linux-kernel - Re: [RFC][PATCH 1/2] sched: Move idle_balance() to post

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Date:	Sat, 5 Jan 2013 15:14:44 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>
Subject: Re: [RFC][PATCH 1/2] sched: Move idle_balance() to post_schedule

2012/12/22 Steven Rostedt <rostedt@...dmis.org>:
> The idle_balance() code is called to do task load balancing just before
> going to idle. This makes sense as the CPU is about to sleep anyway.
> But currently it's called in the middle of the scheduler and in a place
> that must have interrupts disabled. That means, while the load balancing
> is going on, if a task wakes up on this CPU, it wont get to run while
> the interrupts are disabled. The idle task doing the balancing will be
> clueless about it.
>
> There's no real reason that the idle_balance() needs to be called in the
> middle of schedule anyway. The only benefit is that if a task is pulled
> to this CPU, it can be scheduled without the need to schedule the idle
> task. But load balancing and migrating the task makes a switch to idle
> and back negligible.

This cleanup looks nice as it does not only let us enable interrupts
there but also debloats a bit the schedule() code from idle specific
code. So it would be a pity if the optimization that goes away with
your cleanup has any measurable impact. Is there any sensible
benchmark that can be run against this patch? Something that may
involve a lot of back and forth to idle with some bunch of tasks
running around on other CPUs?
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