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 10:45:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Cheng Chao <cs.os.kernel@...il.com>
Cc:     mingo@...nel.org, oleg@...hat.com, 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 Tue, Sep 13, 2016 at 12:03:05PM +0800, Cheng Chao wrote:
> 
> Peter, Is it as a new patch?

I wanted both changes in one pathc, but I fudged my git-diff.

> > ---
> >  kernel/stop_machine.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
> > index ae6f41fb9cba..637798d6b554 100644
> > --- a/kernel/stop_machine.c
> > +++ b/kernel/stop_machine.c
> > @@ -121,6 +121,11 @@ int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
> >  	cpu_stop_init_done(&done, 1);
> >  	if (!cpu_stop_queue_work(cpu, &work))
> >  		return -ENOENT;
> > +	/*
> > +	 * In case @cpu == smp_proccessor_id() we can avoid a sleep+wakeup
> > +	 * by doing a preemption.
> > +	 */
> > +	cond_resched();
> >  	wait_for_completion(&done.completion);
> >  	return done.ret;
> >  }
> > 
> 
> I agree to use cond_resched(). https://lkml.org/lkml/2016/9/12/1228
> 
> so we really don't need "if defined(CONFIG_PREEMPT_NONE)"?

> I also think without the "if defined(CONFIG_PREEMPT_NONE)", 
> the code is more clean,and the logic is still ok.

You really don't need the #ifdef, look at the actual code, not the
Kconfig language.