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Date:	Sat, 26 Jul 2014 10:58:57 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Sergey Oboguev <oboguev.public@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH RFC] sched: deferred set priority (dprio)

On Fri, 2014-07-25 at 12:45 -0700, Sergey Oboguev wrote: 
> [This is a repost of the message from few day ago, with patch file
> inline instead of being pointed by the URL.]
> 
> This patch is intended to improve the support for fine-grain parallel
> applications that may sometimes need to change the priority of their threads at
> a very high rate, hundreds or even thousands of times per scheduling timeslice.
> 
> These are typically applications that have to execute short or very short
> lock-holding critical or otherwise time-urgent sections of code at a very high
> frequency and need to protect these sections with "set priority" system calls,
> one "set priority" call to elevate current thread priority before entering the
> critical or time-urgent section, followed by another call to downgrade thread
> priority at the completion of the section. Due to the high frequency of
> entering and leaving critical or time-urgent sections, the cost of these "set
> priority" system calls may raise to a noticeable part of an application's
> overall expended CPU time. Proposed "deferred set priority" facility allows to
> largely eliminate the cost of these system calls.

So you essentially want to ship preempt_disable() off to userspace?

(smiles wickedly, adds CCs)

-Mike

> Instead of executing a system call to elevate its thread priority, an
> application simply writes its desired priority level to a designated memory
> location in the userspace. When the kernel attempts to preempt the thread...

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