Re: [RFT][PATCH v2 0/3] cpuidle: teo: Do not check timers unconditionally every time

From: Doug Smythies <dsmythies@telus.net>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Linux PM <linux-pm@vger.kernel.org>,
	Kajetan Puchalski <kajetan.puchalski@arm.com>,
	Doug Smythies <dsmythies@telus.net>
Subject: Re: [RFT][PATCH v2 0/3] cpuidle: teo: Do not check timers unconditionally every time
Date: Wed, 9 Aug 2023 17:43:51 -0700	[thread overview]
Message-ID: <CAAYoRsXZsbh0o5fTDjjvTN7bS2EQOY9XWwnM8_MGTiwkAS8pYQ@mail.gmail.com> (raw)
In-Reply-To: <4872c41b-c8fd-1f84-7940-d4944c667e6f@linutronix.de>

On Wed, Aug 9, 2023 at 9:24 AM Anna-Maria Behnsen
<anna-maria@linutronix.de> wrote:
> On Tue, 8 Aug 2023, Doug Smythies wrote:
> > On Tue, Aug 8, 2023 at 9:43 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > > On Tue, Aug 8, 2023 at 5:22 PM Doug Smythies <dsmythies@telus.net> wrote:
>
> [...]
>
> > > > Conclusions: Overall, I am not seeing a compelling reason to
> > > > proceed with this patch set.
> > >
> > > On the other hand, if there is a separate compelling reason to do
> > > that, it doesn't appear to lead to a major regression.
> >
> > Agreed.
> >
>
> Regarding the compelling reason:
>
> On a fully loaded machine with 256 CPUs tick_nohz_next_event() is executed
> ~48000 times per second. With this patchset it is reduced to ~120 times per
> second. The factor for the difference is 400. This is already an
> improvement.
>
> tick_nohz_next_event() marks timer bases idle, whenever possible - even if
> the tick is not stopped afterwards. When a timer is enqueued remote into an
> idle timer base an IPI is sent. Calling tick_nohz_next_event() only when
> the system is not that busy, prevents those unnecessary IPIs.
>
> Beside of those facts, I'm working on the timer pull model [0]. With this,
> non pinned timers can also be expired by other CPUs and do not prevent CPUs
> from going idle. Those timers will be enqueued on the local CPU without any
> heuristics. This helps to improve behavior when a system is idle (regarding
> power). But the call of tick_nohz_next_event() will be more expensive which
> led to a regression during testing. This regression is gone with the new
> teo implementation - it seems that there is also an improvement under
> load. I do not have finalized numbers, as it is still WIP (I came across
> some other possible optimizations during analyzing the regression, which
> I'm evaluating at the moment).
>
> Thanks,
>
>         Anna-Maria
>
>
> [0] https://lore.kernel.org/lkml/20230524070629.6377-1-anna-maria@linutronix.de/

Thank you for the context and the link.

... Doug