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[209.85.208.171]) by smtp.gmail.com with ESMTPSA id s17sm8088541ljo.18.2020.02.24.12.17.06 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 Feb 2020 12:17:07 -0800 (PST) Received: by mail-lj1-f171.google.com with SMTP id n18so11559375ljo.7 for ; Mon, 24 Feb 2020 12:17:06 -0800 (PST) X-Received: by 2002:a2e:461a:: with SMTP id t26mr31070734lja.204.1582574960486; Mon, 24 Feb 2020 12:09:20 -0800 (PST) MIME-Version: 1.0 References: <20200205123216.GO12867@shao2-debian> <20200205125804.GM14879@hirez.programming.kicks-ass.net> <20200221080325.GA67807@shbuild999.sh.intel.com> <20200221132048.GE652992@krava> <20200223141147.GA53531@shbuild999.sh.intel.com> <20200224003301.GA5061@shbuild999.sh.intel.com> <20200224021915.GC5061@shbuild999.sh.intel.com> In-Reply-To: From: Linus Torvalds Date: Mon, 24 Feb 2020 12:09:04 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression To: Feng Tang Cc: Jiri Olsa , Peter Zijlstra , kernel test robot , Ingo Molnar , Vince Weaver , Jiri Olsa , Alexander Shishkin , Arnaldo Carvalho de Melo , Arnaldo Carvalho de Melo , "Naveen N. Rao" , Ravi Bangoria , Stephane Eranian , Thomas Gleixner , LKML , lkp@lists.01.org, andi.kleen@intel.com, "Huang, Ying" Content-Type: multipart/mixed; boundary="0000000000005a186a059f57f234" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --0000000000005a186a059f57f234 Content-Type: text/plain; charset="UTF-8" On Mon, Feb 24, 2020 at 11:24 AM Linus Torvalds wrote: > > I don't know. This does not seem to be a particularly serious load. > But it does feel like it should be possible to combine the two atomic > accesses into one, where you don't need to do the refcount thing > except for the case where sigcount goes from zero to non-zero (and > back to zero again). Ok, that looks just as simple as I thought it would be. TOTALLY UNTESTED patch attached. It may be completely buggy garbage, but it _looks_ trivial enough. Just make the rule be that "if we have any user->sigpending cases, we'll get a ref to the user for the first one, and drop it only when getting rid of the last one". So it might be worth testing this. But again: I have NOT done so. There might be some silly reason why this doesn't work because I just did the tests wrong or missed some case. Or there might be some subtle reason why it doesn't work because I didn't think this through properly. But it _looks_ obvious and simple enough. And it compiles for me. So maybe it works. 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