linux-kernel - Re: [PATCH -mm] percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari ly.fix

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Message-ID: <20121118193235.GA9859@redhat.com>
Date:	Sun, 18 Nov 2012 20:32:35 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mikulas Patocka <mpatocka@...hat.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 -mm]
	percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessari
	ly.fix

On 11/16, Andrew Morton wrote:
>
> On Sun, 11 Nov 2012 19:27:44 +0100
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> >  lib/percpu-rwsem.c |   35 +++++++++++++++++++++++++++++++++--
>
> y'know, this looks like a great pile of useless bloat for single-CPU
> machines.  Maybe add a CONFIG_SMP=n variant which simply calls the
> regular rwsem operations?

Yes, I thought about this and probably I'll send the patch...

But note that the regular down_read() won't be actually faster if
there is no writer, and it doesn't allow to add other features.

I'll try to think, perhaps it would be enough to add a couple of
"ifdef CONFIG_SMP" into this code, say, to avoid __percpu.

Oleg.

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