linux-kernel - Re: [PATCH v4 1/5] sched/topology: Add check to backup comment about hotplug lock

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Message-ID: <20180614141818.GN17720@e108498-lin.cambridge.arm.com>
Date:   Thu, 14 Jun 2018 15:18:19 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, peterz@...radead.org,
        mingo@...hat.com, linux-kernel@...r.kernel.org,
        luca.abeni@...tannapisa.it, claudio@...dence.eu.com,
        tommaso.cucinotta@...tannapisa.it, bristot@...hat.com,
        mathieu.poirier@...aro.org, lizefan@...wei.com,
        cgroups@...r.kernel.org
Subject: Re: [PATCH v4 1/5] sched/topology: Add check to backup comment about
 hotplug lock

On Thursday 14 Jun 2018 at 16:11:18 (+0200), Juri Lelli wrote:
> On 14/06/18 14:58, Quentin Perret wrote:
> 
> [...]
> 
> > Hmm not sure if this can help but I think that rebuild_sched_domains()
> > does _not_ take the hotplug lock before calling partition_sched_domains()
> > when CONFIG_CPUSETS=n. But it does take it for CONFIG_CPUSETS=y.
> 
> Did you mean cpuset_mutex?

Nope, I really meant the cpu_hotplug_lock !

With CONFIG_CPUSETS=n, rebuild_sched_domains() calls
partition_sched_domains() directly:

https://elixir.bootlin.com/linux/latest/source/include/linux/cpuset.h#L255

But with CONFIG_CPUSETS=y, rebuild_sched_domains() calls,
rebuild_sched_domains_locked(), which calls get_online_cpus() which
calls cpus_read_lock(), which does percpu_down_read(&cpu_hotplug_lock).
And all that happens before calling partition_sched_domains().

So yeah, the point I was trying to make is that there is an inconsistency
here, maybe for a good reason ? Maybe related to the issue you're seeing ?

Thanks,
Quentin