From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S265082AbUGZJJI (ORCPT ); Mon, 26 Jul 2004 05:09:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S265086AbUGZJJI (ORCPT ); Mon, 26 Jul 2004 05:09:08 -0400 Received: from mustang.oldcity.dca.net ([216.158.38.3]:7041 "HELO mustang.oldcity.dca.net") by vger.kernel.org with SMTP id S265082AbUGZJJC (ORCPT ); Mon, 26 Jul 2004 05:09:02 -0400 Subject: Re: preempt timing violations From: Lee Revell To: Ingo Molnar Cc: linux-kernel , Andrew Morton In-Reply-To: <20040726085002.GA25519@elte.hu> References: <1090813907.6936.56.camel@mindpipe> <20040726085002.GA25519@elte.hu> Content-Type: text/plain Message-Id: <1090832953.6936.114.camel@mindpipe> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.4.6 Date: Mon, 26 Jul 2004 05:09:14 -0400 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2004-07-26 at 04:50, Ingo Molnar wrote: > * Lee Revell wrote: > > > Latency with 2.6.8-rc2 + voluntary-preempt-I4 is the best so far. > > After extended testing there only seem to be a few hot spots. In > > several minutes I saw an 11ms violation, a 14ms violation, and several > > 2ms violations. > > > > get_user_pages() is much better in 2.6.8-rc1-mm1 than 2.6.8-rc2. Is > > there any chance of getting the fix into mainline? > > in -J3 i've added a cond_resched to the latency-generating point of > get_user_pages(). (The biggest latencies happen via > make_pages_present(), which gets triggered by mlockall()/MAP_LOCKED.) > OK, great, this is the biggest remaing issue with -I4, because jackd does that a lot. > > 2ms non-preemptible critical section violated 1 ms preempt threshold > > starting at unmap_vmas+0x1ff/0x210 and ending at > > unmap_vmas+0x1f5/0x210 > > this is the normal sys_exit()->exit_mmap()->unmap_vmas() path. It's > weird that it generated a 2ms latency. What are the values of > voluntary_preemption and kernel_preemption on your current kernel? With > a 2:0 setting we ought to have a reschedule point every 32 pages. Do > you know which application triggers this latency and is it easy to > reproduce? > 2 and 1. Now that I think about it, this could have happened during bootup, before my rc.local set these. I will try passing them on the kernel command line. Not sure I understand the difference between 2:1 and 2:0. Would the latter make the kernel only preemptible at the voluntary preemption points? > > 14ms non-preemptible critical section violated 1 ms preempt threshold > > starting at tty_write+0x1b6/0x290 and ending at schedule+0x2fd/0x5b0 > > does this one trigger when you are using the VGA console? (or fbcon)? > > it's not immediately obvious to me precisely where this latency comes > from, it would be nice to know how to reproduce it. It think this one was caused by switching virtual consoles. At one point Andrew Morton suggested I remove the (un)lock_kernel calls from do_tty_write. This fixed the problem, with no detectable side effects. Maybe this could be incorporated into voluntary-preempt, it would be useful to have more than one person to test it. Lee