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McKenney" Cc: Steven Rostedt , Joel Fernandes , linux-kernel@vger.kernel.org, Peter Zilstra , Ingo Molnar , Boqun Feng , byungchul.park@lge.com, kernel-team@android.com, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers Subject: Re: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks Message-ID: <20180523215130.GA129848@joelaf.mtv.corp.google.com> References: <20180523063815.198302-1-joel@joelfernandes.org> <20180523063815.198302-2-joel@joelfernandes.org> <20180523155734.GK3803@linux.vnet.ibm.com> <20180523124531.7b0e972a@gandalf.local.home> <20180523170303.GR3803@linux.vnet.ibm.com> <20180523151337.469bba34@gandalf.local.home> <20180523200458.GD3803@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180523200458.GD3803@linux.vnet.ibm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 23, 2018 at 01:04:58PM -0700, Paul E. McKenney wrote: > On Wed, May 23, 2018 at 03:13:37PM -0400, Steven Rostedt wrote: > > On Wed, 23 May 2018 10:03:03 -0700 > > "Paul E. McKenney" wrote: > > > > > > > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > > > > > index 5783bdf86e5a..a28698e44b08 100644 > > > > > > --- a/kernel/rcu/update.c > > > > > > +++ b/kernel/rcu/update.c > > > > > > @@ -743,6 +743,12 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > > */ > > > > > > synchronize_srcu(&tasks_rcu_exit_srcu); > > > > > > > > > > > > + /* > > > > > > + * Wait a little bit incase held tasks are released > > > > > > > > > > in case > > > > > > > > > > > + * during their next timer ticks. > > > > > > + */ > > > > > > + schedule_timeout_interruptible(HZ/10); > > > > > > + > > > > > > /* > > > > > > * Each pass through the following loop scans the list > > > > > > * of holdout tasks, removing any that are no longer > > > > > > @@ -755,7 +761,6 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > > int rtst; > > > > > > struct task_struct *t1; > > > > > > > > > > > > - schedule_timeout_interruptible(HZ); > > > > > > rtst = READ_ONCE(rcu_task_stall_timeout); > > > > > > needreport = rtst > 0 && > > > > > > time_after(jiffies, lastreport + rtst); > > > > > > @@ -768,6 +773,11 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > > > > > check_holdout_task(t, needreport, &firstreport); > > > > > > cond_resched(); > > > > > > } > > > > > > + > > > > > > + if (list_empty(&rcu_tasks_holdouts)) > > > > > > + break; > > > > > > + > > > > > > + schedule_timeout_interruptible(HZ); > > > > > > > > Why is this a full second wait and not the HZ/10 like the others? > > > > > > The idea is to respond quickly on small idle systems and to reduce the > > > number of possibly quite lengthy traversals of the task list otherwise. > > > I actually considered exponential backoff, but decided to keep it simple, > > > at least to start with. > > > > Ah, now it makes sense. Reading what you wrote, we can still do a > > backoff and keep it simple. What about the patch below. It appears to > > have the same performance improvement as Joel's > > Looks plausible to me! > > Joel, do you see any gotchas in Steve's patch? I see one but I hope I'm not day dreaming.. :D > > > > > Is there a better way to do this? Can this be converted into a for-loop? > > > > > Alternatively, would it make sense to have a firsttime local variable > > > > > initialized to true, to keep the schedule_timeout_interruptible() at > > > > > the beginning of the loop, but skip it on the first pass through the loop? > > > > > > > > > > Don't get me wrong, what you have looks functionally correct, but > > > > > duplicating the condition might cause problems later on, for example, > > > > > should a bug fix be needed in the condition. I agree with your suggestions and Steven's patch is better. > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c > > index 68fa19a5e7bd..c6df9fa916cf 100644 > > --- a/kernel/rcu/update.c > > +++ b/kernel/rcu/update.c > > @@ -796,13 +796,22 @@ static int __noreturn rcu_tasks_kthread(void *arg) > > * holdouts. When the list is empty, we are done. > > */ > > lastreport = jiffies; > > - while (!list_empty(&rcu_tasks_holdouts)) { > > + for (;;) { > > bool firstreport; > > bool needreport; > > int rtst; > > struct task_struct *t1; > > + int fract = 15; Shouldn't this assignment be done outside the loop? I believe the variable will be initialized on each iteration. A program like this doesn't terminate: #include int main() { for (;;) { int i = 10; if (!(i--)) break; } return 0; } Otherwise looks good to me, I would initialize fract to 10 so its consistent with "HZ/10" in other parts of the code but I'm ok with either number. thanks! - Joel