linux-kernel - Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu rwsem

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Message-ID: <20170704154208.GC11168@linux-80c1.suse>
Date:   Tue, 4 Jul 2017 08:42:08 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Michal Hocko <mhocko@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu
 rwsem

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