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[49.181.88.204]) by smtp.gmail.com with ESMTPSA id v24-20020a17090ac91800b0024df90a4c58sm11023412pjt.36.2023.05.09.00.10.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 May 2023 00:10:55 -0700 (PDT) Received: from dave by dread.disaster.area with local (Exim 4.92.3) (envelope-from ) id 1pwHUr-00D8HE-7l; Tue, 09 May 2023 17:10:53 +1000 Date: Tue, 9 May 2023 17:10:53 +1000 From: Dave Chinner To: kernel test robot Cc: Dave Chinner , oe-lkp@lists.linux.dev, lkp@intel.com, linux-kernel@vger.kernel.org, "Darrick J. Wong" , linux-xfs@vger.kernel.org, ying.huang@intel.com, feng.tang@intel.com, fengwei.yin@intel.com Subject: Re: [linus:master] [xfs] 2edf06a50f: fsmark.files_per_sec -5.7% regression Message-ID: <20230509071053.GE2651828@dread.disaster.area> References: <202305090905.aff4e0e6-oliver.sang@intel.com> <20230509065433.GT3223426@dread.disaster.area> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230509065433.GT3223426@dread.disaster.area> Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Tue, May 09, 2023 at 04:54:33PM +1000, Dave Chinner wrote: > On Tue, May 09, 2023 at 10:13:19AM +0800, kernel test robot wrote: > > > > > > Hello, > > > > kernel test robot noticed a -5.7% regression of fsmark.files_per_sec on: > > > > > > commit: 2edf06a50f5bbe664283f3c55c480fc013221d70 ("xfs: factor xfs_alloc_vextent_this_ag() for _iterate_ags()") > > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master > > This is just a refactoring patch and doesn't change any logic. > Hence I'm sceptical that it actually resulted in a performance > regression. Indeed, the profile indicates a significant change of > behaviour in the allocator and I can't see how the commit above > would cause anything like that. > > Was this a result of a bisect? If so, what were the original kernel > versions where the regression was detected? Oh, CONFIG_XFS_DEBUG=y, which means: static int xfs_alloc_ag_vextent_lastblock( struct xfs_alloc_arg *args, struct xfs_alloc_cur *acur, xfs_agblock_t *bno, xfs_extlen_t *len, bool *allocated) { int error; int i; #ifdef DEBUG /* Randomly don't execute the first algorithm. */ if (get_random_u32_below(2)) return 0; #endif We randomly chose a near block allocation strategy to use to improve code coverage, not the optimal one for IO performance. Hence the CPU usage and allocation patterns that impact IO performance are simply not predictable or reproducable from run to run. So, yeah, trying to bisect a minor difference in performance as a result of this randomness will not be reliable.... Cheers, Dave. -- Dave Chinner david@fromorbit.com