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McKenney" To: Uladzislau Rezki Cc: Joel Fernandes , "Zhang, Qiang1" , "Zhuo, Qiuxu" , RCU , quic_neeraju@quicinc.com, Boqun Feng , LKML , Oleksiy Avramchenko , Steven Rostedt , Frederic Weisbecker Subject: Re: [PATCH 1/1] Reduce synchronize_rcu() waiting time Message-ID: Reply-To: paulmck@kernel.org References: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> <20230330150933.GB2114899@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Spam-Status: No, score=-5.2 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI,SPF_HELO_NONE, SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 31, 2023 at 12:55:36PM +0200, Uladzislau Rezki wrote: > On Thu, Mar 30, 2023 at 02:16:36PM -0700, Paul E. McKenney wrote: > > On Thu, Mar 30, 2023 at 09:18:44PM +0200, Uladzislau Rezki wrote: > > > On Thu, Mar 30, 2023 at 11:58:41AM -0700, Paul E. McKenney wrote: > > > > On Thu, Mar 30, 2023 at 05:43:15PM +0200, Uladzislau Rezki wrote: > > > > > On Thu, Mar 30, 2023 at 03:09:33PM +0000, Joel Fernandes wrote: > > > > > > On Tue, Mar 28, 2023 at 08:26:13AM -0700, Paul E. McKenney wrote: > > > > > > > On Mon, Mar 27, 2023 at 10:29:31PM -0400, Joel Fernandes wrote: > > > > > > > > Hello, > > > > > > > > > > > > > > > > > On Mar 27, 2023, at 9:06 PM, Paul E. McKenney wrote: > > > > > > > > > > > > > > > > > > On Mon, Mar 27, 2023 at 11:21:23AM +0000, Zhang, Qiang1 wrote: > > > > > > > > >>>> From: Uladzislau Rezki (Sony) > > > > > > > > >>>> Sent: Tuesday, March 21, 2023 6:28 PM > > > > > > > > >>>> [...] > > > > > > > > >>>> Subject: [PATCH 1/1] Reduce synchronize_rcu() waiting time > > > > > > > > >>>> > > > > > > > > >>>> A call to a synchronize_rcu() can be expensive from time point of view. > > > > > > > > >>>> Different workloads can be affected by this especially the ones which use this > > > > > > > > >>>> API in its time critical sections. > > > > > > > > >>>> > > > > > > > > >>> > > > > > > > > >>> This is interesting and meaningful research. ;-) > > > > > > > > >>> > > > > > > > > >>>> For example in case of NOCB scenario the wakeme_after_rcu() callback > > > > > > > > >>>> invocation depends on where in a nocb-list it is located. Below is an example > > > > > > > > >>>> when it was the last out of ~3600 callbacks: > > > > > > > > >>> > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> > > > > > > > > >> Can it be implemented separately as follows? it seems that the code is simpler > > > > > > > > >> (only personal opinion) ????. > > > > > > > > >> > > > > > > > > >> But I didn't test whether this reduce synchronize_rcu() waiting time > > > > > > > > >> > > > > > > > > >> +static void rcu_poll_wait_gp(struct rcu_tasks *rtp) > > > > > > > > >> +{ > > > > > > > > >> + unsigned long gp_snap; > > > > > > > > >> + > > > > > > > > >> + gp_snap = start_poll_synchronize_rcu(); > > > > > > > > >> + while (!poll_state_synchronize_rcu(gp_snap)) > > > > > > > > >> + schedule_timeout_idle(1); > > > > > > > > > > > > > > > > > > I could be wrong, but my guess is that the guys working with > > > > > > > > > battery-powered devices are not going to be very happy with this loop. > > > > > > > > > > > > > > > > > > All those wakeups by all tasks waiting for a grace period end up > > > > > > > > > consuming a surprisingly large amount of energy. > > > > > > > > > > > > > > > > Is that really the common case? On the general topic of wake-ups: > > > > > > > > Most of the time there should be only one > > > > > > > > task waiting synchronously on a GP to end. If that is > > > > > > > > true, then it feels like waking > > > > > > > > up nocb Kthreads which indirectly wake other threads is doing more work than usual? > > > > > > > > > > > > > > A good question, and the number of outstanding synchronize_rcu() > > > > > > > calls will of course be limited by the number of tasks in the system. > > > > > > > But I myself have raised the ire of battery-powered embedded folks with > > > > > > > a rather small number of wakeups, so... > > > > > > > > > > > > But unless I am missing something, even if there is single synchronize_rcu(), > > > > > > you have a flurry of potential wakeups right now, instead of the bare minimum > > > > > > I think. I have not measured how many wake ups, but I'd love to when I get > > > > > > time. Maybe Vlad has some numbers. > > > > > > > > > > > I will measure and have a look at wake-ups. But, what we have for now is > > > > > if there are two callers of synchronize_rcu() on different CPUs, i guess > > > > > two nocb-kthreads have to handle it, thus two nocb-kthreads have to be > > > > > awaken to do the work. This patch needs only one wake-up to serve all > > > > > users. > > > > > > > > One wakeup per synchronize_rcu(), right? > > > > > > > The gp-kthread wake-ups only one work, in its turn a worker wake-ups all > > > registered users of synchronize_rcu() for which a gp was passed. How many > > > users of synchonize_rcu() awaken by one worker depends on how many were > > > registered before initiating a new GP by the gp-kthread. > > > > > > > > Anyway, i will provide some data and analysis of it. > > > > > > > > Looking forward to seeing it! > > > > > > > Good. I will switch fully on it soon. I need to sort out some perf. > > > issues at work. > > > > And if you are looking for reduced wakeups instead of lower latency for > > synchronize_rcu(), I could see where the extra workqueue wakeup might > > be a problem for you. > > > > Assuming that this is all default-off, you could keep a count of the > > number of required wakeups for each grace period (indexed as usual by > > the bottom few bits of the grace-period counter without the low-order > > state bits), and do the wakeups directly from the grace-period kthread > > if there are not all that many of them. > > > At least if there is only one user of synchronize_rcu(), we can wake > it directly, i mean to invoke comlete() from the gp-kthread. I think > we should split such parts into different patches. Agreed, especially given that some experimentation will be required to work out which techniques actually help. > > Except that, given that workqueues try hard to make the handler be on the > > same CPU as the one that did the corresponding schedule_work() invocation, > > it is not clear that this particular wakeup is really costing you enough > > to notice. (That CPU is not idle, after all.) But there is nothing > > quite like measuring the actual energy consumption on real hardware! > > > AFAICR the schedule_work() wants to use current CPU, indeed. For few > users it might be OK to comlete() them directly. Energy wise, +1 wake-up > of our worker to kick all users, if not "direct" option. I do not think > energy is a problem here. Agreed, estimating/modeling energy consumption is as far as I know still is more of an art than a science. I did something like eight or nine rewrites of the old NO_HZ_FULL code learning that one. ;-) Thanx, Paul