linux-kernel - Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next

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Message-ID: <CAGudoHGCZZwQ9EC3aW5bS4Vur7-UHgubLvTQuZa8ct=+m8-fTg@mail.gmail.com>
Date: Sat, 20 Jul 2024 16:22:27 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: "Ma, Yu" <yu.ma@...el.com>
Cc: brauner@...nel.org, jack@...e.cz, edumazet@...gle.com, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	pan.deng@...el.com, tianyou.li@...el.com, tim.c.chen@...el.com, 
	tim.c.chen@...ux.intel.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next_fd()

I think this is getting too much fluff traffic at this point, which is
partially my fault.

I'm buggering off.

Overall the patchset looks good, I don't see any technical reasons to
avoid merging it.

On Sat, Jul 20, 2024 at 2:57 PM Ma, Yu <yu.ma@...el.com> wrote:
>
>
> On 7/20/2024 1:53 AM, Mateusz Guzik wrote:
> > On Wed, Jul 17, 2024 at 4:24 PM Yu Ma <yu.ma@...el.com> 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@...il.com>, and agreed by
> >> Jan Kara <jack@...e.cz>, 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@...e.cz>
> >> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> >> Signed-off-by: Yu Ma <yu.ma@...el.com>
> >> ---
> >>   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
> >>
> >



-- 
Mateusz Guzik <mjguzik gmail.com>