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McKenney" To: Frederic Weisbecker Cc: "Zhang, Qiang1" , "quic_neeraju@quicinc.com" , "joel@joelfernandes.org" , "rcu@vger.kernel.org" , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH v2] rcu: Rework tick dependency setting into rcu_exp_handler() Message-ID: <20230107163142.GC4028633@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20230105034000.971748-1-qiang1.zhang@intel.com> <20230105051146.GN4028633@paulmck-ThinkPad-P17-Gen-1> 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=-7.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS autolearn=ham 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, Jan 06, 2023 at 11:45:46PM +0100, Frederic Weisbecker wrote: > On Fri, Jan 06, 2023 at 02:42:59AM +0000, Zhang, Qiang1 wrote: > > On Thu, Jan 05, 2023 at 11:40:00AM +0800, Zqiang wrote: > > > Currently, when first find out the expedited grace period is not end > > > and timeout occurred, we set tick dependency for CPUs that have not > > > yet reported the quiescent state in the rcu_node structure's->expmask > > > but need to eliminate races between set and clear tick dependency, > > > setting CPU tick dependency need to hold rcu_node structure's->lock. > > > > > > This commit move tick dependency setting into rcu_exp_handler(), set > > > tick dependency when fail to report a quiescent state and clear tick > > > dependency in rcu_report_exp_rdp(). [from Frederic Weisbecker > > > suggestion] > > > > > > Signed-off-by: Zqiang > > > > > >First, a big "thank you" to you an Frederic for investigating this approach! > > > > > >So which is better, this patch or the one that I already have queued? > > > > > >The advantage of the patch of yours that I already have queued is that CPUs that respond in some other way within a millisecond do not get hit with an additional scheduling-clock interrupt. > > > > > >On the other hand, if the CPU goes through a quiescent state before the next scheduling-clock interrupt arrives, rcu_report_exp_cpu_mult() will shut down the tick before it happens. Plus if the CPU waits a full tick before reaching a quiescent state, then the tick_dep_set_cpu() called from > > >synchronize_rcu_expedited_wait() is going to send along an IPI anyway. > > > > Agreed, this new patch is set tick dependency immediately when we can't report a quiescent state > > in rcu_exp_handler(), this seems a little too aggressive. > > > > > > > > > >Except that invoking tick_dep_set_cpu() on the CPU itself will also do an IPI from tick_dep_set_cpu() because of IRQ_WORK_INIT_HARD(), right? > > >Which means that the patch below gets us an extra self-IPI, right? > > >Or am I misreading the code? > > > > Yes, This looks like it will trigger an additional IPI interrupt. > > > > > > > >In addition, doesn't tick_dep_clear_cpu() just clear a bit? Won't that mean that the next scheduling-clock interrupt will happen, just that the one after that won't? (Give or take kernel-to-user or kernel-to-idle transitions that might happen in the meantime.) > > > > Yes, tick_dep_clear_cpu() just only clear a bit. next scheduling-clock interrupt will happen. > > > > So I also want to know which one is better ????? > > Right, I may have misled you with this change. I missed the fact that a chance > is given for 1 jiffy to nohz_full CPUs to report a QS before the tick is forced > there. > > Sorry about that. Your first patch is still a good fix though! And I have it queued, with Frederic's Reviewed-by. And hey, if you don't miss a thing or two once in a while, you are not reviewing sufficiently challenging code. ;-) So I repeat my earlier "thank you" to both of you for looking into this! Thanx, Paul