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Date:   Mon, 15 May 2023 00:13:43 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Kent Overstreet <kent.overstreet@linux.dev>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-bcachefs@vger.kernel.org,
        Kent Overstreet <kent.overstreet@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Uladzislau Rezki <urezki@gmail.com>, linux-mm@kvack.org
Subject: Re: [PATCH 07/32] mm: Bring back vmalloc_exec
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On Mon, May 15, 2023 at 02:18:14AM -0400, Kent Overstreet wrote:
> 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

Sure, given that this is an optimization problem with a very small scope
(decoding 6 fields from a bitstream), I was hoping for something easier and
faster to iterate on than setting up a full kernel + bcachefs test environment
and reverse engineering 500 lines of shell script.  But sure, I can look into
that when I have a chance.

> Your approach wasn't any faster than the existing C version.

Well, it's your implementation of what you thought was "my approach".  It
doesn't quite match what I had suggested.  As I mentioned in my last email, it's
also unclear that your new code is ever actually executed, since you made it
conditional on all fields being byte-aligned...

- Eric