linux-kernel - Re: [fget] 054aa8d439: will-it-scale.per_thread

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Message-ID: <CAG48ez2YE08+vo+E+ZtxeikN4vVCJC+-BrWJUYWb0f0vRA0Uug@mail.gmail.com>
Date:   Sat, 11 Dec 2021 00:29:32 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Feng Tang <feng.tang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>,
        fengwei.yin@...el.com
Subject: Re: [fget] 054aa8d439: will-it-scale.per_thread_ops -5.7% regression

On Fri, Dec 10, 2021 at 10:59 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Fri, Dec 10, 2021 at 1:25 PM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > We could make a special light-weight version of files_lookup_fd_raw(),
> > I guess. We don't need the *whole* "look it up again".  We don't need
> > to re-check the array bounds, and we don't need to do the nospec
> > lookup - we would have triggered a NULL file pointer if that happened
> > the first time around.
> >
> > So all we'd need to do is "check that fdt is the same, and check that
> > fdt->fd[fd] is the same".
>
> This is an ENTIRELY UNTESTED patch to do that.
>
> It basically rewrites __fget_files() from scratch: it really wants to
> do the fd array lookup by hand, in order to cache the intermediate fdt
> pointer, and in order to cache the intermediate speculation-safe fd
> array index etc.
>
> It's not a very complicated function, and rewriting it actually cleans
> up the loop to not need the ugly goto.
>
> I made it use a helper wrapper function for the rcu locking, so that
> the "meat" of the function can just use plain "return NULL" for the
> error cases.
>
> However, not only is it entirely untested, this rewrite also means
> that gcc has now decided that the result is so simple and clear that
> it will inline it into all the callers.
>
> I guess that's a good sign - writing the code in a way that makes the
> compiler say "now it's so trivial that it should be inlined" is
> certainly not a bad thing. But it makes it hard to really compare the
> asm.
>
> I did try a version with "noinline" just to make it more comparable,
> and hey, it all looked sane to me there too.
>
> I added more comments about what is going on.
>
> Again - this is UNTESTED. I've looked at the code, I've looked at the
> diff, and I've looked at the code it generates. It all looks fine to
> me. But I've looked at it so much that I suspect that I'd be entirely
> blind to any completely obvious bug by now.
>
> Comments?

One nit: The original implementation is using rcu_dereference_raw()
because it can run in different contexts, but here plain
rcu_dereference() would probably be more appropriate?

(I was wondering for a bit whether we should also change the
get_mm_exe_file() path, but I guess that's fine because it can only
ever happen for regular executable files and currently there's also no
path to pull out the mm->exe_file and use it for some other syscall?)

> Oliver, does this make any difference in the performance department?
>
>                  Linus