From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: yield() in i2c non-happy paths hits BUG under -rt patch Date: Thu, 19 Nov 2009 15:15:54 +0100 Message-ID: <1258640154.3931.407.camel@laptop> References: <20091107210147.3e754278@hyperion.delvare> <4AF7148C.9090706@thebigcorporation.com> <20091112211255.09cd884a@hyperion.delvare> <20091116155606.GC29479@sirena.org.uk> <20091118010520.4cd397d4@lxorguk.ukuu.org.uk> <20091118175202.490989d8@hyperion.delvare> <20091119130526.23a69b85@hyperion.delvare> <20091119125906.6ad00edd@lxorguk.ukuu.org.uk> <1258636014.4372.328.camel@twins> <20091119150008.6e757c26@hyperion.delvare> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: Alan Cox , Thomas Gleixner , Leon Woestenberg , Mark Brown , Sven-Thorsten Dietrich , linux-i2c@vger.kernel.org, rt-users , "Ben Dooks (embedded platforms)" , LKML To: Jean Delvare Return-path: Received: from casper.infradead.org ([85.118.1.10]:34658 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752678AbZKSOQI (ORCPT ); Thu, 19 Nov 2009 09:16:08 -0500 In-Reply-To: <20091119150008.6e757c26@hyperion.delvare> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Thu, 2009-11-19 at 15:00 +0100, Jean Delvare wrote: > > cond_resched(); > > Are you saying that most calls to yield() should be replaced with calls > to cond_resched()? No, depends on the reason yield() is used. Some cases can be replaced by locking constructs, such as a condition variable. > I admit I a little skeptical. While the description of cond_resched() > ("latency reduction via explicit rescheduling in places that are safe") > sounds promising, following the calls leads me to: > > static inline int need_resched(void) > { > return unlikely(test_thread_flag(TIF_NEED_RESCHED)); > } > > So apparently the condition for need_resched() to do anything is > considered unlikely... suggesting that cond_resched() is a no-op in > most cases? I don't quite get the point of moving away from sched() > because it is a no-op, if we end up with a no-op under a different name. TIF_NEED_RESCHED gets set by the scheduler whenever it decides current needs to get preempted, its unlikely() because that reduces the code impact of cond_resched() and similar in the case we don't schedule, if we do schedule() a mis-predicted branch isn't going to be noticed on the overhead of scheduling. So there's a few cases, 1) PREEMPT=n 2) Voluntary preempt 3) PREEMPT=y 1) non of this has any effect, if the scheduler wants to reschedule a task that's in the kernel, it'll have to wait until it gets back to user-space. 2) uses cond_resched() and similar to have explicit preemption points, so we don't need to wait as long as 1). 3) preempts directly when !preempt_count(), when IRQs are disabled, the IPI that will accompany TIF_NEED_RESCHED will be delayed and local_irq_enable()/restore() will effect a reschedule due to the pending IPI. If preemption was disabled while the IPI hit nothing will happen, but preempt_enable() will do the reschedule once preempt_count reaches 0.