Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next_fd()

Ma, Yu
2024-07-20T12:57:13Z

On 7/20/2024 1:53 AM, Mateusz Guzik wrote:
> On Wed, Jul 17, 2024 at 4:24 PM Yu Ma <yu.ma@intel.com> 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.
> this is stale -- now the fast path searches up to 64 fds in the lower bitmap

Nope, this is still valid, as the searching size of the fast path inside
of find_next_fd() is always 64, it will execute the fast path inside of
find_next_zero_bit().

>
>> (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.
> I think this paragraph is droppable. You already got an ack from Jan
> below, so stating he agrees with the patch is redundant. As for me I
> don't think this warrants mentioning. Just remove it, perhaps
> Christian will be willing to massage it by himself to avoid another
> series posting.

The idea of fast path for the word contains next_fd is from you,
although this patch is small, I think it is reasonable to record here
out of my respect. Appreciate for your guide and comments on this patch,
I've learned a lot on the way of resolving problems :)

Regards

Yu

>> 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: Jan Kara <jack@suse.cz>
>> 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, 9 insertions(+)
>>
>> diff --git a/fs/file.c b/fs/file.c
>> index 1be2a5bcc7c4..729c07a4fc28 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -491,6 +491,15 @@ 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 bitmap
>> + */
>> + 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;
>>
>> bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
>> if (bitbit >= maxfd)
>> --
>> 2.43.0
>>
>