linux-kernel - Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

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Message-ID: <20160714184351.GA18388@redhat.com>
Date:	Thu, 14 Jul 2016 20:43:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, tj@...nel.org,
	paulmck@...ux.vnet.ibm.com, john.stultz@...aro.org,
	dimitrysh@...gle.com, romlem@...gle.com, ccross@...gle.com,
	tkjos@...gle.com
Subject: Re: [PATCH 2/2] locking/percpu-rwsem: Introduce bias knob

On 07/14, Peter Zijlstra wrote:
>
> The current percpu-rwsem read side is entirely free of serializing
> instructions at the cost of having a synchronize_sched() in the write
> path.
>
> The latency of the synchronize_sched() is too high for some users
> (cgroups), so provide a __percpu_init_rwsem(.bias) argument to forgot
> this synchronize_sched() at the cost of forcing all readers into the
> slow path, which has serializing instructions.

Oh well... I personally do not think this is what we want... Can't
we just add the stupid rcu_sync_enter() into cgroup_init() at least
for now? Yes, this means the unnecessary .sync() at boot time, but
it will go away after cleanups I am going to send.

Because, again, we will probably want to change this bias dynamically.

Oleg.