linux-kernel - Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput -16.9% regression

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Date:   Wed, 15 Nov 2023 20:09:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     David Howells <dhowells@...hat.com>,
        kernel test robot <oliver.sang@...el.com>,
        oe-lkp@...ts.linux.dev, lkp@...el.com,
        linux-kernel@...r.kernel.org,
        Christian Brauner <brauner@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Christian Brauner <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        David Laight <David.Laight@...lab.com>, ying.huang@...el.com,
        feng.tang@...el.com, fengwei.yin@...el.com
Subject: Re: [linus:master] [iov_iter] c9eec08bac: vm-scalability.throughput
 -16.9% regression

On Wed, Nov 15, 2023 at 01:38:18PM -0500, Linus Torvalds wrote:
> It's probably completely broken. I particularly worry about "memcpy()"
> being used *during* the instruction rewriting, so the whole approach
> with ALTERNATIVE() is probably entirely broken.

Should we define an alternative_memcpy() which is used *only* during
rewriting so that this becomes a non-issue?

> But I'm cc'ing Borislav, because we've talked about the whole "inline
> memcpy() with alternatives" before, so maybe this gives Borislav some
> ideas for how to do it right.

Yours looks simple enough and makes sense. Lemme poke at it a bit in the
coming days and see what happens.

> Borislav, see
> 
>     https://lore.kernel.org/all/CAHk-=wjCUckvZUQf7gqp2ziJUWxVpikM_6srFdbcNdBJTxExRg@mail.gmail.com/
> 
> for some truly crazy code generation by gcc.

Yeah, lemme show that to gcc folks. That asm is with your compiler,
right? Version?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette