Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 19 Apr 2021 11:56:30 +0100
From:   Vincent Donnefort <vincent.donnefort@arm.com>
To:     Valentin Schneider <valentin.schneider@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>, tglx@linutronix.de,
        mingo@kernel.org, bigeasy@linutronix.de, swood@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, qais.yousef@arm.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs
 hotplug-rollback
Message-ID: <20210419105541.GA40111@e120877-lin.cambridge.arm.com>
References: <20210310145258.899619710@infradead.org>
 <20210310150109.259726371@infradead.org>
 <871rclu3jz.mognet@e113632-lin.i-did-not-set--mail-host-address--so-tickle-me>
 <YHQ3Iy7QfL+0UoM0@hirez.programming.kicks-ass.net>
 <87r1jfmn8d.mognet@arm.com>
 <YHU/a9HvGLYpOLKZ@hirez.programming.kicks-ass.net>
 <YHgAYef83VQhKdC2@hirez.programming.kicks-ass.net>
 <87a6pzmxec.mognet@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87a6pzmxec.mognet@arm.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Precedence: bulk

On Thu, Apr 15, 2021 at 03:32:11PM +0100, Valentin Schneider wrote:
> On 15/04/21 10:59, Peter Zijlstra wrote:
> > Can't make sense of what I did.. I've removed that hunk. Patch now looks
> > like this.
> >
> 
> Small nit below, but regardless feel free to apply to the whole lot:
> Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
> 
> @VincentD, ISTR you had tested the initial version of this with your fancy
> shmancy hotplug rollback stresser. Feel like doing this

I indeed wrote a test to verify all the rollback cases, up and down.

It seems I encounter an intermitent issue while running several iterations of
that test ... but I need more time to debug and figure-out where it is blocking.

> 
> > So instead, make sure balance_push is enabled between
> > sched_cpu_deactivate() and sched_cpu_activate() (eg. when
> > !cpu_active()), and gate it's utility with cpu_dying().
> 
> I'd word that "is enabled below sched_cpu_activate()", since
> sched_cpu_deactivate() is now out of the picture.
> 
> [...]
> > @@ -7639,6 +7639,9 @@ static DEFINE_PER_CPU(struct cpu_stop_wo
> >
> >  /*
> >   * Ensure we only run per-cpu kthreads once the CPU goes !active.
> > + *
> > + * This is active/set between sched_cpu_deactivate() / sched_cpu_activate().
> 
> Ditto
> 
> > + * But only effective when the hotplug motion is down.
> >   */
> >  static void balance_push(struct rq *rq)
> >  {