linux-kernel - [PATCH] lib/raid6: fix abnormally high latency

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Date:   Tue, 14 Dec 2021 10:59:46 +0800
From:   Yajun Deng <yajun.deng@...ux.dev>
To:     stockhausen@...logia.de, neilb@...e.de
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Yajun Deng <yajun.deng@...ux.dev>
Subject: [PATCH] lib/raid6: fix abnormally high latency

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 disable the preemption, this time is too long and
unreasonable. We just need to disable migration. so used migrate_disable()/
migrate_enable() instead of preempt_disable()/preempt_enable(). This is
beneficial for CONFIG_PREEMPT=y or CONFIG_PREEMPT_RT=y, but no effect for
CONFIG_PREEMPT_VOLUNTARY=y.

Fixes: fe5cbc6e06c7 ("md/raid6 algorithms: delta syndrome functions")
Signed-off-by: Yajun Deng <yajun.deng@...ux.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;
-- 
2.32.0