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

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Date:   Sat, 17 Feb 2018 11:50:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Frederic Weisbecker <frederic@...nel.org>
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 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.

Other than that this looks good!

Reviewed-by: Thomas Gleixner <tglx@...utronix.de>