linux-kernel - Re: [GIT PULL] Ceph updates for 5.20-rc1

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Date:   Thu, 11 Aug 2022 14:52:26 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>
Subject: Re: [GIT PULL] Ceph updates for 5.20-rc1

On Thu, Aug 11, 2022 at 2:38 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Exporting the functionality?  Sure, no problem.  Uninlining that one...
> I suspect that it's OK, but I'd like to see profiling data; it's not
> as if it would be hard to return to having it inlined, obviously.

The only case where I think it might matter is in __d_lookup(), and
it's probably not measurable.

Yes, __d_lookup() does matter, but it only matters once you've fallen
out of RCU mode, and at that point the cost of the function call is
likely in the noise.

I don't particularly like how it's inside that dentry hash chain loop,
but realistically by then we've already done a function call for the
dentry lock spinlock, so that loop already has to deal with it.

Again, __d_lookup_rcu() is the place where adding a function call
would matter more, because that one really does show up on profiles
regularly.

And it so carefully tries to avoid function calls (but the
DCACHE_OP_COMPARE case causes problems: at one time a few years ago I
actually wanted to move the DCACHE_OP_COMPARE *out* of the loop
entirely, because it's loop invariant and having that unlikely cause
inside the loop still causes bad things for register allocation).

So I think the uninlining is fine. If I had been really unhappy about
it I would have undone the pull.

It was more the "I was told there would be cake, but there was no
cake" that annoyed me.

            Linus