From: Peter Zijlstra <peterz@infradead.org>
To: Rakib Mullick <rakib.mullick@gmail.com>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org,
paulmck <paulmck@linux.vnet.ibm.com>
Subject: Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down.
Date: Thu, 16 Aug 2012 16:42:18 +0200 [thread overview]
Message-ID: <1345128138.29668.42.camel@twins> (raw)
In-Reply-To: <CADZ9YHjPjExzuJWjziiezBch03Am0imQzDG4EDfG-NTWfz4V8A@mail.gmail.com>
On Thu, 2012-08-16 at 20:28 +0600, Rakib Mullick wrote:
> I'm not sure which parts are missing from Changelog to patch. And this
> patch assumes that, sleeping tasks won't be scattered. From
> select_fallback_rq(), sleeping tasks might get scattered due to
> various cases like. if CPU is down, task isn't allowed to move a
> particular CPU. Other than that, dest cpu supposed to be the same.
Sure but affinities and cpusets can still scatter, and therefore your
logic doesn't hold up, but see below.
> > Furthermore there should be absolutely no impact on load calculation
> > what so ever. nr_uninterruptible is only ever useful as a sum over all
> > cpus, this total sum doesn't change regardless of where you put the
> > value.
> >
> > Worse, there's absolutely no relation to the tasks on the runqueue
> > (sleeping or otherwise) and nr_uninterruptible, so coupling these
> > actions makes no sense what so ever.
> >
> nr_uninterruptible is coupled with tasks on the runqueue to calculate
> nr_active numbers.
It is not.. nr_uninterruptible is incremented on the cpu the task goes
to sleep and decremented on the cpu doing the wakeup.
This means that nr_uninterruptible is a complete mess and any per-cpu
value isn't meaningful at all.
It is quite possible to always have the inc on cpu0 and the decrement on
cpu1, yielding results like:
{1000, -1000} for an effective nr_uninterruptible = 0. Taking either cpu
down will then migrate whatever delta it has to another cpu, but there
might only be a single task, yet the delta is +-1000.
> In calc_load_fold_active(), this nr_active numbers are used to
> calculate delta. This is how I understand this part and seeing some
> impact.
You understand wrong, please re-read the comment added in commit
5167e8d5.
next prev parent reply other threads:[~2012-08-16 14:42 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-08-16 13:45 Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down Rakib Mullick
2012-08-16 13:56 ` Peter Zijlstra
2012-08-16 14:28 ` Rakib Mullick
2012-08-16 14:42 ` Peter Zijlstra [this message]
2012-08-16 15:32 ` Rakib Mullick
2012-08-16 17:46 ` Peter Zijlstra
2012-08-17 13:39 ` Rakib Mullick
2012-08-20 9:26 ` Peter Zijlstra
2012-08-20 16:10 ` Rakib Mullick
2012-08-20 16:16 ` Peter Zijlstra
2012-08-20 16:26 ` Paul E. McKenney
2012-08-27 18:44 ` Paul E. McKenney
2012-08-28 6:57 ` Rakib Mullick
2012-08-28 13:42 ` Paul E. McKenney
2012-08-28 16:52 ` Rakib Mullick
2012-08-28 17:07 ` Paul E. McKenney
2012-08-29 1:05 ` Rakib Mullick
2012-09-04 18:43 ` [tip:sched/core] sched: Fix load avg vs cpu-hotplug tip-bot for Peter Zijlstra
2012-09-05 12:36 ` Peter Zijlstra
2012-09-05 13:29 ` Ingo Molnar
2012-09-05 17:01 ` Peter Zijlstra
2012-09-05 17:34 ` Ingo Molnar
2012-09-05 22:03 ` Peter Zijlstra
2012-09-05 23:39 ` Paul E. McKenney
2012-09-06 3:30 ` Rakib Mullick
2012-09-14 6:14 ` [tip:sched/core] sched: Fix load avg vs. cpu-hotplug tip-bot for Peter Zijlstra
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