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:

quoted

On 20.5.2021 21.47, Will Deacon wrote:

quoted

  /*
   * 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

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