linux-kernel - Re: [PATCH 07/32] mm: Bring back vmalloc

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Message-ID: <ZGHOppBFcKEJkzCe@moria.home.lan>
Date:   Mon, 15 May 2023 02:18:14 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Lorenzo Stoakes <lstoakes@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-bcachefs@...r.kernel.org,
        Kent Overstreet <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec

On Sun, May 14, 2023 at 11:13:46PM -0700, Eric Biggers wrote:
> On Mon, May 15, 2023 at 01:38:51AM -0400, Kent Overstreet wrote:
> > On Sun, May 14, 2023 at 11:43:25AM -0700, Eric Biggers wrote:
> > > I think it would also help if the generated assembly had the handling of the
> > > fields interleaved.  To achieve that, it might be necessary to interleave the C
> > > code.
> > 
> > No, that has negligable effect on performance - as expected, for an out
> > of order processor. < 1% improvement.
> > 
> > It doesn't look like this approach is going to work here. Sadly.
> 
> I'd be glad to take a look at the code you actually tried.  It would be helpful
> if you actually provided it, instead of just this "I tried it, I'm giving up
> now" sort of thing.

https://evilpiepirate.org/git/bcachefs.git/log/?h=bkey_unpack

> I was also hoping you'd take the time to split this out into a userspace
> micro-benchmark program that we could quickly try different approaches on.

I don't need to, because I already have this:
https://evilpiepirate.org/git/ktest.git/tree/tests/bcachefs/perf.ktest

> BTW, even if people are okay with dynamic code generation (which seems
> unlikely?), you'll still need a C version for architectures that you haven't
> implemented the dynamic code generation for.

Excuse me? There already is a C version, and we've been discussing it.
Your approach wasn't any faster than the existing C version.