linux-kernel - Re: [PATCH v4 20/22] rv: Add rtapp

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Message-ID: <84a584isvo.fsf@jogness.linutronix.de>
Date: Fri, 25 Apr 2025 09:54:27 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Nam Cao <namcao@...utronix.de>, Gabriele Monaco <gmonaco@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
 linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 20/22] rv: Add rtapp_sleep monitor

On 2025-04-25, John Ogness <john.ogness@...utronix.de> wrote:
>>> perf stat -e rv:error_sleep stress-ng --cpu-sched 1 -t 10s
>>>   -- shows several errors --
>>
>> This one is a monitor's bug.
>>
>> The monitor mistakenly sees the task getting woken up, *then* sees it going
>> to sleep.
>>
>> This is due to trace_sched_switch() being called with a stale 'prev_state'.
>> 'prev_state' is read at the beginning of __schedule(), but
>> trace_sched_switch() is invoked a bit later. Therefore if task->__state is
>> changed inbetween, 'prev_state' is not the value of task->__state.
>>
>> The monitor checks (prev_state & TASK_INTERRUPTIBLE) to determine if the
>> task is going to sleep. This can be incorrect due to the race above. The
>> monitor sees the task going to sleep, but actually it is just preempted.
>
> If I understand this correctly, trace_sched_switch() is reporting
> accurate state transition information, but by the time it is reported
> that state may have already changed (in which case another
> trace_sched_switch() occurs later).
>
> So in this example, the task did go to sleep. Why do you think it was
> preempted instead?

On 2025-04-25, Gabriele Monaco <gmonaco@...hat.com> wrote:
> Peter's fix [1] landed on next recently, I guess in a couple of days
> you'll get it on the upstream tree and you may not see the problem.

Ah, thanks for pointing that out!

> [1] - https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=8feb053d53194382fcfb68231296fdc220497ea6