linux-kernel - Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next

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Message-ID: <2365dcaf-95d4-462b-9614-83ee9f7c12f6@intel.com>
Date: Sat, 20 Jul 2024 20:57:07 +0800
From: "Ma, Yu" <yu.ma@...el.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, jack@...e.cz, edumazet@...gle.com,
 linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
 pan.deng@...el.com, tianyou.li@...el.com, tim.c.chen@...el.com,
 tim.c.chen@...ux.intel.com, viro@...iv.linux.org.uk, yu.ma@...el.com
Subject: Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next_fd()


On 7/20/2024 1:53 AM, Mateusz Guzik wrote:
> On Wed, Jul 17, 2024 at 4:24 PM Yu Ma <yu.ma@...el.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@...il.com>, and agreed by
>> Jan Kara <jack@...e.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@...e.cz>
>> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@...el.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
>>
>