linux-kernel - Re: [RFC][PATCH 1/7] sched: Fix smp_call_function_single

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Date:   Wed, 27 May 2020 13:28:16 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, Qian Cai <cai@....pw>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [RFC][PATCH 1/7] sched: Fix smp_call_function_single_async()
 usage for ILB

On Wed, May 27, 2020 at 12:23:23PM +0200, Vincent Guittot wrote:
> > -static void nohz_csd_func(void *info)
> > -{
> > -       struct rq *rq = info;
> > +       flags = atomic_fetch_andnot(NOHZ_KICK_MASK, nohz_flags(cpu));
> 
> Why can't this be done in nohz_idle_balance() instead ?
> 
> you are not using flags in nohz_csd_func() and SCHED_SOFTIRQ which
> calls nohz_idle_balance(), happens after nohz_csd_func(), isn't it ?
> 
> In this case, you don't have to use the intermediate variable
> this_rq->nohz_idle_balance

That's in fact to fix the original issue. The softirq was clearing
the nohz_flags but the softirq could be issued from two sources:
the tick and the IPI. And the tick source softirq could then clear
the flags set from the IPI sender before the IPI itself, resulting
in races such as described there: https://lore.kernel.org/lkml/20200521004035.GA15455@lenoir/

Thanks.