linux-kernel - Re: [PATCH 4/8] sched/core: Skip wakeup when task is already running.

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Message-ID: <20140422135339.55a7b799@gandalf.local.home>
Date:	Tue, 22 Apr 2014 13:53:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	bsegall@...gle.com
Cc:	Dongsheng Yang <yangds.fnst@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>, <fweisbec@...il.com>,
	<mingo@...hat.com>, <acme@...stprotocols.net>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/8] sched/core: Skip wakeup when task is already
 running.

On Tue, 22 Apr 2014 10:10:52 -0700
bsegall@...gle.com wrote:


> This is all expected behavior, and the somewhat less than useful trace
> events are expected. A task setting p->state to TASK_RUNNING without
> locks is fine if and only p == current. The standard deschedule loop is

Sure, and if you are not current, then all you need is the rq lock of
the rq that p's CPU is for.

> basically:
> 
> while (1) {
>   set_current_state(TASK_(UN)INTERRUPTIBLE);

Yep, and set_current_state() implies a memory barrier.

>   if (should_still_sleep)
>     schedule();
> }
> set_current_state(TASK_RUNNING);

The above can use __set_current_state() as there's no races to deal
with when setting current's state to RUNNING.

> 
> Which can produce this in a race.
> 
> The only problem this causes is a wasted check_preempt_curr call in the
> racing case, and a somewhat inaccurate sched:sched_wakeup trace event.
> Note that even if you did recheck in ttwu_do_wakeup you could still race
> and get an "inaccurate" trace event. Heck, even if the ttwu is
> _necessary_ because p is currently trying to take rq->lock to
> deschedule, you won't get a matching sched_switch event, because the
> ttwu is running before schedule is.
> 
> You could sorta fix this I guess by tracking every write to p->state
> with trace events, but that would be a somewhat different change, and
> might be considered too expensive for all I know (and the trace events
> could /still/ be resolved in a different order across cpus compared to
> p->state's memory).

Yeah, let's not do that.

-- Steve

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