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Date: Tue, 4 Jul 2017 17:32:37 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch V2 2/2] mm/memory-hotplug: Switch locking to a percpu
 rwsem
In-Reply-To: <20170704152206.GB11168@linux-80c1.suse>
Message-ID: <alpine.DEB.2.20.1707041732030.9000@nanos>
References: <20170704093232.995040438@linutronix.de> <20170704093421.506836322@linutronix.de> <20170704150106.GA11168@linux-80c1.suse> <20170704152206.GB11168@linux-80c1.suse>
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To: Davidlohr Bueso <dave@stgolabs.net>
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, 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 ....

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