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From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Cheng Chao <cs.os.kernel@gmail.com>,
	mingo@kernel.org, tj@kernel.org, akpm@linux-foundation.org,
	chris@chris-wilson.co.uk, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE
Date: Tue, 13 Sep 2016 18:14:27 +0200	[thread overview]
Message-ID: <20160913161427.GA32543@redhat.com> (raw)
In-Reply-To: <20160912150111.GC10121@twins.programming.kicks-ass.net>

On 09/12, Peter Zijlstra wrote:
>
> On Mon, Sep 12, 2016 at 03:05:38PM +0200, Oleg Nesterov wrote:
>
> > But this leads to the question which I wanted to ask many times.
> >
> > Why cond_resched() is not NOP if CONFIG_PREEMPT=y ?
>
> Dunno, nobody bothered to do it? We should keep the might_sleep() of
> course, but the preemption check is pointless.

Yes, agreed, I actually meant _cond_resched().

> > Perhaps we have some users like, just for example,
> > 
> > 	preempt_enable_no_resched();
> > 	cond_resched();
> > 
> > which actually want the should_resched() check even if CONFIG_PREEMPT,
> > but most callers do not?
>
> I would hope not, the few preempt_enable_no_resched() users _should_
> have an actual schedule() call in the _very_ near future.

Me too, and I failed to find something which could be broken... So
perhaps should make it nop and investigate the new bug reports after
that.


Hmm. And  preempt_enable_no_resched_notrace() under TASK_DEAD in
__schedule() should be removed it seems, do_exit() can call __schedule()
directly.

Oleg.

  reply	other threads:[~2016-09-13 16:15 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <OF73BCE1E4.D2224FEC-ON48258025.0022CFA7-48258025.0022E3DC@kedacom.com>
2016-09-09  8:13 ` [PATCH v2] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE Cheng Chao
2016-09-09  8:19   ` chengchao
2016-09-09 13:14   ` Oleg Nesterov
2016-09-09 16:24     ` Peter Zijlstra
2016-09-10  8:52   ` [PATCH v3] " Cheng Chao
2016-09-10  9:51     ` Cheng Chao
2016-09-10 16:33       ` Peter Zijlstra
2016-09-12 11:03     ` Oleg Nesterov
2016-09-13  2:45       ` Cheng Chao
2016-09-12 11:37     ` Peter Zijlstra
2016-09-12 11:41       ` Peter Zijlstra
2016-09-12 13:05         ` Oleg Nesterov
2016-09-12 15:01           ` Peter Zijlstra
2016-09-13 16:14             ` Oleg Nesterov [this message]
2016-09-13 16:37               ` Peter Zijlstra
2016-09-14  2:07                 ` Cheng Chao
2016-09-14  7:50                   ` Peter Zijlstra
2016-09-14 15:45                 ` Oleg Nesterov
2016-09-22 13:59                 ` [tip:sched/core] sched/core: Optimize __schedule() tip-bot for Peter Zijlstra
2016-09-13  4:03         ` [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT_NONE Cheng Chao
2016-09-13  8:45           ` Peter Zijlstra
2016-09-14  2:01     ` [PATCH v4] stop_machine: Avoid a sleep and wakeup in the stop_one_cpu() Cheng Chao
2016-09-14 15:53       ` Oleg Nesterov
2016-09-18  2:07       ` Cheng Chao
2016-09-22 13:59       ` [tip:sched/core] stop_machine: Avoid a sleep and wakeup in stop_one_cpu() tip-bot for Cheng Chao

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