linux-kernel - Re: [RFC]x86: clearing access bit don't flush tlb

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Message-ID: <50EBA9DC.9070400@redhat.com>
Date:	Tue, 08 Jan 2013 00:08:44 -0500
From:	Rik van Riel <riel@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Shaohua Li <shli@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...hat.com, hughd@...gle.com
Subject: Re: [RFC]x86: clearing access bit don't flush tlb

On 01/08/2013 12:03 AM, H. Peter Anvin wrote:
> On 01/07/2013 08:55 PM, Shaohua Li wrote:
>>
>> I searched a little bit, the change (doing TLB flush to clear access bit) is
>> made between 2.6.7 - 2.6.8, I can't find the changelog, but I found a patch:
>> http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.7-rc2/2.6.7-rc2-mm2/broken-out/mm-flush-tlb-when-clearing-young.patch
>>
>> The changelog declaims this is for arm/ppc/ppc64.
>>
>
> Not really.  It says that those have stumbled over it already.  It is
> true in general that this change will make very frequently used pages
> (which stick in the TLB) candidates for eviction.

That is only true if the pages were to stay in the TLB for a
very very long time.  Probably multiple seconds.

> x86 would seem to be just as affected, although possibly with a
> different frequency.
>
> Do we have any actual metrics on anything here?

I suspect that if we do need to force a TLB flush for page
reclaim purposes, it may make sense to do that TLB flush
asynchronously. For example, kswapd could kick off a TLB
flush of every CPU in the system once a second, when the
system is under pageout pressure.

We would have to do this in a smart way, so the kswapds
from multiple nodes do not duplicate the work.

If people want that kind of functionality, I would be
happy to cook up an RFC patch.

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