linux-kernel - Re: [PATCH 4/6] sched/isolation: Offload residual 1Hz scheduler tick

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Date:   Tue, 20 Feb 2018 16:44:07 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 4/6] sched/isolation: Offload residual 1Hz scheduler tick

On Sat, Feb 17, 2018 at 11:50:52AM +0100, Thomas Gleixner wrote:
> On Thu, 15 Feb 2018, Frederic Weisbecker wrote:
> 
> > When a CPU runs in full dynticks mode, a 1Hz tick remains in order to
> > keep the scheduler stats alive. However this residual tick is a burden
> > for bare metal tasks that can't stand any interruption at all, or want
> > to minimize them.
> > 
> > The usual boot parameters "nohz_full=" or "isolcpus=nohz" will now
> > outsource these scheduler ticks to the global workqueue so that a
> > housekeeping CPU handles those remotely. The sched_class::task_tick()
> > implementations have been audited and look safe to be called remotely
> > as the target runqueue and its current task are passed in parameter
> > and don't seem to be accessed locally.
> 
> That scares me a bit. Not for the current state of affairs, but we want to
> ensure that this still works in 2 years from now
> 
> So at least you want to add a comment to task_tick() which explains the
> constraints which come with the remote tick.

Good point, I'm adding that.

> 
> Other than that this looks good!
> 
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Thanks!