linux-kernel - Re: [PATCH] fs/select.c: batch user writes in do_sys

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Message-ID: <87wo22n5ez.fsf@dja-thinkpad.axtens.net>
Date:   Thu, 13 Aug 2020 21:36:52 +1000
From:   Daniel Axtens <dja@...ens.net>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/select.c: batch user writes in do_sys_poll

Hi,

>> Seem like this could simply use a copy_to_user to further simplify
>> things?
>
> I'll benchmark it and find out.

I tried this:

        for (walk = head; walk; walk = walk->next) {
-               struct pollfd *fds = walk->entries;
-               int j;
-
-               for (j = 0; j < walk->len; j++, ufds++)
-                       if (__put_user(fds[j].revents, &ufds->revents))
-                               goto out_fds;
+               if (copy_to_user(ufds, walk->entries,
+                                sizeof(struct pollfd) * walk->len))
+                       goto out_fds;
+               ufds += walk->len;
        }

With that approach, the poll2 microbenchmark (which polls 128 fds) is
about as fast as v1.

However, the poll1 microbenchmark, which polls just 1 fd, regresses a
touch (<1% - ~2%) compared to the current code, although it's largely
within the noise. Thoughts?

Kind regards,
Daniel

>> Also please don't pointlessly add overly long lines.
>
> Weird, I ran the commit through checkpatch and it didn't pick it
> up. I'll check the next version more carefully.
>
> Regards,
> Daniel