linux-kernel - Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

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Date:	Tue, 23 Oct 2012 22:45:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anton Arapov <anton@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] percpu-rw-semaphores: use light/heavy barriers

On Tue, 2012-10-23 at 21:23 +0200, Oleg Nesterov wrote:
> I have to admit, I have
> no idea how much cli/sti is slower compared to preempt_disable/enable.
> 
A lot.. esp on stupid hardware (insert pentium-4 reference), but I think
its more expensive for pretty much all hardware, preempt_disable() is
only a non-atomic cpu local increment and a compiler barrier, enable the
same and a single conditional.
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