Re: [PATCH v4 3/3] fs/file.c: add fast path in find_next_fd()

Ma, Yu
2024-07-16T12:37:10Z

On 7/16/2024 7:19 PM, Jan Kara wrote:
> On Fri 12-07-24 22:39:17, Yu Ma 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.
>> (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@gmail.com>, and agreed by
>> Jan Kara <jack@suse.cz>, which is more generic and scalable than previous
>> versions. 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: Tim Chen <tim.c.chen@linux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@intel.com>
> Looks good. Just some code style nits below.

Copy that, thanks Honza, I'll revise and send out updated version soon.

>
>> diff --git a/fs/file.c b/fs/file.c
>> index 1be2a5bcc7c4..a3ce6ba30c8c 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -488,9 +488,20 @@ struct files_struct init_files = {
>>
>> static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
>> {
>> + 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));
> ^^ Either
> (BITS_PER_LONG-1) or (BITS_PER_LONG - 1) please. Your combination looks
> particularly weird :)
>
>> + if (bit < BITS_PER_LONG) {
>> + return bit + bitbit * BITS_PER_LONG;
>> + }
> No need for braces around the above block.
>
>> unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
>> unsigned int maxbit = maxfd / BITS_PER_LONG;
> We keep declarations at the beginning of the block. Usually it keeps the
> code more readable and the compiler should be clever enough to perform the
> loads & arithmetics only when needed.
>
> After fixing these style nits feel free to add:
>
> Reviewed-by: Jan Kara <jack@suse.cz>
>
> Honza

Yes, I'll polish this part of code accordingly, thanks for all the
comments here :)