Re: [PATCH 2/3] sched/deadline: fix bandwidth check/update when migrating tasks between exclusive cpusets

[Peter Zijlstra]

On Tue, Oct 07, 2014 at 09:59:54AM +0100, Juri Lelli wrote:
> Hi Peter,
> 
> On 19/09/14 22:25, Peter Zijlstra wrote:
> > On Fri, Sep 19, 2014 at 10:22:40AM +0100, Juri Lelli wrote:
> >> Exclusive cpusets are the only way users can restrict SCHED_DEADLINE tasks
> >> affinity (performing what is commonly called clustered scheduling).
> >> Unfortunately, such thing is currently broken for two reasons:
> >>
> >>  - No check is performed when the user tries to attach a task to
> >>    an exlusive cpuset (recall that exclusive cpusets have an
> >>    associated maximum allowed bandwidth).
> >>
> >>  - Bandwidths of source and destination cpusets are not correctly
> >>    updated after a task is migrated between them.
> >>
> >> This patch fixes both things at once, as they are opposite faces
> >> of the same coin.
> >>
> >> The check is performed in cpuset_can_attach(), as there aren't any
> >> points of failure after that function. The updated is split in two
> >> halves. We first reserve bandwidth in the destination cpuset, after
> >> we pass the check in cpuset_can_attach(). And we then release
> >> bandwidth from the source cpuset when the task's affinity is
> >> actually changed. Even if there can be time windows when sched_setattr()
> >> may erroneously fail in the source cpuset, we are fine with it, as
> >> we can't perfom an atomic update of both cpusets at once.
> > 
> > The thing I cannot find is if we correctly deal with updates to the
> > cpuset. Say we first setup 2 (exclusive) sets A:cpu0 B:cpu1-3. Then
> > assign tasks and then update the cpu masks like: B:cpu2,3, A:cpu1,2.
> > 
> 
> So, what follows should address the problem you describe.
> 
> Assuming you intended that we try to update masks as A:cpu0,3 and
> B:cpu1,2, with what below we are able to check that removing cpu3
> from B doesn't break guarantees. After that cpu3 can be put in A.
> 
> Does it make any sense?

Yeah, I think that about covers is. Could you write a changelog with it?

The reason I hadn't applied your patch #2 yet is because I thought it
triggered the splat reported in this thread. But later emails seem to
suggest this is a separate/pre-existing issue?


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