linux-kernel - Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments in a structure

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Date:   Fri, 21 Jun 2019 10:15:01 +0200
From:   Christoph Hellwig <hch@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Christoph Hellwig <hch@....de>,
        James Hogan <jhogan@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-mips@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        linuxppc-dev@...ts.ozlabs.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Burton <paul.burton@...s.com>,
        Paul Mackerras <paulus@...ba.org>, sparclinux@...r.kernel.org,
        the arch/x86 maintainers <x86@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 16/16] mm: pass get_user_pages_fast iterator arguments
 in a structure

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.
> 
> 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?
> 
> 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).

I've not seen any difference in the GUP bench output here ar all.

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.