* [dm-crypt] [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc()
@ 2021-08-08 13:42 Arne Welzel
2021-08-10 18:21 ` [dm-crypt] " Mikulas Patocka
0 siblings, 1 reply; 4+ messages in thread
From: Arne Welzel @ 2021-08-08 13:42 UTC (permalink / raw)
To: dm-devel, dm-crypt; +Cc: mpatocka, snitzer, agk, Arne Welzel, DJ Gregor
On many core systems using dm-crypt, heavy spinlock contention in
percpu_counter_compare() can be observed when the dmcrypt page allocation
limit for a given device is reached or close to be reached. This is due
to percpu_counter_compare() taking a spinlock to compute an exact
result on potentially many CPUs at the same time.
Switch to non-exact comparison of allocated and allowed pages by using
the value returned by percpu_counter_read_positive().
This may over/under estimate the actual number of allocated pages by at
most (batch-1) * num_online_cpus() (assuming my understanding of the
percpu_counter logic is proper).
Currently, batch is bounded by 32. The system on which this issue was
first observed has 256 CPUs and 512G of RAM. With a 4k page size, this
change may over/under estimate by 31MB. With ~10G (2%) allowed for dmcrypt
allocations, this seems an acceptable error. Certainly preferred over
running into the spinlock contention.
This behavior was separately/artificially reproduced on an EC2 c5.24xlarge
instance system with 96 CPUs and 192GB RAM as follows, but can be
provokes on systems with less available CPUs.
* Disable swap
* Tune vm settings to promote regular writeback
$ echo 50 > /proc/sys/vm/dirty_expire_centisecs
$ echo 25 > /proc/sys/vm/dirty_writeback_centisecs
$ echo $((128 * 1024 * 1024)) > /proc/sys/vm/dirty_background_bytes
* Create 8 dmcrypt devices based on files on a tmpfs
* Create and mount an ext4 filesystem on each crypt devices
* Run stress-ng --hdd 8 within one of above filesystems
Total %system usage shown via sysstat goes to ~35%, write througput on the
underlying loop device is ~2GB/s. perf profiling an individual kworker
kcryptd thread shows the following in the profile, indicating it hits
heavy spinlock contention in percpu_counter_compare():
99.98% 0.00% kworker/u193:46 [kernel.kallsyms] [k] ret_from_fork
|
---ret_from_fork
kthread
worker_thread
|
--99.92%--process_one_work
|
|--80.52%--kcryptd_crypt
| |
| |--62.58%--mempool_alloc
| | |
| | --62.24%--crypt_page_alloc
| | |
| | --61.51%--__percpu_counter_compare
| | |
| | --61.34%--__percpu_counter_sum
| | |
| | |--58.68%--_raw_spin_lock_irqsave
| | | |
| | | --58.30%--native_queued_spin_lock_slowpath
| | |
| | --0.69%--cpumask_next
| | |
| | --0.51%--_find_next_bit
| |
| |--10.61%--crypt_convert
| | |
| | |--6.05%--xts_crypt
...
After apply this change, %system usage is lowered to ~7% and
write throughput on the loopback interface increases to 2.7GB/s.
The profile shows mempool_alloc() as ~8% rather than ~62% in the
profile and not hitting the percpu_counter() spinlock anymore.
|--8.15%--mempool_alloc
| |
| |--3.93%--crypt_page_alloc
| | |
| | --3.75%--__alloc_pages
| | |
| | --3.62%--get_page_from_freelist
| | |
| | --3.22%--rmqueue_bulk
| | |
| | --2.59%--_raw_spin_lock
| |
| | --2.57%--native_queued_spin_lock_slowpath
| |
| --3.05%--_raw_spin_lock_irqsave
| |
| --2.49%--native_queued_spin_lock_slowpath
Suggested-by: DJ Gregor <dj@corelight.com>
Signed-off-by: Arne Welzel <arne.welzel@corelight.com>
---
drivers/md/dm-crypt.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 50f4cbd600d5..2ae481610f12 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c
@@ -2661,7 +2661,12 @@ static void *crypt_page_alloc(gfp_t gfp_mask, void *pool_data)
struct crypt_config *cc = pool_data;
struct page *page;
- if (unlikely(percpu_counter_compare(&cc->n_allocated_pages, dm_crypt_pages_per_client) >= 0) &&
+ /*
+ * Note, percpu_counter_read_positive() may over (and under) estimate
+ * the current usage by at most (batch - 1) * num_online_cpus() pages,
+ * but avoids potential spinlock contention of an exact result.
