linux-kernel - Re: sched/isolation: tick_take_do_timer_from_boot() calls smp_call_function

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Date: Sun, 26 May 2024 22:52:30 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Nicholas Piggin <npiggin@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, Phil Auld <pauld@...hat.com>,
	Chris von Recklinghausen <crecklin@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: sched/isolation: tick_take_do_timer_from_boot() calls
 smp_call_function_single() with irqs disabled

Le Sun, May 26, 2024 at 09:27:58PM +0200, Oleg Nesterov a écrit :
> > So up to this point the tick is never stopped neither on housekeeping
> > nor on NOHZ FULL CPUs:
> >
> > tick_nohz_full_update_tick()
> >   if (!tick_sched_flag_test(ts, TS_FLAG_NOHZ))
> >     return;
> 
> OK... But tick_nohz_idle_update_tick() doesn't check TS_FLAG_NOHZ and
> the tick_nohz_full_cpu() check can't help at boot time.

Yes but tick_nohz_idle_update_tick() is only called when the tick is already
stopped. And for the tick to be already stopped, TS_FLAG_NOHZ must have been
set.

> And I still don't understand why we can rely on can_stop_idle_tick() even
> in tick_nohz_idle_stop_tick().

Not sure I follow you on this one...

> 
> I'll try to read this code again tomorrow, but it will never fit my poor
> little brain ;)

You understood more than I ever did in just a few hours :-)

But yes the tick code was much more simple before I put my own hands
inside. nohz_full and cpuidle did not arrange it.

Thanks.