2021-12-16 09:27:51

by Yajun Deng

Subject: [PATCH v2] lib/raid6: Reduce high latency by using migrate instead of preempt

We found an abnormally high latency when executing modprobe raid6_pq, the
latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.

This is caused by ksoftirqd fail to scheduled due to disable preemption,
this time is too long and unreasonable.

Reduce high latency by using migrate_disabl()/emigrate_enable() instead of
preempt_disable()/preempt_enable().

How to reproduce:
- Install cyclictest
sudo apt install rt-tests
- Run cyclictest example in one terminal
sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m
- Modprobe raid6_pq in another terminal
sudo modprobe raid6_pq

This patch beneficial for CONFIG_PREEMPT=y and CONFIG_PREEMPT_RT=y, but
no effect for CONFIG_PREEMPT_VOLUNTARY=y.

Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
Fixes: cc4589ebfae6 ("Rename raid6 files now they're in a 'raid6' directory.")
Link: https://lore.kernel.org/linux-raid/[email protected]/T/#t
Signed-off-by: Yajun Deng <[email protected]>
---
lib/raid6/algos.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index 6d5e5000fdd7..21611d05c34c 100644
--- a/lib/raid6/algos.c
+++ b/lib/raid6/algos.c
@@ -162,7 +162,7 @@ static inline const struct raid6_calls *raid6_choose_gen(

perf = 0;

- preempt_disable();
+ migrate_disable();
j0 = jiffies;
while ((j1 = jiffies) == j0)
cpu_relax();
@@ -171,7 +171,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
(*algo)->gen_syndrome(disks, PAGE_SIZE, *dptrs);
perf++;
}
- preempt_enable();
+ migrate_enable();

if (perf > bestgenperf) {
bestgenperf = perf;
@@ -186,7 +186,7 @@ static inline const struct raid6_calls *raid6_choose_gen(

perf = 0;

- preempt_disable();
+ migrate_disable();
j0 = jiffies;
while ((j1 = jiffies) == j0)
cpu_relax();
@@ -196,7 +196,7 @@ static inline const struct raid6_calls *raid6_choose_gen(
PAGE_SIZE, *dptrs);
perf++;
}
- preempt_enable();
+ migrate_enable();

if (best == *algo)
bestxorperf = perf;
--
2.32.0

2021-12-16 19:23:29

by Song Liu

Subject: Re: [PATCH v2] lib/raid6: Reduce high latency by using migrate instead of preempt

On Thu, Dec 16, 2021 at 1:27 AM Yajun Deng <[email protected]> wrote:
>
> We found an abnormally high latency when executing modprobe raid6_pq, the
> latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
> 67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
>
> This is caused by ksoftirqd fail to scheduled due to disable preemption,
> this time is too long and unreasonable.
>
> Reduce high latency by using migrate_disabl()/emigrate_enable() instead of
> preempt_disable()/preempt_enable().
>
> How to reproduce:
> - Install cyclictest
> sudo apt install rt-tests
> - Run cyclictest example in one terminal
> sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m
> - Modprobe raid6_pq in another terminal
> sudo modprobe raid6_pq
>
> This patch beneficial for CONFIG_PREEMPT=y and CONFIG_PREEMPT_RT=y, but
> no effect for CONFIG_PREEMPT_VOLUNTARY=y.
>
> Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
> Fixes: cc4589ebfae6 ("Rename raid6 files now they're in a 'raid6' directory.")
> Link: https://lore.kernel.org/linux-raid/[email protected]/T/#t
> Signed-off-by: Yajun Deng <[email protected]>

Updated the patch and CC'ed stable. It should make to LTS kernels.

Thanks,
Song

2021-12-17 21:55:55

by Daniel Vacek

Subject: Re: [PATCH v2] lib/raid6: Reduce high latency by using migrate instead of preempt

NACK. This is a BUG.

--nX

On Fri, Dec 17, 2021 at 1:02 AM Song Liu <[email protected]> wrote:
>
> On Thu, Dec 16, 2021 at 1:27 AM Yajun Deng <[email protected]> wrote:
> >
> > We found an abnormally high latency when executing modprobe raid6_pq, the
> > latency is greater than 1.2s when CONFIG_PREEMPT_VOLUNTARY=y, greater than
> > 67ms when CONFIG_PREEMPT=y, and greater than 16ms when CONFIG_PREEMPT_RT=y.
> >
> > This is caused by ksoftirqd fail to scheduled due to disable preemption,
> > this time is too long and unreasonable.
> >
> > Reduce high latency by using migrate_disabl()/emigrate_enable() instead of
> > preempt_disable()/preempt_enable().
> >
> > How to reproduce:
> > - Install cyclictest
> > sudo apt install rt-tests
> > - Run cyclictest example in one terminal
> > sudo cyclictest -S -p 95 -d 0 -i 1000 -D 24h -m
> > - Modprobe raid6_pq in another terminal
> > sudo modprobe raid6_pq
> >
> > This patch beneficial for CONFIG_PREEMPT=y and CONFIG_PREEMPT_RT=y, but
> > no effect for CONFIG_PREEMPT_VOLUNTARY=y.
> >
> > Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
> > Fixes: cc4589ebfae6 ("Rename raid6 files now they're in a 'raid6' directory.")
> > Link: https://lore.kernel.org/linux-raid/[email protected]/T/#t
> > Signed-off-by: Yajun Deng <[email protected]>
>
> Updated the patch and CC'ed stable. It should make to LTS kernels.
>
> Thanks,
> Song