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

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Message-ID: <e7718d5c-e160-4850-b980-3c8c227159a5@intel.com>
Date: Thu, 4 Jul 2024 22:50:58 +0800
From: "Ma, Yu" <yu.ma@...el.com>
To: Jan Kara <jack@...e.cz>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, mjguzik@...il.com,
 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 7/4/2024 6:03 PM, Jan Kara wrote:
> On Wed 03-07-24 10:33:11, 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.
>>
>> 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>
> Nice! The patch looks good to me. Feel free to add:
>
> Reviewed-by: Jan Kara <jack@...e.cz>
>
> One style nit below:
>
>> 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;
> Empty line here please to separate variable declaration and code...

Thanks Honza, copy that :)

>
>> +	bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
>> +	if (bit < BITS_PER_LONG)
>> +		return bit;
>> +
>>   	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;
> 									Honza