linux-kernel - Re: [patch 20/20] cpu/hotplug: Convert hotplug locking to percpu rwsem

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Message-ID: <20170417065022.ucihdhfazq7bxib4@hirez.programming.kicks-ass.net>
Date:   Mon, 17 Apr 2017 08:50:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 20/20] cpu/hotplug: Convert hotplug locking to percpu
 rwsem

On Sat, Apr 15, 2017 at 07:01:27PM +0200, Thomas Gleixner wrote:
> There are no more (known) nested calls to get_online_cpus() so it's
> possible to remove the nested call magic and convert the mutex to a
> percpu-rwsem, which speeds up get/put_online_cpus() significantly for the
> uncontended case.
> 
> The contended case (write locked for hotplug operations) is slow anyway, so
> the slightly more expensive down_write of the percpu rwsem does not matter.

So the previous thing was reader biassed and thus prone to writer
starvation. So a slightly more expensive write path doesn't matter; esp.
as its now fair and provides a guarantee it will happen, unlike the
previous one.