From: Vincent Guittot <email@example.com>
To: Odin Ugedal <firstname.lastname@example.org>
Cc: Ingo Molnar <email@example.com>,
Peter Zijlstra <firstname.lastname@example.org>,
Juri Lelli <email@example.com>,
Dietmar Eggemann <firstname.lastname@example.org>,
Steven Rostedt <email@example.com>,
Ben Segall <firstname.lastname@example.org>, Mel Gorman <email@example.com>,
Daniel Bristot de Oliveira <firstname.lastname@example.org>,
"open list:CONTROL GROUP (CGROUP)" <email@example.com>,
Subject: Re: [PATCH 1/3] sched/fair: Add tg_load_contrib cfs_rq decay checking
Date: Wed, 26 May 2021 12:50:35 +0200 [thread overview]
Message-ID: <CAKfTPtAFn3=anfTCxKTDXF0wpttpEiAhksLvcEPdSiYZTj38_A@mail.gmail.com> (raw)
On Tue, 25 May 2021 at 16:30, Vincent Guittot
> On Tue, 25 May 2021 at 12:34, Odin Ugedal <firstname.lastname@example.org> wrote:
> > Hi,
> > tir. 25. mai 2021 kl. 11:58 skrev Vincent Guittot <email@example.com>:
> > > Could you give more details about how cfs_rq->avg.load_avg = 4 but
> > > cfs_rq->avg.load_sum = 0 ?
> > >
> > > cfs_rq->avg.load_sum is decayed and can become null when crossing
> > > period which implies an update of cfs_rq->avg.load_avg. This means
> > > that your case is generated by something outside the pelt formula ...
> > > like maybe the propagation of load in the tree. If this is the case,
> > > we should find the error and fix it
> > Ahh, yeah, that could probably be described better.
> > It is (as far as I have found out) because the pelt divider is changed,
> > and the output from "get_pelt_divider(&cfs_rq->avg)" is changed, resulting
> > in a different value being removed than added.
> ok so IIUC, it happens during the adding/removing/propagating
> entities' load in the cfs_rq.
> > Inside pelt itself, this cannot happen. When pelt changes the load_sum, it
> > recalculates the load_avg based on load_sum, and not the delta, afaik.
> > And as you say, the "issue" therefore (as I see it) outside of PELT. Due to
> > how the pelt divider is changed, I assume it is hard to pinpoint where the issue
> > is. I can try to find a clear path where where we can see what is added
> > and what is removed from both cfs_rq->avg.load_sum and cfs_rq->avg.load_avg,
> > to better be able to pinpoint what is happening.
> > Previously I thought this was a result of precision loss due to division and
> > multiplication during load add/remove inside fair.c, but I am not sure that
> > is the issue, or is it?
> I don't think the precision looss is the problem because
> adding/removing load in fair.c could truncate load_sum but it stays
> sync with load_avg. I will have a llo to see if i can see something
I have added a trace in cfs_rq_is_decayed() but I'm not able to
reproduce a situation where load_sum == 0 but not load_avg even with
the script in the cover letter
> > If my above line of thought makes sense, do you still view this as an error
> > outside PELT, or do you see another possible/better solution?
> > Will investigate further.
> > Thanks
> > Odin
next prev parent reply other threads:[~2021-05-26 10:51 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-18 12:51 [PATCH 0/3] sched/fair: Fix load decay issues related to throttling Odin Ugedal
2021-05-18 12:52 ` [PATCH 1/3] sched/fair: Add tg_load_contrib cfs_rq decay checking Odin Ugedal
2021-05-25 9:58 ` Vincent Guittot
2021-05-25 10:33 ` Odin Ugedal
2021-05-25 14:30 ` Vincent Guittot
2021-05-26 10:50 ` Vincent Guittot [this message]
2021-05-27 7:50 ` Odin Ugedal
2021-05-27 9:35 ` Vincent Guittot
2021-05-27 9:45 ` Odin Ugedal
2021-05-27 10:49 ` Vincent Guittot
2021-05-27 11:04 ` Odin Ugedal
2021-05-27 12:37 ` Vincent Guittot
2021-05-27 12:37 ` Odin Ugedal
2021-05-27 12:39 ` Odin Ugedal
2021-05-27 12:49 ` Vincent Guittot
2021-05-18 12:52 ` [PATCH 2/3] sched/fair: Correctly insert cfs_rq's to list on unthrottle Odin Ugedal
2021-05-28 14:24 ` Vincent Guittot
2021-05-28 15:06 ` Odin Ugedal
2021-05-28 15:27 ` Vincent Guittot
2021-05-29 9:33 ` Odin Ugedal
2021-05-31 12:14 ` Vincent Guittot
2021-05-18 12:52 ` [PATCH 3/3] sched/fair: Fix ascii art by relpacing tabs Odin Ugedal
2021-05-27 13:27 ` Vincent Guittot
2021-06-01 14:04 ` [tip: sched/core] " tip-bot2 for Odin Ugedal
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