederic Weisbecker <frederic@kernel.org> writes:
> On Thu, Jul 02, 2020 at 05:14:23PM +0200, Thomas Gleixner wrote:
>> 
>> The reason for this < 2 is historical and goes back to the oddities of
>> the original timer wheel before the big rewrite.
>
> Ok. And is it still needed today or can we now forward even with a 1 delta?

I haven't found anything which requires it. But be aware of dragons ....