linux-kernel - Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer

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Date:	Tue, 21 Apr 2015 23:32:23 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	viresh kumar <viresh.kumar@...aro.org>
cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linaro-kernel@...ts.linaro.org, linux-kernel@...r.kernel.org,
	Steven Miao <realmz6@...il.com>, shashim@...eaurora.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate
 running timer

On Fri, 17 Apr 2015, viresh kumar wrote:
> Does this look any better ?

Yes, but:
 
> 1.) Should the above list_empty(migration_list) block be added out of the
> 
> 	while (time_after_eq(jiffies, base->timer_jiffies))
> 
>     block ? So that we check it only once per timer interrupt.

That's what I suggested.
 
>     Also ->running_timer is set to the last serviced timer and a
>     del_timer_sync() might be waiting to remove it. But we continue with
>     the migration list first, without clearing it first. Not sure if this
>     is important at all..

Of course it is and doing it outside of the loop solves that issue.
 
> 2.) By the time we finish serving all pending timers, local CPU might not be
>     idle anymore OR the target CPU may become idle.

That's another good reason to move that migration list outside of the
while loop.
 
> 2.) It might be better to update preferred_target every time we choose a
> difference base. This may help us avoid calling get_nohz_timer_target()
> in the second if block above.

Yuck no. You can do that in that migration code and not add more
pointless crap to the normal case.

Are you realizing that __mod_timer() is a massive hotpath for network
heavy workloads?

This stuff needs to be debloated and not mindlessly packed with more
corner case features.

Thanks,

	tglx
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