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[49.180.121.96]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2ead03de6f6sm1762377a91.34.2024.11.20.12.37.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 20 Nov 2024 12:37:20 -0800 (PST) Received: from dave by dread.disaster.area with local (Exim 4.98) (envelope-from ) id 1tDrRt-000000010Wl-2glT; Thu, 21 Nov 2024 07:37:17 +1100 Date: Thu, 21 Nov 2024 07:37:17 +1100 From: Dave Chinner To: Uros Bizjak Cc: linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, Chandan Babu R , "Darrick J. Wong" , Christoph Hellwig , Dave Chinner Subject: Re: [PATCH] xfs: Use xchg() in xlog_cil_insert_pcp_aggregate() Message-ID: References: <20241120150725.3378-1-ubizjak@gmail.com> Precedence: bulk X-Mailing-List: linux-xfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20241120150725.3378-1-ubizjak@gmail.com> On Wed, Nov 20, 2024 at 04:06:22PM +0100, Uros Bizjak wrote: > try_cmpxchg() loop with constant "new" value can be substituted > with just xchg() to atomically get and clear the location. > > 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 > 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 > Cc: Chandan Babu R > Cc: "Darrick J. Wong" > Cc: Christoph Hellwig > Cc: Dave Chinner > --- > 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); > } Looks fine. Reviewed-by: Dave Chinner -- Dave Chinner david@fromorbit.com