linux-kernel - Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs without ERMS

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Message-ID: <CAGudoHGsuARwuMWa=2U9D2AY4bQPv0t1HOZ0_TocNLsB5yAf9g@mail.gmail.com>
Date:   Sun, 3 Sep 2023 22:58:32 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        bp@...en8.de
Subject: Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs
 without ERMS

On 9/3/23, Mateusz Guzik <mjguzik@...il.com> wrote:
> On Sun, Sep 03, 2023 at 01:08:03PM -0700, Linus Torvalds wrote:
>> On Sun, 3 Sept 2023 at 11:49, Linus Torvalds
>> <torvalds@...ux-foundation.org> wrote:
>> >
>> > So I have no idea why you claim that "currently they have no choice".
>> > glibc is simply being incredibly stupid, and using newfstatat() for no
>> > good reason.
>>
>> Do you have any good benchmark that shows the effects of this?
>>
>> And if you do, does the attached patch (ENTIRELY UNTESTED!) fix the
>> silly glibc mis-feature?
>>
>
> "real fstat" is syscall(5, fd, &sb).
>
> Sapphire Rapids, will-it-scale, ops/s
>
> stock fstat	5088199
> patched fstat	7625244	(+49%)
> real fstat	8540383	(+67% / +12%)
>
> It dodges lockref et al, but it does not dodge SMAP which accounts for
> the difference.
>

I'm going to ask glibc people what's up with the change later.

I'll note statx does not have a dedicated entry point (I checked again
;)) which elides path lookup. It *did* have a way, but it got removed
in 1e2f82d1e9d1 ("statx: Kill fd-with-NULL-path support in favour of
AT_EMPTY_PATH").

One frequent consumer I know of is Rust.

Would be nice to have a non-hacky way to sort it out.

-- 
Mateusz Guzik <mjguzik gmail.com>