linux-kernel - Re: [RFC PATCH 00/11] sched: CFS low-latency features

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Message-ID: <1282930072.1975.2962.camel@laptop>
Date:	Fri, 27 Aug 2010 19:27:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tony Lindgren <tony@...mide.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [RFC PATCH 00/11] sched: CFS low-latency features

On Fri, 2010-08-27 at 12:09 -0400, Mathieu Desnoyers wrote:
> > The specification also doesn't cover the case where the handler takes
> > more time to execute than the timer interval.
> 
> Why should it ? It seems valid for a workload to result in spawning many threads
> bound to more than a single core on a multi-core system. So concurrency
> management should be performed by the application. 

The problem with allowing concurrency is that the moment you want to do
that you get the spawner context and error propagation problems.

Thus we're limited to spawning a single thread at timer_create() and
handling expirations in there. At that point you have to specify what
happens to an expiration while the handler is running, will it queue
handlers or will you have to carefully craft your handler using
timer_getoverrun().

But I really don't understand why POSIX would provide this composite
functionality instead of providing the separate bits to implement this,
which I think is only SIGEV_THREAD_ID which is missing.

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