Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next_fd()

[Ma, Yu]

On 7/20/2024 1:53 AM, Mateusz Guzik wrote:
> On Wed, Jul 17, 2024 at 4:24 PM Yu Ma <yu.ma@xxxxxxxxx> wrote:
> > Skip 2-levels searching via find_next_zero_bit() when there is free slot in the
> > word contains next_fd, as:
> > (1) next_fd indicates the lower bound for the first free fd.
> > (2) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
> > searching.
> this is stale -- now the fast path searches up to 64 fds in the lower bitmap

Nope, this is still valid, as the searching size of the fast path inside 
of find_next_fd() is always 64, it will execute the fast path inside of 
find_next_zero_bit().


> > (3) After fdt is expanded (the bitmap size doubled for each time of expansion),
> > it would never be shrunk. The search size increases but there are few open fds
> > available here.
> >
> > This fast path is proposed by Mateusz Guzik <mjguzik@xxxxxxxxx>, and agreed by
> > Jan Kara <jack@xxxxxxx>, which is more generic and scalable than previous
> > versions.
> I think this paragraph is droppable. You already got an ack from Jan
> below, so stating he agrees with the patch is redundant. As for me I
> don't think this warrants mentioning. Just remove it, perhaps
> Christian will be willing to massage it by himself to avoid another
> series posting.

The idea of fast path for the word contains next_fd is from you, 
although this patch is small, I think it is reasonable to record here 
out of my respect. Appreciate for your guide and comments on this patch, 
I've learned a lot on the way of resolving problems :)


Regards

Yu

> > And on top of patch 1 and 2, it improves pts/blogbench-1.1.0 read by
> > 8% and write by 4% on Intel ICX 160 cores configuration with v6.10-rc7.
> >
> > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> > ---
> >   fs/file.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/fs/file.c b/fs/file.c
> > index 1be2a5bcc7c4..729c07a4fc28 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -491,6 +491,15 @@ static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
> >          unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
> >          unsigned int maxbit = maxfd / BITS_PER_LONG;
> >          unsigned int bitbit = start / BITS_PER_LONG;
> > +       unsigned int bit;
> > +
> > +       /*
> > +        * Try to avoid looking at the second level bitmap
> > +        */
> > +       bit = find_next_zero_bit(&fdt->open_fds[bitbit], BITS_PER_LONG,
> > +                                start & (BITS_PER_LONG - 1));
> > +       if (bit < BITS_PER_LONG)
> > +               return bit + bitbit * BITS_PER_LONG;
> >
> >          bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
> >          if (bitbit >= maxfd)
> > --
> > 2.43.0