linux-kernel - Re: [PATCH V2 0/7] sched/deadline: fix cpusets bandwidth accounting

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Message-ID: <20180205204849.GA2621@xps15>
Date:   Mon, 5 Feb 2018 13:48:49 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Luca Abeni <luca.abeni@...tannapisa.it>
Cc:     peterz@...radead.org, lizefan@...wei.com, mingo@...hat.com,
        rostedt@...dmis.org, claudio@...dence.eu.com, bristot@...hat.com,
        tommaso.cucinotta@...tannapisa.it, juri.lelli@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2 0/7] sched/deadline: fix cpusets bandwidth accounting

On Fri, Feb 02, 2018 at 02:17:50PM +0100, Luca Abeni wrote:
> Hi Mathieu,
> 
> On Thu,  1 Feb 2018 09:51:02 -0700
> Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
> 
> > This is the follow-up patchset to [1] that attempt to fix a problem
> > reported by Steve Rostedt [2] where DL bandwidth accounting is not
> > recomputed after CPUset and CPU hotplug operations.  When CPU hotplug and
> > some CUPset manipulation take place root domains are destroyed and new ones
> > created, loosing at the same time DL accounting information pertaining to
> > utilisation.  Please see [1] for a full description of the approach.
> 
> I do not know the cgroup / cpuset code too much, so I have no useful
> comments on your patches... But I think this patchset is a nice
> improvemnt respect to the current situation.
> 
> [...]
> > A notable addition is patch 7/7 - it addresses a problem seen when hot
> > plugging out a CPU where a DL task is running (see changelog for full
> > details).  The issue is unrelated to this patchset and will manifest
> > itself on a mainline kernel.
> 
> I think I introduced this bug with my reclaiming patches, so I am
> interested.
> When a cpu is hot-plugged out, which code in the kernel is responsible
> for migrating the tasks that are executing on such CPU?

sched_cpu_deactivate()
    cpuset_cpu_inactive()
        cpuset_update_active_cpus()
            cpuset_hotplug_workfn()
                hotplug_update_tasks_legacy()
                    hotplug_update_tasks()
                        set_cpus_allowed_ptr()
                            __set_cpus_allowed_ptr()


> I was sure I
> was handling all the relevant codepaths, but this bug clearly shows
> that I was wrong.

I remember reviewing your patchset and I too thought you had tackled all the
cases.  In function __set_cpus_allowed_ptr() you'll notice two cases are
handled, i.e the task is running or suspended.  I suspect the former to be the
culprit but haven't investigated fully.

Regards,
Mathieu
                                


> 
> 
> 			Thanks,
> 				Luca