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

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

On 6/15/2024 2:31 PM, Mateusz Guzik wrote:
> On Fri, Jun 14, 2024 at 12:34:14PM -0400, 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) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
>> searching.
>>
>> With the fast path added in alloc_fd() through one-time bitmap searching,
>> pts/blogbench-1.1.0 read is improved by 20% and write by 10% on Intel ICX 160
>> cores configuration with v6.8-rc6.
>>
>> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@intel.com>
>> ---
>>   fs/file.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/file.c b/fs/file.c
>> index 3b683b9101d8..e8d2f9ef7fd1 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -510,8 +510,13 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>>   	if (fd < files->next_fd)
>>   		fd = files->next_fd;
>>   
>> -	if (fd < fdt->max_fds)
>> +	if (fd < fdt->max_fds) {
>> +		if (~fdt->open_fds[0]) {
>> +			fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);
>> +			goto success;
>> +		}
>>   		fd = find_next_fd(fdt, fd);
>> +	}
>>   
>>   	/*
>>   	 * N.B. For clone tasks sharing a files structure, this test
>> @@ -531,7 +536,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>>   	 */
>>   	if (error)
>>   		goto repeat;
>> -
>> +success:
>>   	if (start <= files->next_fd)
>>   		files->next_fd = fd + 1;
>>   
> As indicated in my other e-mail it may be a process can reach a certain
> fd number and then lower its rlimit(NOFILE). In that case the max_fds
> field can happen to be higher and the above patch will fail to check for
> the (fd < end) case.

Thanks for the good catch, replied in that mail thread for details.

>
>> -- 
>> 2.43.0
>>