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

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Message-ID: <20160912150111.GC10121@twins.programming.kicks-ass.net>
Date:   Mon, 12 Sep 2016 17:01:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 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.

> 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.