Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751271AbdHCPFS (ORCPT ); Thu, 3 Aug 2017 11:05:18 -0400 Received: from foss.arm.com ([217.140.101.70]:41668 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751010AbdHCPFR (ORCPT ); Thu, 3 Aug 2017 11:05:17 -0400 References: <20170630004912.GA2457@destiny> <20170630142815.GA9743@destiny> <1498842140.15161.66.camel@gmail.com> <20170630175540.GA2097@destiny> <87zibgrkgw.fsf@arm.com> <20170803131537.GB17196@destiny> User-agent: mu4e 0.9.17; emacs 25.1.1 From: Brendan Jackman To: Josef Bacik Cc: Mike Galbraith , Joel Fernandes , Peter Zijlstra , LKML , Juri Lelli , Dietmar Eggemann , Patrick Bellasi , Chris Redpath Subject: Re: wake_wide mechanism clarification In-reply-to: <20170803131537.GB17196@destiny> Date: Thu, 03 Aug 2017 16:05:11 +0100 Message-ID: <87wp6kr8t4.fsf@arm.com> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6393 Lines: 142 On Thu, Aug 03 2017 at 13:15, Josef Bacik wrote: > On Thu, Aug 03, 2017 at 11:53:19AM +0100, Brendan Jackman wrote: >> >> Hi, >> >> On Fri, Jun 30 2017 at 17:55, Josef Bacik wrote: >> > On Fri, Jun 30, 2017 at 07:02:20PM +0200, Mike Galbraith wrote: >> >> On Fri, 2017-06-30 at 10:28 -0400, Josef Bacik wrote: >> >> > On Thu, Jun 29, 2017 at 08:04:59PM -0700, Joel Fernandes wrote: >> >> > >> >> > > That makes sense that we multiply slave's flips by a factor because >> >> > > its low, but I still didn't get why the factor is chosen to be >> >> > > llc_size instead of something else for the multiplication with slave >> >> > > (slave * factor). >> >> >> >> > Yeah I don't know why llc_size was chosen... >> >> >> >> static void update_top_cache_domain(int cpu) >> >> { >> >> struct sched_domain_shared *sds = NULL; >> >> struct sched_domain *sd; >> >> int id = cpu; >> >> int size = 1; >> >> >> >> sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES); >> >> if (sd) { >> >> id = cpumask_first(sched_domain_span(sd)); >> >> size = cpumask_weight(sched_domain_span(sd)); >> >> sds = sd->shared; >> >> } >> >> >> >> rcu_assign_pointer(per_cpu(sd_llc, cpu), sd); >> >> per_cpu(sd_llc_size, cpu) = size; >> >> >> >> The goal of wake wide was to approximate when pulling would be a futile >> >> consolidation effort and counterproductive to scaling. 'course with >> >> ever increasing socket size, any 1:N waker is ever more likely to run >> >> out of CPU for its one and only self (slamming into scaling wall) >> >> before it needing to turn its minions loose to conquer the world. >> >> >> >> Something else to consider: network interrupt waking multiple workers >> >> at high frequency. If the waking CPU is idle, do you really want to >> >> place a worker directly in front of a tattoo artist, or is it better >> >> off nearly anywhere but there? >> >> >> >> If the box is virtual, with no topology exposed (or real but ancient) >> >> to let select_idle_sibling() come to the rescue, two workers can even >> >> get tattooed simultaneously (see sync wakeup). >> >> >> > >> > Heuristics are hard, news at 11. I think messing with wake_wide() itself is too >> > big of a hammer, we probably need a middle ground. I'm messing with it right >> > now so it's too early to say for sure, but i _suspect_ the bigger latencies we >> > see are not because we overload the cpu we're trying to pull to, but because >> > when we fail to do the wake_affine() we only look at siblings of the affine_sd >> > instead of doing the full "find the idlest cpu in the land!" thing. >> >> This is the problem I've been hitting lately. My use case is 1 task per >> CPU on ARM big.LITTLE (asymmetrical CPU capacity). The workload is 1 >> task per CPU, they all do X amount of work then pthread_barrier_wait >> (i.e. sleep until the last task finishes its X and hits the barrier). On >> big.LITTLE, the tasks which get a "big" CPU finish faster, and then >> those CPUs pull over the tasks that are still running: >> >> v CPU v ->time-> >> >> ------------- >> 0 (big) 11111 /333 >> ------------- >> 1 (big) 22222 /444| >> ------------- >> 2 (LITTLE) 333333/ >> ------------- >> 3 (LITTLE) 444444/ >> ------------- >> >> Now when task 4 hits the barrier (at |) and wakes the others up, there >> are 4 tasks with prev_cpu= and 0 tasks with >> prev_cpu=. Assuming that those wakeups happen on CPU4, >> regardless of wake_affine, want_affine means that we'll only look in >> sd_llc (cpus 0 and 1), so tasks will be unnecessarily coscheduled on the >> bigs until the next load balance, something like this: >> >> v CPU v ->time-> >> >> ------------------------ >> 0 (big) 11111 /333 31313\33333 >> ------------------------ >> 1 (big) 22222 /444|424\4444444 >> ------------------------ >> 2 (LITTLE) 333333/ \222222 >> ------------------------ >> 3 (LITTLE) 444444/ \1111 >> ------------------------ >> ^^^ >> underutilization >> >> > I _think_ >> > the answer is to make select_idle_sibling() try less hard to find something >> > workable and only use obviously idle cpu's in the affine sd, and fall back to >> > the full load balance esque search. >> >> So this idea of allowing select_idle_sibling to fail, and falling back >> to the slow path, would help me too, I think. > > Unfortunately this statement of mine was wrong, I had it in my head that we > would fall back to a find the idlest cpu thing provided we failed to wake > affine, but we just do select_idle_sibling() and expect the load balancer to > move things around as needed. Ah yes, when wake_affine() returns false, we still do select_idle_sibling (except in prev_cpu's sd_llc instead of smp_processor_id()'s), and that is the problem faced by my workload. I thought you were suggesting to change the flow so that select_idle_sibling can say "I didn't find any idle siblings - go to the find_idlest_group path". >> This is also why I was playing with your >> don't-affine-recently-balanced-tasks patch[1], which also helps my case >> since it prevents want_affine for tasks 3 and 4 (which were recently >> moved by an active balance). >> >> [1] https://marc.info/?l=linux-kernel&m=150003849602535&w=2 >> (also linked elsewhere in this thread) >> > > Would you try peter's sched/experimental branch and see how that affects your > workload? I'm still messing with my patches and I may drop this one as it now > appears to be too aggressive with the new set of patches. Thanks, Sure, I'll take a look at those, thanks. I guess the idea of caching values in LB and then using them in wakeup[2] is a lighter-handed way of achieving the same thing as last_balance_ts? It won't solve my problem directly since we'll still only look in sd_llc, but I think it could be a basis for a way to say "go find_idlest_group path on these tasks" at the beginning of select_task_rq_fair. [2] https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?h=sched/experimental&id=5b4ed509027a5b6f495e6fe871cae850d5762bef Thanks, Brendan