Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc_fd()

Tim Chen
2024-06-27T16:21:41Z

On Thu, 2024-06-27 at 14:09 +0200, Jan Kara wrote:
> On Wed 26-06-24 09:52:50, Tim Chen wrote:
> > On Wed, 2024-06-26 at 09:43 -0700, Tim Chen wrote:
> > > On Wed, 2024-06-26 at 13:54 +0200, Jan Kara wrote:
> > > >
> > > >
> > > > Indeed, thanks for correcting me! next_fd is just a lower bound for the
> > > > first free fd.
> > > >
> > > > > The conditions
> > > > > should either be like it is in patch or if (!start && !test_bit(0,
> > > > > fdt->full_fds_bits)), the latter should also have the bitmap loading cost,
> > > > > but another point is that a bit in full_fds_bits represents 64 bits in
> > > > > open_fds, no matter fd >64 or not, full_fds_bits should be loaded any way,
> > > > > maybe we can modify the condition to use full_fds_bits ?
> > > >
> > > > So maybe I'm wrong but I think the biggest benefit of your code compared to
> > > > plain find_next_fd() is exactly in that we don't have to load full_fds_bits
> > > > into cache. So I'm afraid that using full_fds_bits in the condition would
> > > > destroy your performance gains. Thinking about this with a fresh head how
> > > > about putting implementing your optimization like:
> > > >
> > > > --- a/fs/file.c
> > > > +++ b/fs/file.c
> > > > @@ -490,6 +490,20 @@ static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
> > > > unsigned int maxbit = maxfd / BITS_PER_LONG;
> > > > unsigned int bitbit = start / BITS_PER_LONG;
> > > >
> > > > + /*
> > > > + * Optimistically search the first long of the open_fds bitmap. It
> > > > + * saves us from loading full_fds_bits into cache in the common case
> > > > + * and because BITS_PER_LONG > start >= files->next_fd, we have quite
> > > > + * a good chance there's a bit free in there.
> > > > + */
> > > > + if (start < BITS_PER_LONG) {
> > > > + unsigned int bit;
> > > > +
> > > > + bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
> > >
> > > Say start is 31 (< BITS_PER_LONG)
> > > bit found here could be 32 and greater than start. Do we care if we return bit > start?
> >
> > Sorry, I mean to say that we could find a bit like 30 that is less than
> > start instead of the other way round.
>
> Well, I propose calling find_next_zero_bit() with offset set to 'start' so
> it cannot possibly happen that the returned bit number is smaller than
> start... But maybe I'm missing something?

Yes, you're right. the search begin at start bit so it is okay.

Tim

>
> Honza