Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Date:   Wed, 27 Apr 2022 12:50:20 +0100
From:   Aaron Tomlin <atomlin@redhat.com>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Christoph Lameter <cl@gentwo.de>, frederic@kernel.org,
        tglx@linutronix.de, mingo@kernel.org, pauld@redhat.com,
        neelx@redhat.com, oleksandr@natalenko.name,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH v3] tick/sched: Ensure quiet_vmstat() is called when
 the idle tick was stopped too
Message-ID: <20220427115020.kyaxc5j67lq5zrfq@ava.usersys.com>
References: <20220422193647.3808657-1-atomlin@redhat.com>
 <alpine.DEB.2.22.394.2204250919400.2367@gentwo.de>
 <20220425113909.u3smtztp66svlw4o@ava.usersys.com>
 <alpine.DEB.2.22.394.2204251406370.13839@gentwo.de>
 <20220425132700.GK2731@worktop.programming.kicks-ass.net>
 <20220425141717.vw2jfnn3zp6c5ib2@ava.usersys.com>
 <Ymb018EaVlOUfx87@fuller.cnet>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <Ymb018EaVlOUfx87@fuller.cnet>
Precedence: bulk

On Mon 2022-04-25 16:21 -0300, Marcelo Tosatti wrote:
> > Hi Peter, Christoph,
> > 
> > Indeed. Which was why I decided, initially, against the general-purpose
> > case/or approach. Personally, I would prefer to keep this somewhat
> > restrictive to nohz.
> 
> Is there anything that prevents a nohz full CPU from running an
> application with short and frequent idling?

Hi Marcelo,

I'm not sure I understand the question; albeit, if I understand correctly,
yes: the scheduling-clock tick, if it was stopped.
Yet I believe this behaviour is correct. Consider the following example:

  When a CFS task is moved/or migrated to a nohz_full CPU that was
  previously idle and had its tick stopped, if its the only task on the
  run-queue then it is possible that the idle task may not restart the
  tick (see __tick_nohz_full_update_tick()).  Thus once the CFS task exits
  manual intervention i.e. a reschedule IPI to wake the idle task, would be
  required to run again, on the same CPU.


Kind regards,

-- 
Aaron Tomlin