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[209.85.208.176]) by smtp.gmail.com with ESMTPSA id j2sm653606lji.115.2020.07.24.19.46.38 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 24 Jul 2020 19:46:38 -0700 (PDT) Received: by mail-lj1-f176.google.com with SMTP id s9so11808083ljm.11 for ; Fri, 24 Jul 2020 19:46:38 -0700 (PDT) X-Received: by 2002:a2e:86c4:: with SMTP id n4mr5814202ljj.312.1595645198300; Fri, 24 Jul 2020 19:46:38 -0700 (PDT) MIME-Version: 1.0 References: <20200723124749.GA7428@redhat.com> <20200724152424.GC17209@redhat.com> In-Reply-To: From: Linus Torvalds Date: Fri, 24 Jul 2020 19:46:21 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page To: Hugh Dickins Cc: Oleg Nesterov , Michal Hocko , Linux-MM , LKML , Andrew Morton , Tim Chen , Michal Hocko Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 24, 2020 at 7:08 PM Hugh Dickins wrote: > > But whatever, what happens on the next run, with these latest patches, > will be more important; and I'll follow this next run with a run on > the baseline without them, to compare results. So the loads you are running are known to have sensitivity to this particular area, and are why you've done your patches to the page wait bit code? Because yes, I think that last version in particular might make a big difference with the "let people continue even if they only saw the wakeup event, and never actually tested and saw the bit clear". Of course, there's a possibility that "big difference" ends up being a negative one. I think it will make the page wait queues shorter (which is good for that latency and lockup thing), but waking things up more aggressively _may_ also end up adding more CPU load, if they then all decide to retry the operation for whatever reason. And hey, it's also possible that it makes no difference at all, because your load mainly tests the exclusive "lock_page()" case, which won't have changed. And that's all assuming they don't show some instability, of course. But the code actually seems fairly simple now, and the basic synchronization hasn't changed, just a behavioral optimization. Famous last words. Linus