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

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Message-ID: <20160714185605.GL30154@twins.programming.kicks-ass.net>
Date:	Thu, 14 Jul 2016 20:56:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 Thu, Jul 14, 2016 at 08:43:51PM +0200, Oleg Nesterov wrote:
> 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.

Those would have to hit the same merge window though; some people (like
Arjan) really care about boot times and hunt and kill people adding
pointless delays..

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

Hmm, how so?