linux-kernel - Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access

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Date:	Fri, 26 Oct 2012 09:16:33 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Michel Lespinasse <walken@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 05/31] x86/mm: Reduce tlb flushes from ptep_set_access_flags()

On 10/26/2012 08:48 AM, Andi Kleen wrote:
> Michel Lespinasse <walken@...gle.com> writes:
>
>> On Thu, Oct 25, 2012 at 9:23 PM, Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>>> On Thu, Oct 25, 2012 at 8:57 PM, Rik van Riel <riel@...hat.com> wrote:
>>>>
>>>> That may not even be needed.  Apparently Intel chips
>>>> automatically flush an entry from the TLB when it
>>>> causes a page fault.  I assume AMD chips do the same,
>>>> because flush_tlb_fix_spurious_fault evaluates to
>>>> nothing on x86.
>>>
>>> Yes. It's not architected as far as I know, though. But I agree, it's
>>> possible - even likely - we could avoid TLB flushing entirely on x86.
>>
>> Actually, it is architected on x86. This was first described in the
>> intel appnote 317080 "TLBs, Paging-Structure Caches, and Their
>> Invalidation", last paragraph of section 5.1. Nowadays, the same
>> contents are buried somewhere in Volume 3 of the architecture manual
>> (in my copy: 4.10.4.1 Operations that Invalidate TLBs and
>> Paging-Structure Caches)
>
> This unfortunately would only work for processes with no threads
> because it only works on the current logical CPU.

That is fine.

Potentially triggering a spurious page fault on
another CPU is bound to be better than always
doing a synchronous remote TLB flush, waiting
for who knows how many CPUs to acknowledge the
IPI...

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