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

Jan Kara
2024-06-26T11:54:29Z

On Tue 25-06-24 23:33:34, Ma, Yu wrote:
> On 6/25/2024 8:53 PM, Jan Kara wrote:
> > On Tue 25-06-24 13:52:57, Jan Kara wrote:
> > > On Sat 22-06-24 11:49:02, Yu Ma wrote:
> > > > There is available fd in the lower 64 bits of open_fds bitmap for most cases
> > > > when we look for an available fd slot. Skip 2-levels searching via
> > > > find_next_zero_bit() for this common fast path.
> > > >
> > > > Look directly for an open bit in the lower 64 bits of open_fds bitmap when a
> > > > free slot is available there, as:
> > > > (1) The fd allocation algorithm would always allocate fd from small to large.
> > > > Lower bits in open_fds bitmap would be used much more frequently than higher
> > > > bits.
> > > > (2) 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.
> > > > (3) find_next_zero_bit() itself has a fast path inside to speed up searching
> > > > when size<=64.
> > > >
> > > > Besides, "!start" is added to fast path condition to ensure the allocated fd is
> > > > greater than start (i.e. >=0), given alloc_fd() is only called in two scenarios:
> > > > (1) Allocating a new fd (the most common usage scenario) via
> > > > get_unused_fd_flags() to find fd start from bit 0 in fdt (i.e. start==0).
> > > > (2) Duplicating a fd (less common usage) via dup_fd() to find a fd start from
> > > > old_fd's index in fdt, which is only called by syscall fcntl.
> > > >
> > > > With the fast path added in alloc_fd(), pts/blogbench-1.1.0 read is improved
> > > > by 17% and write by 9% on Intel ICX 160 cores configuration with v6.10-rc4.
> > > >
> > > > Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> > > > Signed-off-by: Yu Ma <yu.ma@intel.com>
> > > > ---
> > > > fs/file.c | 35 +++++++++++++++++++++--------------
> > > > 1 file changed, 21 insertions(+), 14 deletions(-)
> > > >
> > > > diff --git a/fs/file.c b/fs/file.c
> > > > index a3b72aa64f11..50e900a47107 100644
> > > > --- a/fs/file.c
> > > > +++ b/fs/file.c
> > > > @@ -515,28 +515,35 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> > > > if (fd < files->next_fd)
> > > > fd = files->next_fd;
> > > > - if (fd < fdt->max_fds)
> > > > + error = -EMFILE;
> > > > + if (likely(fd < fdt->max_fds)) {
> > > > + if (~fdt->open_fds[0] && !start) {
> > > > + fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);
> > > So I don't think this is quite correct. If files->next_fd is set, we could
> > > end up calling find_next_zero_bit() starting from quite high offset causing
> > > a regression? Also because we don't expand in this case, we could cause access
> > > beyond end of fdtable?
> > OK, I've misunderstood the next_fd logic. next_fd is the lowest 0-bit in
> > the open_fds bitmap so if next_fd is big, the ~fdt->open_fds[0] should
> > be false. As such the above condition could be rewritten as:
> >
> > if (!start && files->next_fd < BITS_PER_LONG)
> >
> > to avoid loading the first bitmap long if we know it is full? Or we could
> > maybe go as far as:
> >
> > if (!start && fd < BITS_PER_LONG && !test_bit(fd, fdt->open_fds))
> > goto fastreturn;
> >
> > because AFAIU this should work in exactly the same cases as your code?
> >
> > Honza
>
> Thanks Honza for the good concern and suggestions here, while both above
> conditions are not enough to ensure that there is available fd in the first
> 64 bits of open_fds. As next_fd just means there is no available fd before
> next_fd, just imagine that fd from 0 to 66 are already occupied, now fd=3 is
> returned back, then next_fd would be set as 3 per fd recycling logic (i.e.
> in __put_unused_fd()), next time when alloc_fd() being called, it would
> return fd=3 to the caller and set next_fd=4. Then next time when alloc_fd()
> being called again, next_fd==4, but actually it's already been occupied. So
> find_next_zero_bit() is needed to find the real 0 bit anyway.

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);
+ if (bit < BITS_PER_LONG)
+ return bit;
+ }
+
bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
if (bitbit >= maxfd)
return maxfd;

Plus your optimizations with likely / unlikely. This way the code flow in
alloc_fd() stays more readable, we avoid loading the first open_fds long
into cache if it is full, and we should get all the performance benefits?

Honza

> > > > + goto fastreturn;
> > > > + }
> > > > fd = find_next_fd(fdt, fd);
> > > > + }
> > > > +
> > > > + if (unlikely(fd >= fdt->max_fds)) {
> > > > + error = expand_files(files, fd);
> > > > + if (error < 0)
> > > > + goto out;
> > > > + /*
> > > > + * If we needed to expand the fs array we
> > > > + * might have blocked - try again.
> > > > + */
> > > > + if (error)
> > > > + goto repeat;
> > > > + }
> > > > +fastreturn:
> > > > /*
> > > > * N.B. For clone tasks sharing a files structure, this test
> > > > * will limit the total number of files that can be opened.
> > > > */
> > > > - error = -EMFILE;
> > > > - if (fd >= end)
> > > > + if (unlikely(fd >= end))
> > > > goto out;
> > > > - error = expand_files(files, fd);
> > > > - if (error < 0)
> > > > - goto out;
> > > > -
> > > > - /*
> > > > - * If we needed to expand the fs array we
> > > > - * might have blocked - try again.
> > > > - */
> > > > - if (error)
> > > > - goto repeat;
> > > > -
> > > > if (start <= files->next_fd)
> > > > files->next_fd = fd + 1;
> > > > --
> > > > 2.43.0
> > > >
> > > --
> > > Jan Kara <jack@suse.com>
> > > SUSE Labs, CR
> > >
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR