From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754673AbeDTK6n (ORCPT ); Fri, 20 Apr 2018 06:58:43 -0400 Received: from merlin.infradead.org ([205.233.59.134]:48646 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754462AbeDTK6l (ORCPT ); Fri, 20 Apr 2018 06:58:41 -0400 Date: Fri, 20 Apr 2018 12:58:27 +0200 From: Peter Zijlstra To: Nicholas Piggin Cc: linux-kernel@vger.kernel.org, Ingo Molnar , "Rafael J. Wysocki" , "Paul E . McKenney" Subject: Re: [RFC PATCH] kernel/sched/core: busy wait before going idle Message-ID: <20180420105827.GK4064@hirez.programming.kicks-ass.net> References: <20180415133149.24112-1-npiggin@gmail.com> <20180420074456.GA4064@hirez.programming.kicks-ass.net> <20180420190126.1644f4cd@roar.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180420190126.1644f4cd@roar.ozlabs.ibm.com> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 20, 2018 at 07:01:47PM +1000, Nicholas Piggin wrote: > On Fri, 20 Apr 2018 09:44:56 +0200 > Peter Zijlstra wrote: > > > On Sun, Apr 15, 2018 at 11:31:49PM +1000, Nicholas Piggin wrote: > > > This is a quick hack for comments, but I've always wondered -- > > > if we have a short term polling idle states in cpuidle for performance > > > -- why not skip the context switch and entry into all the idle states, > > > and just wait for a bit to see if something wakes up again. > > > > Is that context switch so expensive? > > I guess relatively much more than taking one branch mispredict on the > loop exit when the task wakes. 10s of cycles vs 1000s? Sure, just wondering how much. And I'm assuming you're looking at Power here, right? > > And what kernel did you test on? We recently merged a bunch of patches > > from Rafael that avoided disabling the tick for short idle predictions. > > This also has a performance improvements for such workloads. Did your > > kernel include those? > > Yes that actually improved profiles quite a lot, but these numbers were > with those changes. I'll try to find some fast disks or network and get > some more more interesting numbers. OK, good that you have those patches in. That ensures you're not trying to fix something that's possibly already addressed elsewhere. > > > It's not uncommon to see various going-to-idle work in kernel profiles. > > > This might be a way to reduce that (and just the cost of switching > > > registers and kernel stack to idle thread). This can be an important > > > path for single thread request-response throughput. > > > > So I feel that _if_ we do a spin here, it should only be long enough to > > amortize the schedule switch context. > > > > However, doing busy waits here has the downside that the 'idle' time is > > not in fact fed into the cpuidle predictor. > > That's why I cc'ed Rafael :) > > Yes the latency in my hack is probably too long, but I think if we did > this, the cpuile predictor could become involved here. There is no > fundamental reason it has to wait for the idle task to be context > switched for that... it's already become involved in core scheduler > code. Yes, cpuidle/cpufreq are getting more and more intergrated so there is no objection from that point. Growing multiple 'idle' points otoh is a little dodgy and could cause some maintenance issues. Of course, this loop would have the same idle-duration problems as the poll_state.c one. We should probably use that code. Also, do we want to ask the estimator before doing this? If it predicts a very long idle time, spinning here is just wasting cycles.