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

["Ma, Yu" <yu.ma@intel.com>]

On 7/5/2024 5:55 AM, Jan Kara wrote:
> On Thu 04-07-24 19:44:10, 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;
>>> +
>>>          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
>>>
>> I had something like this in mind:
>> diff --git a/fs/file.c b/fs/file.c
>> index a3b72aa64f11..4d3307e39db7 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -489,6 +489,16 @@ 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 map.
>> +        */
>> +       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;
>> +       }
> Drat, you're right. I missed that Ma did not add the proper offset to
> open_fds. *This* is what I meant :)
>
> 								Honza

Just tried this on v6.10-rc6, the improvement on top of patch 1 and 
patch 2 is 7% for read and 3% for write, less than just check first word.

Per my understanding, its performance would be better if we can find 
free bit in the same word of next_fd with high possibility, but next_fd 
just represents the lowest possible free bit. If fds are open/close 
frequently and randomly, that might not always be the case, next_fd may 
be distributed randomly, for example, 0-65 are occupied, fd=3 is 
returned, next_fd will be set to 3, next time when 3 is allocated, 
next_fd will be set to 4, while the actual first free bit is 66 , when 
66 is allocated, and fd=5 is returned, then the above process would be 
went through again.

Yu