linux-kernel - Re: [PATCH v3 1/1] percpu_rw_semaphore: reimplement to not block the readers unnecessarily

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Date:	Wed, 7 Nov 2012 14:17:01 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.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 v3 1/1] percpu_rw_semaphore: reimplement to not block
 the readers unnecessarily



On Wed, 7 Nov 2012, Oleg Nesterov wrote:

> On 11/07, Mikulas Patocka wrote:
> >
> > It looks sensible.
> >
> > Here I'm sending an improvement of the patch - I changed it so that there
> > are not two-level nested functions for the fast path and so that both
> > percpu_down_read and percpu_up_read use the same piece of code (to reduce
> > cache footprint).
> 
> IOW, the only change is that you eliminate "static update_fast_ctr()"
> and fold it into down/up_read which takes the additional argument.
> 
> Honestly, personally I do not think this is better, but I won't argue.
> I agree with everything but I guess we need the ack from Paul.

If you look at generated assembly (for x86-64), the footprint of my patch 
is 78 bytes shared for both percpu_down_read and percpu_up_read.

The footprint of your patch is 62 bytes for update_fast_ctr, 46 bytes for 
percpu_down_read and 20 bytes for percpu_up_read.

Mikulas
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