Messages in this thread | | | Date | Thu, 16 Nov 2023 14:38:24 +0800 | Subject | Re: [PATCH 3/4] sched/fair: introduce core_vruntime and core_min_vruntime | From | cruzzhao <> |
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在 2023/11/15 下午11:22, Peter Zijlstra 写道: > On Wed, Nov 15, 2023 at 09:42:13PM +0800, cruzzhao wrote: >> >> >> 在 2023/11/15 下午8:20, Peter Zijlstra 写道: >>> On Wed, Nov 15, 2023 at 07:33:40PM +0800, Cruz Zhao wrote: >>>> To compare the priority of sched_entity from different cpus of a core, >>>> we introduce core_vruntime to struct sched_entity and core_min_vruntime >>>> to struct cfs_rq. >>>> >>>> cfs_rq->core->core_min_vruntime records the min vruntime of the cfs_rqs >>>> of the same task_group among the core, and se->core_vruntime is the >>>> vruntime relative to se->cfs_rq->core->core_min_vruntime. >>> >>> But that makes absolutely no sense. vruntime of different RQs can >>> advance at wildly different rates. Not to mention there's this random >>> offset between them. >>> >>> No, this cannot be. >> >> Force idle vruntime snapshot does the same thing, comparing >> sea->vruntime - cfs_rqa->min_vruntime_fi with seb->vruntime - >> cfs_rqb->min_vruntime_fi, while sea and seb may have wildly different rates. > > But that subtracts a from a and b from b, it doesn't mix a and b. > > Note that se->vruntime - cfs_rq->min_vruntime is a very poor > approximation of lag. We have actual lag now. > > Note that: > > (sea - seb) + (min_fib - min_fia) = > (sea - min_fia) + (min_fib - seb) = > (sea - min_fia) - (seb - min_fib) = > 'lag'a - 'lag'b > > It doesn't mix absolute a and b terms anywhere. > >> Actually, cfs_rq->core->core_min_vruntime does the same thing as >> cfs_rq->min_vruntime_fi, providing a baseline, but >> cfs_rq->core->core_min_vruntime is more accurate. > > min(cfs_rqa, cfs_rqb) is nonsense. And I can't see how min_vruntime_fi > would do anything like that. > >> I've tried to implement a fair enough mechanism of core_vruntime, but >> it's too complex because of the weight, and it costs a lot. So this is a >> compromise solution. > > 'this' is complete nonsense and not motivated by any math. > >> BTW, is there any other solutions to solve this problem? > > Well, this is where it all started: > > https://lkml.kernel.org/r/20200506143506.GH5298%40hirez.programming.kicks-ass.net > > The above lag thing is detailed in a follow up: > > https://lkml.kernel.org/r/20200515103844.GG2978%40hirez.programming.kicks-ass.net
Many thanks, I'll study the discussion about this.
> > Anyway, I think the first of those links has the start of the > multi-queue formalism, see the S_k+l bits. Work that out and see where > it ends. > > I did go a bit further, but I've forgotten everything since, it's been > years. > > Anyway, nothing like this goes in without a fairly solid bit of math in > the changelog to justify it. > > Also, I think Joel complained about something like this at some point, > and he wanted to update the core tree more often, because IIRc his > observation was that things got stale or something.
Many thanks for reviewing. I'll think about this more comprehensively.
Best, Cruz Zhao
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