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Date:   Sat, 22 Jun 2019 09:55:09 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in
 a structure
To:     Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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        James Hogan <jhogan@kernel.org>,
        Khalid Aziz <khalid.aziz@oracle.com>,
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        the arch/x86 maintainers <x86@kernel.org>,
        Yoshinori Sato <ysato@users.sourceforge.jp>
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Christoph Hellwig's on June 21, 2019 6:15 pm:
> On Thu, Jun 20, 2019 at 10:21:46AM -0700, Linus Torvalds wrote:
>> Hmm. Honestly, I've never seen anything like that in any kernel profiles=
.
>>=20
>> Compared to the problems I _do_ see (which is usually the obvious
>> cache misses, and locking), it must either be in the noise or it's
>> some problem specific to whatever CPU you are doing performance work
>> on?
>>=20
>> I've occasionally seen pipeline hiccups in profiles, but it's usually
>> been either some serious glass jaw of the core, or it's been something
>> really stupid we did (or occasionally that the compiler did: one in
>> particular I remember was how there was a time when gcc would narrow
>> stores when it could, so if you set a bit in a word, it would do it
>> with a byte store, and then when you read the whole word afterwards
>> you'd get a major pipeline stall and it happened to show up in some
>> really hot paths).
>=20
> I've not seen any difference in the GUP bench output here ar all.
>=20
> But I'm fine with skipping this patch for now, I have a potential
> series I'm looking into that would benefit a lot from it, but we
> can discusss it in that context and make sure all the other works gets in
> in time.
>=20

If you can, that would be good. I don't like to object based on
handwaving so I'll see if I can find any benchmarks that will give
better confidence. Those old TPC-C tests were good, and there was
some DB2 workload that was the reason I added gup fast in the first
place. I'll do some digging.

Thanks,
Nick
=