+ */
+ if (unlikely(percpu_counter_read_positive(&cc->n_allocated_pages) > dm_crypt_pages_per_client) &&
likely(gfp_mask & __GFP_NORETRY))
return NULL;
--
2.20.1
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^ permalink raw reply related [flat|nested] 4+ messages in thread* [dm-crypt] Re: [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc()
2021-08-08 13:42 [dm-crypt] [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc() Arne Welzel
@ 2021-08-10 18:21 ` Mikulas Patocka
2021-08-12 19:47 ` Arne Welzel
0 siblings, 1 reply; 4+ messages in thread
From: Mikulas Patocka @ 2021-08-10 18:21 UTC (permalink / raw)
To: Arne Welzel; +Cc: dm-devel, dm-crypt, snitzer, agk, DJ Gregor
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
On Sun, 8 Aug 2021, Arne Welzel wrote:
> On many core systems using dm-crypt, heavy spinlock contention in
> percpu_counter_compare() can be observed when the dmcrypt page allocation
> limit for a given device is reached or close to be reached. This is due
> to percpu_counter_compare() taking a spinlock to compute an exact
> result on potentially many CPUs at the same time.
>
> Switch to non-exact comparison of allocated and allowed pages by using
> the value returned by percpu_counter_read_positive().
>
> This may over/under estimate the actual number of allocated pages by at
> most (batch-1) * num_online_cpus() (assuming my understanding of the
> percpu_counter logic is proper).
>
> Currently, batch is bounded by 32. The system on which this issue was
> first observed has 256 CPUs and 512G of RAM. With a 4k page size, this
> change may over/under estimate by 31MB. With ~10G (2%) allowed for dmcrypt
> allocations, this seems an acceptable error. Certainly preferred over
> running into the spinlock contention.
>
> This behavior was separately/artificially reproduced on an EC2 c5.24xlarge
> instance system with 96 CPUs and 192GB RAM as follows, but can be
> provokes on systems with less available CPUs.
>
> * Disable swap
> * Tune vm settings to promote regular writeback
> $ echo 50 > /proc/sys/vm/dirty_expire_centisecs
> $ echo 25 > /proc/sys/vm/dirty_writeback_centisecs
> $ echo $((128 * 1024 * 1024)) > /proc/sys/vm/dirty_background_bytes
>
> * Create 8 dmcrypt devices based on files on a tmpfs
> * Create and mount an ext4 filesystem on each crypt devices
> * Run stress-ng --hdd 8 within one of above filesystems
>
> Total %system usage shown via sysstat goes to ~35%, write througput on the
> underlying loop device is ~2GB/s. perf profiling an individual kworker
> kcryptd thread shows the following in the profile, indicating it hits
> heavy spinlock contention in percpu_counter_compare():
>
> 99.98% 0.00% kworker/u193:46 [kernel.kallsyms] [k] ret_from_fork
> |
> ---ret_from_fork
> kthread
> worker_thread
> |
> --99.92%--process_one_work
> |
> |--80.52%--kcryptd_crypt
> | |
> | |--62.58%--mempool_alloc
> | | |
> | | --62.24%--crypt_page_alloc
> | | |
> | | --61.51%--__percpu_counter_compare
> | | |
> | | --61.34%--__percpu_counter_sum
> | | |
> | | |--58.68%--_raw_spin_lock_irqsave
> | | | |
> | | | --58.30%--native_queued_spin_lock_slowpath
> | | |
> | | --0.69%--cpumask_next
> | | |
> | | --0.51%--_find_next_bit
> | |
> | |--10.61%--crypt_convert
> | | |
> | | |--6.05%--xts_crypt
> ...
>
> After apply this change, %system usage is lowered to ~7% and
> write throughput on the loopback interface increases to 2.7GB/s.
> The profile shows mempool_alloc() as ~8% rather than ~62% in the
> profile and not hitting the percpu_counter() spinlock anymore.
