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[96.246.38.36]) by smtp.gmail.com with ESMTPSA id t11-v6sm9584120qkt.28.2018.08.06.08.37.52 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 06 Aug 2018 08:37:52 -0700 (PDT) Date: Mon, 6 Aug 2018 11:40:51 -0400 From: Johannes Weiner To: Peter Zijlstra Cc: Ingo Molnar , Andrew Morton , Linus Torvalds , Tejun Heo , Suren Baghdasaryan , Daniel Drake , Vinayak Menon , Christopher Lameter , Mike Galbraith , Shakeel Butt , Peter Enderborg , linux-mm@kvack.org, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO Message-ID: <20180806154051.GA14209@cmpxchg.org> References: <20180801151958.32590-1-hannes@cmpxchg.org> <20180801151958.32590-9-hannes@cmpxchg.org> <20180803165641.GA2476@hirez.programming.kicks-ass.net> <20180806150550.GA9888@cmpxchg.org> <20180806152528.GM2494@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180806152528.GM2494@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Aug 06, 2018 at 05:25:28PM +0200, Peter Zijlstra wrote: > On Mon, Aug 06, 2018 at 11:05:50AM -0400, Johannes Weiner wrote: > > Argh, that's right. This needs an explicit count if we want to access > > it locklessly. And you already said you didn't like that this is the > > only state not derived purely from the task counters, so maybe this is > > the way to go after all. > > > > How about something like this (untested)? > > > > +static inline void psi_switch(struct rq *rq, struct task_struct *prev, > > + struct task_struct *next) > > +{ > > + if (psi_disabled) > > + return; > > + > > + if (unlikely(prev->flags & PF_MEMSTALL)) > > + psi_task_change(prev, rq_clock(rq), TSK_RECLAIMING, 0); > > + if (unlikely(next->flags & PF_MEMSTALL)) > > + psi_task_change(next, rq_clock(rq), 0, TSK_RECLAIMING); > > +} > > > Urgh... can't say I really like that. > > I would really rather do that scheduler_tick() thing to avoid the remote > update. The tick is a lot less hot than the switch path and esp. > next->flags might be a cold line (prev->flags is typically the same line > as prev->state so we already have that, but I don't think anybody now > looks at next->flags or its line, so that'd be cold load). Okay, the tick updater sounds like a much better option then. HZ frequency should produce more than recent enough data. That means we will retain the not-so-nice PF_MEMSTALL flag test under rq lock, but it'll eliminate most of that memory ordering headache. I'll do that. Thanks!