linux-kernel - Re: sched_yield() call on Linux Kernel 2.6.39 is not behaving correct

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Message-ID: <1420694794.5279.33.camel@marge.simpson.net>
Date:	Thu, 08 Jan 2015 06:26:34 +0100
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Yogesh Ahire <yogesh02061983@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: sched_yield() call on Linux Kernel 2.6.39 is not behaving
 correct

On Wed, 2015-01-07 at 16:30 -0500, Yogesh Ahire wrote: 
> Hi All,
> 
> I have a system with multiple CPU cores.  I have multiple threads
> assigned to particular CPU. Among these threads the main thread calls
> sched_yield() if it has nothing to do, I am hoping that doing so will
> give chance to other threads to run. But the strange behavior of
> sched_yield() is , even if there are ready-to-runs tasks on this CPU
> waiting for their turn, the task which calls sched_yield() is always
> running ( get scheduled) and not giving chance to any other task to
> run. It is consuming 100% of CPU. Is sched_yield() is broken on 2.6
> Kernel?

Nope, your expectation is likely busted.  sched_yield() for a fair class
task is merely a resched check in CFS.  IFF there's a runnable task
that's more deserving by the CFS definition thereof, it'll initiate a
context switch, otherwise it's a non-free noop.  For realtime class
tasks, behavior is predictable, the scheduler WILL initiate a context
switch IFF there is a runnable task of >= yielding tasks priority on the
same CPU.  If you depend on a context switch happening in any other
circumstance, you're gonna be seriously disappointed.

-Mike

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