On Wed, 23 May 2018 08:57:34 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> On Tue, May 22, 2018 at 11:38:12PM -0700, Joel Fernandes wrote: > > From: "Joel Fernandes (Google)" <j...@joelfernandes.org> > > > > RCU tasks callbacks can take at least 1 second before the callbacks are > > executed. This happens even if the hold-out tasks enter their quiescent > > states > > quickly. I noticed this when I was testing trampoline callback execution. > > > > To test the trampoline freeing, I wrote a simple script: > > cd /sys/kernel/debug/tracing/ > > echo '__schedule_bug:traceon' > set_ftrace_filter; > > echo '!__schedule_bug:traceon' > set_ftrace_filter; > > > > In the background I had simple bash while loop: > > while [ 1 ]; do x=1; done & > > > > Total time of completion of above commands in seconds: > > > > With this patch: > > real 0m0.179s > > user 0m0.000s > > sys 0m0.054s > > > > Without this patch: > > real 0m1.098s > > user 0m0.000s > > sys 0m0.053s > > > > That's a greater than 6X speed up in performance. In order to accomplish > > this, I am waiting for HZ/10 time before entering the hold-out checking > > loop. The loop still preserves its checking of held tasks every 1 second > > as before, in case this first test doesn't succeed. > > > > Cc: Steven Rostedt <rost...@goodmis.org> > > Given an ack from Steven, I would be happy to take this, give or take > some nits below. I'm currently testing it, and trying to understand it better. > > Thanx, Paul > > > Cc: Peter Zilstra <pet...@infradead.org> > > Cc: Ingo Molnar <mi...@redhat.com> > > Cc: Boqun Feng <boqun.f...@gmail.com> > > Cc: Paul McKenney <paul...@linux.vnet.ibm.com> > > Cc: byungchul.p...@lge.com > > