linux-kernel - Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match

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Date:   Sun, 4 Sep 2022 12:44:36 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, mgorman@...e.de,
        ebiederm@...ssion.com, bigeasy@...utronix.de,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 3/6] sched: Change wait_task_inactive()s match_state


* Peter Zijlstra <peterz@...radead.org> wrote:

> Make wait_task_inactive()'s @match_state work like ttwu()'s @state.
> 
> That is, instead of an equal comparison, use it as a mask. This allows
> matching multiple block conditions.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/sched/core.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3295,7 +3295,7 @@ unsigned long wait_task_inactive(struct
>  		 * is actually now running somewhere else!
>  		 */
>  		while (task_running(rq, p)) {
> -			if (match_state && unlikely(READ_ONCE(p->__state) != match_state))
> +			if (match_state && !(READ_ONCE(p->__state) & match_state))
>  				return 0;

We lose the unlikely annotation there - but I guess it probably never 
really mattered anyway?

Suggestion #1:

- Shouldn't we rename task_running() to something like task_on_cpu()? The 
  task_running() primitive is similar to TASK_RUNNING but is not based off 
  any TASK_FLAGS.

Suggestion #2:

- Shouldn't we eventually standardize on task->on_cpu on UP kernels too? 
  They don't really matter anymore, and doing so removes #ifdefs and makes 
  the code easier to read.


>  			cpu_relax();
>  		}
> @@ -3310,7 +3310,7 @@ unsigned long wait_task_inactive(struct
>  		running = task_running(rq, p);
>  		queued = task_on_rq_queued(p);
>  		ncsw = 0;
> -		if (!match_state || READ_ONCE(p->__state) == match_state)
> +		if (!match_state || (READ_ONCE(p->__state) & match_state))
>  			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
>  		task_rq_unlock(rq, p, &rf);

Suggestion #3:

- Couldn't the following users with a 0 mask:

    drivers/powercap/idle_inject.c:         wait_task_inactive(iit->tsk, 0);
    fs/coredump.c:                  wait_task_inactive(ptr->task, 0);

  Use ~0 instead (exposed as TASK_ANY or so) and then we can drop the
  !match_state special case?

  They'd do something like:

    drivers/powercap/idle_inject.c:         wait_task_inactive(iit->tsk, TASK_ANY);
    fs/coredump.c:                  wait_task_inactive(ptr->task, TASK_ANY);

  It's not an entirely 100% equivalent transformation though, but looks OK 
  at first sight: ->__state will be some nonzero mask for genuine tasks 
  waiting to schedule out, so any match will be functionally the same as a 
  0 flag telling us not to check any of the bits, right? I might be missing 
  something though.

Thanks,

	Ingo