linux-kernel - Re: [PATCH 3/5] tick/broadcast: Prefer per-cpu oneshot wakeup timers to broadcast

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Message-ID: <20210521112503.GA11850@willie-the-truck>
Date:   Fri, 21 May 2021 12:25:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Mika Penttilä <mika.penttila@...tfour.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        Lorenzo Colitti <lorenzo@...gle.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>, kernel-team@...roid.com
Subject: Re: [PATCH 3/5] tick/broadcast: Prefer per-cpu oneshot wakeup timers
 to broadcast

On Fri, May 21, 2021 at 05:25:41AM +0300, Mika Penttilä wrote:
> On 20.5.2021 21.47, Will Deacon wrote:
> >   /*
> >    * Conditionally install/replace broadcast device
> >    */
> > -void tick_install_broadcast_device(struct clock_event_device *dev)
> > +void tick_install_broadcast_device(struct clock_event_device *dev, int cpu)
> >   {
> >   	struct clock_event_device *cur = tick_broadcast_device.evtdev;
> > +	if (tick_set_oneshot_wakeup_device(dev, cpu))
> > +		return;
> > +
> >   	if (!tick_check_broadcast_device(cur, dev))
> >   		return;
> 
> Does this disable hpet registering as a global broadcast device on x86 ? I
> think it starts with cpumask = cpu0 so it qualifies for a percpu wakeup
> timer.

Well spotted, I think you're probably right. I'll try to reproduce on my
laptop to confirm, but I hadn't noticed the tricks played with the cpumask
on x86.

I'll probably need to rework things so that we install the broadcast timer
first, but prefer global devices.

Will