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McKenney" , Tim Chen Subject: Re: [PATCH v8 -tip 06/26] sched: Add core wide task selection and scheduling. Message-ID: <20201023213118.GD3563800@google.com> References: <20201020014336.2076526-1-joel@joelfernandes.org> <20201020014336.2076526-7-joel@joelfernandes.org> <20201023135129.GS2611@hirez.programming.kicks-ass.net> <20201023135400.GA2651@hirez.programming.kicks-ass.net> <20201023175724.GA3563800@google.com> <20201023192654.GH2974@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201023192654.GH2974@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 23, 2020 at 09:26:54PM +0200, Peter Zijlstra wrote: > On Fri, Oct 23, 2020 at 01:57:24PM -0400, Joel Fernandes wrote: > > On Fri, Oct 23, 2020 at 03:54:00PM +0200, Peter Zijlstra wrote: > > > On Fri, Oct 23, 2020 at 03:51:29PM +0200, Peter Zijlstra wrote: > > > > On Mon, Oct 19, 2020 at 09:43:16PM -0400, Joel Fernandes (Google) wrote: > > > > > + /* > > > > > + * If this sibling doesn't yet have a suitable task to > > > > > + * run; ask for the most elegible task, given the > > > > > + * highest priority task already selected for this > > > > > + * core. > > > > > + */ > > > > > + p = pick_task(rq_i, class, max); > > > > > + if (!p) { > > > > > + /* > > > > > + * If there weren't no cookies; we don't need to > > > > > + * bother with the other siblings. > > > > > + * If the rest of the core is not running a tagged > > > > > + * task, i.e. need_sync == 0, and the current CPU > > > > > + * which called into the schedule() loop does not > > > > > + * have any tasks for this class, skip selecting for > > > > > + * other siblings since there's no point. We don't skip > > > > > + * for RT/DL because that could make CFS force-idle RT. > > > > > + */ > > > > > + if (i == cpu && !need_sync && class == &fair_sched_class) > > > > > + goto next_class; > > > > > + > > > > > + continue; > > > > > + } > > > > > > > > I'm failing to understand the class == &fair_sched_class bit. > > > > The last line in the comment explains it "We don't skip for RT/DL because > > that could make CFS force-idle RT.". > > Well, yes, but it does not explain how this can come about, now does it. Sorry, I should have made it a separate commit with the below explanation. Oh well, live and learn! > > Even if need_sync == false, we need to go look at other CPUs (non-local > > CPUs) to see if they could be running RT. > > > > Say the RQs in a particular core look like this: > > Let CFS1 and CFS2 be 2 tagged CFS tags. Let RT1 be an untagged RT task. > > > > rq0 rq1 > > CFS1 (tagged) RT1 (not tag) > > CFS2 (tagged) > > > > Say schedule() runs on rq0. Now, it will enter the above loop and > > pick_task(RT) will return NULL for 'p'. It will enter the above if() block > > and see that need_sync == false and will skip RT entirely. > > > > The end result of the selection will be (say prio(CFS1) > prio(CFS2)): > > rq0 rq1 > > CFS1 IDLE > > > > When it should have selected: > > rq0 r1 > > IDLE RT > > > > I saw this issue on real-world usecases in ChromeOS where an RT task gets > > constantly force-idled and breaks RT. The "class == &fair_sched_class" bit > > cures it. > > Ah, I see. The thing is, this looses the optimization for a bunch of > valid (and arguably common) scenarios. The problem is that the moment we > end up selecting a task with a cookie we've invalidated the premise > under which we ended up with the selected task. > > How about this then? This does look better. It makes sense and I think it will work. I will look more into it and also test it. BTW, as further optimization in the future, isn't it better for the schedule() loop on 1 HT to select for all HT *even if* need_sync == false to begin with? i.e. no cookied tasks are runnable. That way the pick loop in schedule() running on other HTs can directly pick what was pre-selected for it via: if (rq->core->core_pick_seq == rq->core->core_task_seq && rq->core->core_pick_seq != rq->core_sched_seq && rq->core_pick) .. which I think is more efficient. Its just a thought and may not be worth doing. thanks, - Joel > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -4709,6 +4709,7 @@ pick_next_task(struct rq *rq, struct tas > need_sync = !!rq->core->core_cookie; > > /* reset state */ > +reset: > rq->core->core_cookie = 0UL; > for_each_cpu(i, smt_mask) { > struct rq *rq_i = cpu_rq(i); > @@ -4748,14 +4749,8 @@ pick_next_task(struct rq *rq, struct tas > /* > * If there weren't no cookies; we don't need to > * bother with the other siblings. > - * If the rest of the core is not running a tagged > - * task, i.e. need_sync == 0, and the current CPU > - * which called into the schedule() loop does not > - * have any tasks for this class, skip selecting for > - * other siblings since there's no point. We don't skip > - * for RT/DL because that could make CFS force-idle RT. > */ > - if (i == cpu && !need_sync && !p->core_cookie) > + if (i == cpu && !need_sync) > goto next_class; > > continue; > @@ -4765,7 +4760,17 @@ pick_next_task(struct rq *rq, struct tas > * Optimize the 'normal' case where there aren't any > * cookies and we don't need to sync up. > */ > - if (i == cpu && !need_sync && !p->core_cookie) { > + if (i == cpu && !need_sync) { > + if (p->core_cookie) { > + /* > + * This optimization is only valid as > + * long as there are no cookies > + * involved. > + */ > + need_sync = true; > + goto reset; > + } > + > next = p; > goto done; > } > @@ -4805,7 +4810,6 @@ pick_next_task(struct rq *rq, struct tas > */ > need_sync = true; > } > - > } > } > next_class:; >