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Date: Tue, 4 Jul 2017 08:42:08 -0700
From: Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu
 rwsem
Message-ID: <20170704154208.GC11168@linux-80c1.suse>
References: <20170704093232.995040438@linutronix.de>
 <20170704093421.506836322@linutronix.de>
 <20170704150106.GA11168@linux-80c1.suse>
 <20170704152206.GB11168@linux-80c1.suse>
 <alpine.DEB.2.20.1707041732030.9000@nanos>
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To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org, Andrey Ryabinin <aryabinin@virtuozzo.com>, Michal Hocko <mhocko@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Vlastimil Babka <vbabka@suse.cz>, Vladimir Davydov <vdavydov.dev@gmail.com>, Peter Zijlstra <peterz@infradead.org>

On Tue, 04 Jul 2017, Thomas Gleixner wrote:

>On Tue, 4 Jul 2017, Davidlohr Bueso wrote:
>> On Tue, 04 Jul 2017, Davidlohr Bueso wrote:
>>
>> > As a side effect you end up optimizing get/put_online_mems() at the cost
>> > of more overhead for the actual hotplug operation, which is rare and of less
>> > performance importance.
>>
>> So nm this, the reader side actually gets _more_ expensive with pcpu-rwsems
>> due to at least two full barriers for each get/put operation.
>
>Compared to a mutex_lock/unlock() pair on a global mutex ....

Ah, right, I was thrown off the:

    if (mem_hotplug.active_writer == current)
       return;

checks, which is only true within hotplug_begin/end. So normally we'd take
the lock, which was my first impression. Sorry for the noise.

Thanks,
Davidlohr

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