linux-kernel - Re: [PATCH] sched: next buddy hint on sleep and preempt path

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Message-ID: <1299061903.1310.1.camel@laptop>
Date:	Wed, 02 Mar 2011 11:31:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Venkatesh Pallipadi <venki@...gle.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched: next buddy hint on sleep and preempt path

On Tue, 2011-03-01 at 15:33 -0800, Venkatesh Pallipadi wrote:
> When a task in a taskgroup sleeps, pick_next_task starts all the way back at
> the root and picks the task/taskgroup with the min vruntime across all
> runnable tasks. But, when there are many frequently sleeping tasks
> across different taskgroups, it makes better sense to stay with same taskgroup
> for its slice period (or until all tasks in the taskgroup sleeps) instead of
> switching cross taskgroup on each sleep after a short runtime.
> This helps specifically where taskgroups corresponds to a process with
> multiple threads. The change reduces the number of CR3 switches in this case.

I wasn't expecting this approach to this problem, and was dreading a
pick_next_task() rewrite, but aside from all the mentioned problems it
does look quite nice :-)

It doesn't avoid iterating the whole hierarchy every schedule, but like
you say, it should avoid the expensive cr3 switches.

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