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

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Message-ID: <20150422152940.GC3007@worktop.Skamania.guest>
Date:	Wed, 22 Apr 2015 17:29:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	viresh kumar <viresh.kumar@...aro.org>,
	Ingo Molnar <mingo@...hat.com>, 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 Tue, Apr 21, 2015 at 02:54:55PM -0700, Eric Dumazet wrote:
> On Tue, 2015-04-21 at 23:32 +0200, Thomas Gleixner wrote:
> 
> > 
> > Are you realizing that __mod_timer() is a massive hotpath for network
> > heavy workloads?
> 
> BTW I was considering using mod_timer_pinned() from these networking
> timers (ie sk_reset_timer())
> 
> get_nohz_timer_target() sounds cool for laptop users, but is one cause
> for bad responses to DDOS, when the selected cpu gets stressed.
> 
> This is the reason I used mod_timer_pinned() in commit 789f558cfb3680ae
> ("tcp/dccp: get rid of central timewait timer")

Hmm, that sounds unfortunate, this would wreck life for the power aware
laptop/tablet etc.. people.

There is already a sysctl to manage this, is that not enough to mitigate
this problem on the server side of things?
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