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Subject: Re: [PATCH] cgroup/cpuset: fix circular locking dependency
To: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, avagin@gmail.com,
        mingo@kernel.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, sramana@codeaurora.org
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From: Prateek Sood <prsood@codeaurora.org>
Message-ID: <7c25aac5-ef23-26c6-5096-4f1ab1b53b70@codeaurora.org>
Date: Sat, 16 Dec 2017 00:34:18 +0530
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On 12/13/2017 09:36 PM, Tejun Heo wrote:
> Hello, Prateek.
> 
> On Wed, Dec 13, 2017 at 01:20:46PM +0530, Prateek Sood wrote:
>> This change makes the usage of cpuset_hotplug_workfn() from cpu
>> hotplug path synchronous. For memory hotplug it still remains
>> asynchronous.
> 
> Ah, right.
> 
>> Memory migration happening from cpuset_hotplug_workfn() is
>> already asynchronous by queuing cpuset_migrate_mm_workfn() in
>> cpuset_migrate_mm_wq.
>>
>> cpuset_hotplug_workfn()
>>    cpuset_hotplug_workfn(()
>>       cpuset_migrate_mm()
>>          queue_work(cpuset_migrate_mm_wq)
>>
>> It seems that memory migration latency might not have
>> impact with this change.
>>
>> Please let me know if you meant something else by cpuset
>> migration taking time when memory migration is turned on.
> 
> No, I didn't.  I was just confused about which part became
> synchronous.  So, I don't have anything against making the cpu part
> synchronous, but let's not do that as the fix to the deadlocks cuz,
> while we can avoid them by changing cpuset, I don't think cpuset is
> the root cause for them.  If there are benefits to making cpuset cpu
> migration synchronous, let's do that for those benefits.

Making CPU part synchronous can only be achieved if we retain the
patch for inverting the locking order for cpuset_mutex and
cpu_hotplug_lock.

> 
> Thanks.
> 

Peter,

Do you suggest taking both the patches:
1) Inverting locking order of cpuset_mutex and cpu_hotplug_lock.
2) Make cpuset hotplug work synchronous.

or  

 https://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git/commit/?h=for-4.15-fixes&id=e8b3f8db7aad99fcc5234fc5b89984ff6620de3d

I would leave it for you and TJ to decide on this.

Thanks


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