linux-kernel - [PATCH] softirq: Do not loop if running under a real-time task

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Message-Id: <20230306154548.655799-1-oss@malat.biz>
Date:   Mon,  6 Mar 2023 16:45:48 +0100
From:   Petr Malat <oss@...at.biz>
To:     linux-kernel@...r.kernel.org
Cc:     paulmck@...nel.org, tglx@...utronix.de, bigeasy@...utronix.de,
        nsaenzju@...hat.com, frederic@...nel.org,
        Petr Malat <oss@...at.biz>
Subject: [PATCH] softirq: Do not loop if running under a real-time task

Softirq processing can be a source of a scheduling jitter if it executes
in a real-time task as in that case need_resched() is false unless there
is another runnable task with a higher priority. This is especially bad
if the softirq processing runs in a migration thread, which has priority
99 and usually runs for a short time.

One option would be to not restart the softirq processing if there is
another runnable task to allow the high prio task to finish and yield the
CPU, the second one is to not restart if softirq executes in a real-time
task. Usually, real-time tasks don't want to be interrupted, so implement
the second option.

Signed-off-by: Petr Malat <oss@...at.biz>
---
 kernel/softirq.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/softirq.c b/kernel/softirq.c
index c8a6913c067d..6a66d28bf020 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -478,7 +478,8 @@ asmlinkage __visible void do_softirq(void)
 
 /*
  * We restart softirq processing for at most MAX_SOFTIRQ_RESTART times,
- * but break the loop if need_resched() is set or after 2 ms.
+ * but break the loop after 2 ms or if need_resched() is set or if we
+ * execute in a real-time task.
  * The MAX_SOFTIRQ_TIME provides a nice upper bound in most cases, but in
  * certain cases, such as stop_machine(), jiffies may cease to
  * increment and so we need the MAX_SOFTIRQ_RESTART limit as
@@ -589,6 +590,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
 	pending = local_softirq_pending();
 	if (pending) {
 		if (time_before(jiffies, end) && !need_resched() &&
+		    (current->prio >= MAX_RT_PRIO || current == __this_cpu_read(ksoftirqd)) &&
 		    --max_restart)
 			goto restart;
 
-- 
2.30.2