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From: Eric Dumazet <eric.dumazet@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
	viresh kumar <viresh.kumar@linaro.org>,
	Ingo Molnar <mingo@redhat.com>,
	linaro-kernel@lists.linaro.org, linux-kernel@vger.kernel.org,
	Steven Miao <realmz6@gmail.com>,
	shashim@codeaurora.org
Subject: Re: [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer
Date: Wed, 22 Apr 2015 12:59:03 -0700	[thread overview]
Message-ID: <1429732743.18561.134.camel@edumazet-glaptop2.roam.corp.google.com> (raw)
In-Reply-To: <alpine.DEB.2.11.1504222021240.13914@nanos>

On Wed, 2015-04-22 at 20:56 +0200, Thomas Gleixner wrote:
> On Wed, 22 Apr 2015, Eric Dumazet wrote:
> > Check commit 4a8e320c929991c9480 ("net: sched: use pinned timers")
> > for a specific example of the problems that can be raised.
> 
> If you have a problem with the core timer code then it should be fixed
> there and not worked around in some place which will ruin stuff for
> power saving interested users. I'm so tired of this 'I fix it in my
> sandbox' attitude, really. If the core code has a shortcoming we fix
> it there right away because you are probably not the only one who runs
> into that shortcoming. So if we don't fix it in the core we end up
> with a metric ton of slightly different (or broken) workarounds which
> affect the workload/system characteristics of other people.
> 
> Just for the record. Even the changelog of this commit is blatantly
> wrong:
> 
>   "We can see that timers get migrated into a single cpu, presumably
>    idle at the time timers are set up."

Spare me the obvious typo. A 'not' is missing.

> 
> The timer migration moves timers to non idle cpus to leave the idle
> ones alone for power saving sake.
> 
> I can see and understand the reason why you want to avoid that, but I
> have to ask the question whether this pinning is the correct behaviour
> under all workloads and system characteristics. If yes, then the patch
> is the right answer, if no, then it is simply the wrong approach.
> 
> > but /proc/sys/kernel/timer_migration adds a fair overhead in many
> > workloads.
> > 
> > get_nohz_timer_target() has to touch 3 cache lines per cpu...
> 
> And this is something we can fix and completely avoid if we think
> about it. Looking at the code I have to admit that the out of line
> call and the sysctl variable lookup is silly. But its not rocket
> science to fix this.

Awesome.



  reply	other threads:[~2015-04-22 19:59 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-31  6:55 [PATCH 0/2] timer: Migrate running timers Viresh Kumar
2015-03-31  6:55 ` [PATCH 1/2] timer: Avoid waking up an idle-core by migrate running timer Viresh Kumar
2015-03-31 14:53   ` Peter Zijlstra
2015-04-14 23:13   ` Thomas Gleixner
2015-04-17  8:12     ` viresh kumar
2015-04-17  8:32       ` Ingo Molnar
2015-04-21 21:32       ` Thomas Gleixner
2015-04-21 21:54         ` Eric Dumazet
2015-04-22 15:29           ` Peter Zijlstra
2015-04-22 16:02             ` Eric Dumazet
2015-04-22 18:56               ` Thomas Gleixner
2015-04-22 19:59                 ` Eric Dumazet [this message]
2015-04-22 21:56                   ` Thomas Gleixner
2015-04-23  6:57                     ` Eric Dumazet
2015-04-23 12:45                       ` Thomas Gleixner
2015-04-25 18:37                         ` Eric Dumazet
2015-05-05 13:00                           ` Thomas Gleixner
2015-05-06 16:33                             ` Eric Dumazet
2015-04-15 15:54   ` Thomas Gleixner
2015-03-31  6:55 ` [PATCH 2/2] timer: Replace base-> 'running_timer' with 'busy' Viresh Kumar
2015-03-31 15:01 ` [PATCH 0/2] timer: Migrate running timers Viresh Kumar

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