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

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Date:	Fri, 27 Aug 2010 17:41:26 +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 11:21 -0400, Mathieu Desnoyers wrote:
>        SIGEV_THREAD
>               Upon  timer  expiration,  invoke  sigev_notify_function as if it
>               were the start function of a new thread.  (Among the implementa‐
>               tion  possibilities  here are that each timer notification could
>               result in the creation of a new thread, or that a single  thread
>               is  created  to  receive  all  notifications.)   The function is
>               invoked   with   sigev_value   as   its   sole   argument.    If
>               sigev_notify_attributes  is  not  NULL,  it  should  point  to a
>               pthread_attr_t structure that defines  attributes  for  the  new
>               thread (see pthread_attr_init(3).
> 
> So basically, it's the glibc implementation that is broken, not the standard.

The standard is broken too, what context will the new thread inherit?
The pthread_attr_t stuff tries to cover some of that, but pthread_attr_t
doesn't cover all inherited task attributes, and allows for some very
'interesting' bugs [1].

The specification also doesn't cover the case where the handler takes
more time to execute than the timer interval.

[1] - consider the case where pthread_attr_t includes the stack and we
use a spawn thread on expire policy and then run into the situation
where the handler is delayed past the next expiration.
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