linux-kernel - Re: [RFC PATCH] ARM: mm: disable kmap_high

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Date:	Thu, 7 Mar 2013 17:12:58 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ARM: mm: disable kmap_high_get() for SMP

Hello, Nicolas.

On Tue, Mar 05, 2013 at 05:36:12PM +0800, Nicolas Pitre wrote:
> On Mon, 4 Mar 2013, Joonsoo Kim wrote:
> 
> > With SMP and enabling kmap_high_get(), it makes users of kmap_atomic()
> > sequential ordered, because kmap_high_get() use global kmap_lock().
> > It is not welcome situation, so turn off this optimization for SMP.
> 
> I'm not sure I understand the problem.
> 
> The lock taken by kmap_high_get() is released right away before that 
> function returns and therefore this is not actually serializing 
> anything.

Yes, you understand what I want to say correctly.
Sorry for bad explanation.

Following is reasons why I send this patch with RFC tag.

If we have more cpus, performance degration is possible although
it is very short time to holding the lock in kmap_high_get().

And kmap has maximum 512 entries(512 * 4K = 2M) and some mobile devices 
has 2G memory(highmem 1G>), so probability for finding matched entry
is approximately < 1/512. This probability can be more decreasing
for device which have more memory. So I think that waste time to find
matched entry is more than saved time.

Above is my humble opinion, so please let me know what I am missing.

Thanks.

> 
> 
> Nicolas
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