From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Rostedt Subject: Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature Date: Mon, 7 Nov 2016 15:44:01 -0500 Message-ID: <20161107154401.16f0214e@gandalf.local.home> References: <20161107133207.4282de69@gandalf.local.home> <20161107144738.4811a5dd@gandalf.local.home> <20161107150003.66777b43@gandalf.local.home> <1e79f711-95f1-da2f-f572-1ac4329c8be7@bristot.me> <20161107151617.486b1b42@gandalf.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Christoph Lameter , Daniel Bristot de Oliveira , Ingo Molnar , Peter Zijlstra , linux-rt-users , LKML To: Daniel Bristot de Oliveira Return-path: Received: from smtprelay0228.hostedemail.com ([216.40.44.228]:36941 "EHLO smtprelay.hostedemail.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751512AbcKGUoF (ORCPT ); Mon, 7 Nov 2016 15:44:05 -0500 In-Reply-To: Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Mon, 7 Nov 2016 21:33:02 +0100 Daniel Bristot de Oliveira wrote: > On 11/07/2016 09:16 PM, Steven Rostedt wrote: > > I'm confused? Are you saying that RR tasks don't get throttled in the > > current code? That sounds like a bug to me. > > If the RT_RUNTIME_SHARING is enabled, the CPU in which the RR tasks are > running (and pinned) will borrow RT runtime from another CPU, allowing > the RR tasks to run forever. For example: > > [root@kiron debug]# cat /proc/sched_debug | grep rt_runtime > .rt_runtime : 950.000000 > .rt_runtime : 950.000000 > .rt_runtime : 950.000000 > .rt_runtime : 950.000000 > [root@kiron debug]# echo RT_RUNTIME_SHARE > sched_features > [root@kiron debug]# taskset -c 2 chrt -r 5 /home/bristot/f & > [1] 23908 > [root@kiron debug]# taskset -c 2 chrt -r 5 /home/bristot/f & > [2] 23915 > [root@kiron debug]# cat /proc/sched_debug | grep rt_runtime > .rt_runtime : 900.000000 > .rt_runtime : 950.000000 > .rt_runtime : 1000.000000 > .rt_runtime : 950.000000 > > You see? the rt_runtime of the CPU 2 was borrowed time from CPU 0. > > It is not a BUG but a feature (no jokes haha). With RT_RUNTIME_SHARE, > the rt_runtime is such a global runtime. It works fine for tasks that > can migrate... but that is not the case for per-cpu kworkers. This still looks like a bug, or not the expected result. Perhaps we shouldn't share when tasks are pinned. It doesn't make sense. It's like pinning two deadline tasks to the same CPU and giving them 100% of that CPU and saying that it's really just 1/nr_cpus of usage, which would have the same effect. OK, it appears this is specific to RT_RUNTIME_SHARE which is what causes this strange behavior, and even more rational to make this a default option and perhaps even turn RT_RUNTIME_SHARE off by default. -- Steve