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.
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 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(), the latency won't greater than 100us.
This patch beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no effect for CONFIG_PREEMPT_VOLUNTARY=y.
Cc: stable@vger.kernel.org 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/b06c5e3ef3413f12a2c2b2a241005af9@linux.de... # v1 Signed-off-by: Yajun Deng yajun.deng@linux.dev --- 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;