>
> |--8.15%--mempool_alloc
> | |
> | |--3.93%--crypt_page_alloc
> | | |
> | | --3.75%--__alloc_pages
> | | |
> | | --3.62%--get_page_from_freelist
> | | |
> | | --3.22%--rmqueue_bulk
> | | |
> | | --2.59%--_raw_spin_lock
> | |
> | | --2.57%--native_queued_spin_lock_slowpath
> | |
> | --3.05%--_raw_spin_lock_irqsave
> | |
> | --2.49%--native_queued_spin_lock_slowpath
>
> Suggested-by: DJ Gregor <dj@corelight.com>
> Signed-off-by: Arne Welzel <arne.welzel@corelight.com>
> ---
> drivers/md/dm-crypt.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index 50f4cbd600d5..2ae481610f12 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -2661,7 +2661,12 @@ static void *crypt_page_alloc(gfp_t gfp_mask, void *pool_data)
> struct crypt_config *cc = pool_data;
> struct page *page;
>
> - if (unlikely(percpu_counter_compare(&cc->n_allocated_pages, dm_crypt_pages_per_client) >= 0) &&
> + /*
> + * Note, percpu_counter_read_positive() may over (and under) estimate
> + * the current usage by at most (batch - 1) * num_online_cpus() pages,
> + * but avoids potential spinlock contention of an exact result.
> + */
> + if (unlikely(percpu_counter_read_positive(&cc->n_allocated_pages) > dm_crypt_pages_per_client) &&
> likely(gfp_mask & __GFP_NORETRY))
> return NULL;
>
> --
> 2.20.1
>
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^ permalink raw reply [flat|nested] 4+ messages in thread
* [dm-crypt] Re: [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc()
2021-08-10 18:21 ` [dm-crypt] " Mikulas Patocka
@ 2021-08-12 19:47 ` Arne Welzel
2021-08-12 20:37 ` Mikulas Patocka
0 siblings, 1 reply; 4+ messages in thread
From: Arne Welzel @ 2021-08-12 19:47 UTC (permalink / raw)
To: Mikulas Patocka; +Cc: dm-devel, dm-crypt, snitzer, agk, DJ Gregor
Mikulas,
On Tue, 10 Aug 2021, Mikulas Patocka wrote:
> Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
>
thank you for the review. After looking at the submitted patch again,
seems more proper to use >= as the condition:
> > + if (unlikely(percpu_counter_read_positive(&cc->n_allocated_pages) > dm_crypt_pages_per_client) &&
^^
>=
Would it be okay if I resend the patch with this changed and add your
Reviewed-by still? Would also fix some wording in the description and
dedent the perf report output somewhat.
Thanks,
Arne
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^ permalink raw reply [flat|nested] 4+ messages in thread* [dm-crypt] Re: [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc()
2021-08-12 19:47 ` Arne Welzel
@ 2021-08-12 20:37 ` Mikulas Patocka
0 siblings, 0 replies; 4+ messages in thread
From: Mikulas Patocka @ 2021-08-12 20:37 UTC (permalink / raw)
To: Arne Welzel; +Cc: dm-devel, dm-crypt, snitzer, agk, DJ Gregor
On Thu, 12 Aug 2021, Arne Welzel wrote:
> Mikulas,
>
> On Tue, 10 Aug 2021, Mikulas Patocka wrote:
>
> > Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
> >
>
> thank you for the review. After looking at the submitted patch again,
> seems more proper to use >= as the condition:
>
> > > + if (unlikely(percpu_counter_read_positive(&cc->n_allocated_pages) > dm_crypt_pages_per_client) &&
> ^^
> >=
> Would it be okay if I resend the patch with this changed and add your
> Reviewed-by still? Would also fix some wording in the description and
> dedent the perf report output somewhat.
>
> Thanks,
> Arne
OK - you can resend the patch with my "Reviewed-by".
Mikulas
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^ permalink raw reply [flat|nested] 4+ messages in thread
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2021-08-08 13:42 [dm-crypt] [PATCH] dm crypt: Avoid percpu_counter spinlock contention in crypt_page_alloc() Arne Welzel
2021-08-10 18:21 ` [dm-crypt] " Mikulas Patocka
2021-08-12 19:47 ` Arne Welzel
2021-08-12 20:37 ` Mikulas Patocka
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