Re: [PATCH] fs/select.c: batch user writes in do_sys_poll

[Daniel Axtens <dja@axtens.net>]

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