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

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Message-ID: <87tumw9oy4.ffs@nanos.tec.linutronix.de>
Date:   Fri, 21 May 2021 15:49:39 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Will Deacon <will@...nel.org>,
        Mika Penttilä <mika.penttila@...tfour.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>,
        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 12:25, Will Deacon wrote:
> 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.

HPET has cpumask(0) but does not have CLOCK_EVT_FEAT_PERCPU set. The
feature flag is a clear indicator for per cpu.

Thanks,

        tglx