Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next_fd()

Ma, Yu
2024-07-03T14:28:40Z

On 7/3/2024 10:17 PM, Mateusz Guzik wrote:
> On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@intel.com> 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) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
>> searching.
>>
>> As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@suse.cz>,
>> update the fast path from alloc_fd() to find_next_fd(). With which, on top of
>> patch 1 and 2, pts/blogbench-1.1.0 read is improved by 13% and write by 7% on
>> Intel ICX 160 cores configuration with v6.10-rc6.
>>
>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@intel.com>
>> ---
>> fs/file.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>>
>> diff --git a/fs/file.c b/fs/file.c
>> index a15317db3119..f25eca311f51 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -488,6 +488,11 @@ struct files_struct init_files = {
>>
>> static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
>> {
>> + unsigned int bit;
>> + bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
>> + if (bit < BITS_PER_LONG)
>> + return bit;
>> +
> The rest of the patchset looks good on cursory read.
>
> As for this one, the suggestion was to make it work across the entire range.
>
> Today I wont have time to write and test what we proposed, but will
> probably find some time tomorrow. Perhaps Jan will do the needful(tm)
> in the meantime.
>
> That said, please stay tuned for a patch. :)

Sure, understood, Guzik, thanks for the quick feedback and consideration
of chances to make it better and more versatile. I'll also give a try to
double check previous proposal on entire fds range.

>> 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;
>> --
>> 2.43.0
>>
>