linux-kernel - Re: [PATCH] fs: use wq_has_sleeper() in end_dir

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Message-ID: <apadfckxgxx46eten4sftyiay5nnbuopnph5oagnch6lyrtd3r@cgpwxge6bzs3>
Date: Mon, 17 Mar 2025 18:03:04 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: use wq_has_sleeper() in end_dir_add()

On Mon 17-03-25 00:24:21, Mateusz Guzik wrote:
> The routine is used a lot, while the wakeup almost never has anyone to
> deal with.
> 
> wake_up_all() takes an irq-protected spinlock, wq_has_sleeper() "only"
> contains a full fence -- not free by any means, but still cheaper.
> 
> Sample result tracing waiters using a custom probe during -j 20 kernel
> build (0 - no waiters, 1 - waiters):
> 
> @[
>     wakeprobe+5
>     __wake_up_common+63
>     __wake_up+54
>     __d_add+234
>     d_splice_alias+146
>     ext4_lookup+439
>     path_openat+1746
>     do_filp_open+195
>     do_sys_openat2+153
>     __x64_sys_openat+86
>     do_syscall_64+82
>     entry_SYSCALL_64_after_hwframe+118
> ]:
> [0, 1)             13999 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [1, ...)               1 |                                                    |
> 
> So that 14000 calls in total from this backtrace, where only one time
> had a waiter.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/dcache.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index df8833fe9986..bd5aa136153a 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -2497,7 +2497,8 @@ static inline void end_dir_add(struct inode *dir, unsigned int n,
>  {
>  	smp_store_release(&dir->i_dir_seq, n + 2);
>  	preempt_enable_nested();
> -	wake_up_all(d_wait);
> +	if (wq_has_sleeper(d_wait))
> +		wake_up_all(d_wait);
>  }
>  
>  static void d_wait_lookup(struct dentry *dentry)
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR