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[209.85.167.44]) by smtp.gmail.com with ESMTPSA id m28sm322489lfj.173.2021.05.24.20.11.53 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 May 2021 20:11:53 -0700 (PDT) Received: by mail-lf1-f44.google.com with SMTP id q1so1268742lfo.3 for ; Mon, 24 May 2021 20:11:53 -0700 (PDT) X-Received: by 2002:a05:6512:36c5:: with SMTP id e5mr12425672lfs.41.1621912313387; Mon, 24 May 2021 20:11:53 -0700 (PDT) MIME-Version: 1.0 References: <20210525031636.GB7744@xsang-OptiPlex-9020> In-Reply-To: <20210525031636.GB7744@xsang-OptiPlex-9020> From: Linus Torvalds Date: Mon, 24 May 2021 17:11:37 -1000 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [mm/gup] 57efa1fe59: will-it-scale.per_thread_ops -9.2% regression To: kernel test robot Cc: Jason Gunthorpe , John Hubbard , Jan Kara , Peter Xu , Andrea Arcangeli , "Aneesh Kumar K.V" , Christoph Hellwig , Hugh Dickins , Jann Horn , Kirill Shutemov , Kirill Tkhai , Leon Romanovsky , Michal Hocko , Oleg Nesterov , Andrew Morton , LKML , lkp@lists.01.org, kernel test robot , "Huang, Ying" , Feng Tang , zhengjun.xing@intel.com Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 24, 2021 at 5:00 PM kernel test robot wrote: > > FYI, we noticed a -9.2% regression of will-it-scale.per_thread_ops due to commit: > commit: 57efa1fe5957694fa541c9062de0a127f0b9acb0 ("mm/gup: prevent gup_fast from racing with COW during fork") Hmm. This looks like one of those "random fluctuations" things. It would be good to hear if other test-cases also bisect to the same thing, but this report already says: > In addition to that, the commit also has significant impact on the following tests: > > +------------------+---------------------------------------------------------------------------------+ > | testcase: change | will-it-scale: will-it-scale.per_thread_ops 3.7% improvement | which does kind of reinforce that "this benchmark gives unstable numbers". The perf data doesn't even mention any of the GUP paths, and on the pure fork path the biggest impact would be: (a) maybe "struct mm_struct" changed in size or had a different cache layout (b) two added (nonatomic) increment operations in the fork path due to the seqcount and I'm not seeing what would cause that 9% change. Obviously cache placement has done it before. If somebody else sees something that I'm missing, please holler. But I'll ignore this as "noise" otherwise. Linus