linux-kernel - Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

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Message-ID: <87jznh3o10.fsf@somnus>
Date: Tue, 06 Feb 2024 11:06:19 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Eric Dumazet <edumazet@...gle.com>, "Rafael J . Wysocki"
 <rafael.j.wysocki@...el.com>, Arjan van de Ven <arjan@...radead.org>,
 "Paul E . McKenney" <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>,
 Steven Rostedt <rostedt@...dmis.org>, Sebastian Siewior
 <bigeasy@...utronix.de>, Giovanni Gherdovich <ggherdovich@...e.cz>, Lukasz
 Luba <lukasz.luba@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Srinivas Pandruvada <srinivas.pandruvada@...el.com>, K Prateek Nayak
 <kprateek.nayak@....com>
Subject: Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model

Frederic Weisbecker <frederic@...nel.org> writes:

> Le Mon, Feb 05, 2024 at 02:29:34PM +0100, Anna-Maria Behnsen a écrit :
>> Anna-Maria Behnsen <anna-maria@...utronix.de> writes:
>> 
>> > Frederic Weisbecker <frederic@...nel.org> writes:
>> >
>> >> Le Thu, Feb 01, 2024 at 05:15:37PM +0100, Anna-Maria Behnsen a écrit :
>> >>> Frederic Weisbecker <frederic@...nel.org> writes:
>> >>> 
>> >>> > Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit :
>> >>> > Heh, I was about to say that it's impossible that timer_base_is_idle()
>> >>> > at this stage but actually if we run in nohz_full...
>> >>> >
>> >>> > It happens so that nohz_full is deactivated until rcutree_online_cpu()
>> >>> > which calls tick_dep_clear() but it's a pure coincidence that might
>> >>> > disappear one day. So yes, let's keep it that way.
>> >>> 
>> >>> I instrumented the code (with NOHZ FULL and NOHZ_IDLE) to make sure the
>> >>> timer migration hierarchy state 'idle' is in sync with the timer base
>> >>> 'idle'. And this was one part where it was possible that it runs out of
>> >>> sync as I remember correctly. But if I understood you correctly, this
>> >>> shouldn't happen at the moment?
>> >>
>> >> Well, it's not supposed to :-)
>> >
>> > Hmm, let me double check this and run the tests on the instrumented
>> > version...
>> 
>> I added a prinkt() to verify what I think I remember. I was able to see
>> the prints. So it seems, that the coincidence that nohz_full is
>> deactivated until rcutree_online_cpu() already disappeared.
>
> Nice, then I guess it can become a WARN_ON.

Either I misunderstood something, or wasn't able to explain what I
wanted to say.

I understood, that nohz full is disabled (by coincidence) until
rcutree_online_cpu() which comes after the timer migration CPU hotplug
AP. This means, that the check whether timer base is idle or not,
shouldn't be required in tmigr_cpu_online() to keep cpu idle or mark it
active in the hierarchy. But we could keep it in case coincidence
disappears. No?

So I added a printk() when timer base is idle in tmigr_cpu_online(). And
I was able to see the prints. This means, nohz full is _not_ disabled
when executing tmigr_cpu_online(), or am I wrong?

So when I replace the printk() with a WARN_ON() it will definitely
trigger. So I'm not sure if this is what you want to have :)

Thanks,

        Anna-Maria