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

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Message-ID: <alpine.DEB.2.20.1707041732030.9000@nanos>
Date:   Tue, 4 Jul 2017 17:32:37 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Davidlohr Bueso <dave@...olabs.net>
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, 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 ....