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

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Date:   Thu, 13 Aug 2020 14:00:44 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Daniel Axtens <dja@...ens.net>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/select.c: batch user writes in do_sys_poll

On Thu, Aug 13, 2020 at 09:36:52PM +1000, Daniel Axtens wrote:
> 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?

I'd go with copy_to_user() here; post such variant and I'll throw it into
-next after -rc1.