linux-kernel - Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful works for CONFIG_PREEMPT

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Date:   Tue, 13 Sep 2016 18:14:27 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Cheng Chao <cs.os.kernel@...il.com>, mingo@...nel.org,
        tj@...nel.org, akpm@...ux-foundation.org, chris@...is-wilson.co.uk,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] stop_machine: Make migration_cpu_stop() does useful
        works for CONFIG_PREEMPT_NONE

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.