From: Pavan Kondeti <firstname.lastname@example.org> To: Qais Yousef <email@example.com> Cc: Ingo Molnar <firstname.lastname@example.org>, Peter Zijlstra <email@example.com>, Steven Rostedt <firstname.lastname@example.org>, Dietmar Eggemann <email@example.com>, Juri Lelli <firstname.lastname@example.org>, Vincent Guittot <email@example.com>, Ben Segall <firstname.lastname@example.org>, Mel Gorman <email@example.com>, LKML <firstname.lastname@example.org> Subject: Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup Date: Thu, 27 Feb 2020 09:06:08 +0530 [thread overview] Message-ID: <20200227033608.GN28029@codeaurora.org> (raw) In-Reply-To: <email@example.com> On Wed, Feb 26, 2020 at 04:02:48PM +0000, Qais Yousef wrote: > On 02/25/20 09:25, Pavan Kondeti wrote: > > > I haven't been staring at this code for as long as you, but since we have > > > logic at wakeup to do a push, I think we need something here anyway for unfit > > > tasks. > > > > > > Fixing select_task_rq_rt() to better balance tasks will help a lot in general, > > > but if that was enough already then why do we need to consider a push at the > > > wakeup at all then? > > > > > > AFAIU, in SMP the whole push-pull mechanism is racy and we introduce redundancy > > > at taking the decision on various points to ensure we minimize this racy nature > > > of SMP systems. Anything could have happened between the time we called > > > select_task_rq_rt() and the wakeup, so we double check again before we finally > > > go and run. That's how I interpret it. > > > > > > I am open to hear about other alternatives first anyway. Your help has been > > > much appreciated so far. > > > > > > > The search inside find_lowest_rq() happens without any locks so I believe it > > is expected to have races like this. In fact there is a comment in the code > > saying "This test is optimistic, if we get it wrong the load-balancer > > will have to sort it out" in select_task_rq_rt(). However, the push logic > > as of today works only for overloaded case. In that sense, your patch fixes > > this race for b.L systems. At the same time, I feel like tracking nonfit tasks > > just to fix this race seems to be too much. I will leave this to Steve and > > others to take a decision. > > I do think without this tasks can end up on the wrong CPU longer than they > should. Keep in mind that if a task is boosted to run on a big core, it still > have to compete with non-boosted tasks who can run on a any cpu. So this > opportunistic push might be necessary. > > For 5.6 though, I'll send an updated series that removes the fitness check from > task_woken_rt() && switched_to_rt() and carry on with this discussion for 5.7. > > > > > I thought of suggesting to remove the below check from select_task_rq_rt() > > > > p->prio < cpu_rq(target)->rt.highest_prio.curr > > > > which would then make the target CPU overloaded and the push logic would > > spread the tasks. That works for a b.L system too. However there seems to > > be a very good reason for doing this. see > > https://lore.kernel.org/patchwork/patch/539137/ > > > > The fact that a CPU is part of lowest_mask but running a higher prio RT > > task means there is a race. Should we retry one more time to see if we find > > another CPU? > > Isn't this what I did in v1? > > https://firstname.lastname@example.org/ > Yes, that patch allows overloading the CPU When the priorities are same. I think, We should also consider when a low prio task and high prio task are waking at the same time and high prio task winning the race. Thanks, Pavan -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
next prev parent reply other threads:[~2020-02-27 3:36 UTC|newest] Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-23 18:39 [PATCH v2 0/6] RT Capacity Awareness Fixes & Improvements Qais Yousef 2020-02-23 18:39 ` [PATCH v2 1/6] sched/rt: cpupri_find: implement fallback mechanism for !fit case Qais Yousef 2020-02-23 18:39 ` [PATCH v2 2/6] sched/rt: Re-instate old behavior in select_task_rq_rt Qais Yousef 2020-02-25 15:21 ` Dietmar Eggemann 2020-02-26 11:34 ` Qais Yousef 2020-02-23 18:39 ` [PATCH v2 3/6] sched/rt: Optimize cpupri_find on non-heterogenous systems Qais Yousef 2020-02-23 18:39 ` [PATCH v2 4/6] sched/rt: allow pulling unfitting task Qais Yousef 2020-02-23 18:40 ` [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup Qais Yousef 2020-02-24 6:10 ` Pavan Kondeti 2020-02-24 12:11 ` Qais Yousef 2020-02-24 16:04 ` Pavan Kondeti 2020-02-24 17:41 ` Qais Yousef 2020-02-25 3:55 ` Pavan Kondeti 2020-02-26 16:02 ` Qais Yousef 2020-02-27 3:36 ` Pavan Kondeti [this message] 2020-02-27 10:29 ` Qais Yousef 2020-02-23 18:40 ` [PATCH v2 6/6] sched/rt: Remove unnecessary assignment in inc/dec_rt_migration Qais Yousef 2020-02-23 23:16 ` Dietmar Eggemann 2020-02-24 12:31 ` Qais Yousef 2020-02-24 13:03 ` Dietmar Eggemann 2020-02-24 13:47 ` Qais Yousef
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20200227033608.GN28029@codeaurora.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --subject='Re: [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).