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McKenney" To: Joel Fernandes 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: <2cd8f407-2b77-48b1-9f17-9aa8e4ce9c64@paulmck-laptop> Reply-To: paulmck@kernel.org References: 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 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. > 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