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[35.188.239.129]) by smtp.gmail.com with ESMTPSA id u23-20020a37ab17000000b0071f0d0aaef7sm16228569qke.80.2023.03.30.08.11.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Mar 2023 08:11:15 -0700 (PDT) Date: Thu, 30 Mar 2023 15:11:15 +0000 From: Joel Fernandes To: "Paul E. McKenney" Cc: "Zhang, Qiang1" , Uladzislau Rezki , "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: <20230330151115.GC2114899@google.com> References: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 28, 2023 at 03:14:32PM -0700, Paul E. McKenney 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... > > > > And on larger systems there can be a tradeoff between contention on > > the one hand and number of wakeups on the other. > > > > The original nocb implementation in fact had the grace-period kthead > > waking up all of what are now called rcuoc kthreads. The indirect scheme > > reduced the total number of wakeups by up to 50% and also reduced the > > CPU consumption of the grace-period kthread, which otherwise would have > > become a bottleneck on large systems. > > > > And also, a scheme that directly wakes tasks waiting in synchronize_rcu() > > might well use the same ->nocb_gp_wq[] waitqueues that are used by the > > rcuog kthreads, if that is what you were getting at. > > And on small systems, you might of course have the rcuog kthread directly > invoke callbacks if there are not very many of them. This would of > course need to be done quite carefully to avoid any number of races > with the rcuoc kthreads. You could do the same thing on a large system, > but on a per-rcuog basis. > > I vaguely recall discussing this in one of our sessions, but who knows? > > Would this really be of benefit? Or did you have something else in mind? Yes, this is what I was also referring to. Not sure about benefit, depends on workloads and measurement. thanks, - Joel > > Thanx, Paul > > > > I am curious to measure how much does Vlad patch reduce wakeups in the common case. > > > > Sounds like a good thing to measure! > > > > > I was also wondering how Vlad patch effects RCU-barrier ordering. I guess > > > we want the wake up to happen in the order of > > > other callbacks also waiting. > > > > OK, I will bite. Why would rcu_barrier() need to care about the > > synchronize_rcu() invocations if they no longer used call_rcu()? > > > > > One last note, most battery powered systems are perhaps already using expedited RCU ;-) > > > > Good point. And that does raise the question of exactly what workloads > > and systems want faster wakeups from synchronize_rcu() and cannot get > > this effect from expedited grace periods. > > > > Thanx, Paul > > > > > Thoughts? > > > > > > - Joel > > > > > > > > > > > Thanx, Paul > > > > > > > >> +} > > > >> + > > > >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func); > > > >> +DEFINE_RCU_TASKS(rcu_poll, rcu_poll_wait_gp, call_rcu_poll, > > > >> + "RCU Poll"); > > > >> +void call_rcu_poll(struct rcu_head *rhp, rcu_callback_t func) > > > >> +{ > > > >> + call_rcu_tasks_generic(rhp, func, &rcu_poll); > > > >> +} > > > >> +EXPORT_SYMBOL_GPL(call_rcu_poll); > > > >> + > > > >> +void synchronize_rcu_poll(void) > > > >> +{ > > > >> + synchronize_rcu_tasks_generic(&rcu_poll); > > > >> +} > > > >> +EXPORT_SYMBOL_GPL(synchronize_rcu_poll); > > > >> + > > > >> +static int __init rcu_spawn_poll_kthread(void) > > > >> +{ > > > >> + cblist_init_generic(&rcu_poll); > > > >> + rcu_poll.gp_sleep = HZ / 10; > > > >> + rcu_spawn_tasks_kthread_generic(&rcu_poll); > > > >> + return 0; > > > >> +} > > > >> > > > >> Thanks > > > >> Zqiang > > > >> > > > >> > > > >>>> > > > >>>> > > > >>>> <...>-29 [001] d..1. 21950.145313: rcu_batch_start: rcu_preempt > > > >>>> CBs=3613 bl=28 > > > >>>> ... > > > >>>> <...>-29 [001] ..... 21950.152578: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=00000000b2d6dee8 func=__free_vm_area_struct.cfi_jt > > > >>>> <...>-29 [001] ..... 21950.152579: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=00000000a446f607 func=__free_vm_area_struct.cfi_jt > > > >>>> <...>-29 [001] ..... 21950.152580: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=00000000a5cab03b func=__free_vm_area_struct.cfi_jt > > > >>>> <...>-29 [001] ..... 21950.152581: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=0000000013b7e5ee func=__free_vm_area_struct.cfi_jt > > > >>>> <...>-29 [001] ..... 21950.152582: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=000000000a8ca6f9 func=__free_vm_area_struct.cfi_jt > > > >>>> <...>-29 [001] ..... 21950.152583: rcu_invoke_callback: rcu_preempt > > > >>>> rhp=000000008f162ca8 func=wakeme_after_rcu.cfi_jt > > > >>>> <...>-29 [001] d..1. 21950.152625: rcu_batch_end: rcu_preempt CBs- > > > >>>> invoked=3612 idle=.... > > > >>>> > > > >>>> > > > >>> > > > >>> Did the results above tell us that CBs-invoked=3612 during the time 21950.145313 ~ 21950.152625? > > > >>> > > > >>> Yes. > > > >>> > > > >>> > > > >>> If possible, may I know the steps, commands, and related parameters to produce the results above? > > > >>> Thank you! > > > >>> > > > >>> Build the kernel with CONFIG_RCU_TRACE configuration. Update your "set_event" > > > >>> file with appropriate traces: > > > >>> > > > >>> > > > >>> XQ-DQ54:/sys/kernel/tracing # echo rcu:rcu_batch_start rcu:rcu_batch_end rcu:rcu_invoke_callback > set_event > > > >>> > > > >>> XQ-DQ54:/sys/kernel/tracing # cat set_event > > > >>> rcu:rcu_batch_start > > > >>> rcu:rcu_invoke_callback > > > >>> rcu:rcu_batch_end > > > >>> XQ-DQ54:/sys/kernel/tracing # > > > >>> > > > >>> > > > >>> Collect traces as much as you want: XQ-DQ54:/sys/kernel/tracing # echo 1 > tracing_on; sleep 10; echo 0 > tracing_on > > > >>> Next problem is how to parse it. Of course you will not be able to parse > > > >>> megabytes of traces. For that purpose i use a special C trace parser. > > > >>> If you need an example please let me know i can show here. > > > >>> > > > >>> -- > > > >>> Uladzislau Rezki