linux-kernel - Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next

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Message-ID: <9c57d47b-8e77-49f6-ab31-8899d6b845b8@intel.com>
Date: Wed, 3 Jul 2024 22:28:33 +0800
From: "Ma, Yu" <yu.ma@...el.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: viro@...iv.linux.org.uk, 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, yu.ma@...el.com
Subject: Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next_fd()


On 7/3/2024 10:17 PM, Mateusz Guzik wrote:
> On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@...el.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@...e.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@...ux.intel.com>
>> Signed-off-by: Yu Ma <yu.ma@...el.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;
>> +
> The rest of the patchset looks good on cursory read.
>
> As for this one, the suggestion was to make it work across the entire range.
>
> Today I wont have time to write and test what we proposed, but will
> probably find some time tomorrow. Perhaps Jan will do the needful(tm)
> in the meantime.
>
> That said, please stay tuned for a patch. :)

Sure, understood, Guzik, thanks for the quick feedback and consideration 
of chances to make it better and more versatile. I'll also give a try to 
double check previous proposal on entire fds range.

>>          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
>>
>