linux-kernel - Re: [PATCH] xfs: Use xchg() in xlog_cil_insert_pcp

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Message-ID: <ad32f0aa-79df-41b2-90d0-9d98de695a18@riscstar.com>
Date: Wed, 20 Nov 2024 09:34:11 -0600
From: Alex Elder <elder@...cstar.com>
To: Uros Bizjak <ubizjak@...il.com>, linux-xfs@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: Chandan Babu R <chandan.babu@...cle.com>,
 "Darrick J. Wong" <djwong@...nel.org>, Christoph Hellwig
 <hch@...radead.org>, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] xfs: Use xchg() in xlog_cil_insert_pcp_aggregate()

On 11/20/24 9:06 AM, Uros Bizjak wrote:
> try_cmpxchg() loop with constant "new" value can be substituted
> with just xchg() to atomically get and clear the location.

You're right.  With a constant new value (0), there is no need
to loop to ensure we get a "stable" update.

Is the READ_ONCE() is still needed?

					-Alex

> The code on x86_64 improves from:
> 
>      1e7f:	48 89 4c 24 10       	mov    %rcx,0x10(%rsp)
>      1e84:	48 03 14 c5 00 00 00 	add    0x0(,%rax,8),%rdx
>      1e8b:	00
> 			1e88: R_X86_64_32S	__per_cpu_offset
>      1e8c:	8b 02                	mov    (%rdx),%eax
>      1e8e:	41 89 c5             	mov    %eax,%r13d
>      1e91:	31 c9                	xor    %ecx,%ecx
>      1e93:	f0 0f b1 0a          	lock cmpxchg %ecx,(%rdx)
>      1e97:	75 f5                	jne    1e8e <xlog_cil_commit+0x84e>
>      1e99:	48 8b 4c 24 10       	mov    0x10(%rsp),%rcx
>      1e9e:	45 01 e9             	add    %r13d,%r9d
> 
> to just:
> 
>      1e7f:	48 03 14 cd 00 00 00 	add    0x0(,%rcx,8),%rdx
>      1e86:	00
> 			1e83: R_X86_64_32S	__per_cpu_offset
>      1e87:	31 c9                	xor    %ecx,%ecx
>      1e89:	87 0a                	xchg   %ecx,(%rdx)
>      1e8b:	41 01 cb             	add    %ecx,%r11d
> 
> No functional change intended.
> 
> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Chandan Babu R <chandan.babu@...cle.com>
> Cc: "Darrick J. Wong" <djwong@...nel.org>
> Cc: Christoph Hellwig <hch@...radead.org>
> Cc: Dave Chinner <dchinner@...hat.com>
> ---
>   fs/xfs/xfs_log_cil.c | 5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 80da0cf87d7a..9d667be1d909 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -171,11 +171,8 @@ xlog_cil_insert_pcp_aggregate(
>   	 */
>   	for_each_cpu(cpu, &ctx->cil_pcpmask) {
>   		struct xlog_cil_pcp	*cilpcp = per_cpu_ptr(cil->xc_pcp, cpu);
> -		int			old = READ_ONCE(cilpcp->space_used);
>   
> -		while (!try_cmpxchg(&cilpcp->space_used, &old, 0))
> -			;
> -		count += old;
> +		count += xchg(&cilpcp->space_used, 0);
>   	}
>   	atomic_add(count, &ctx->space_used);
>   }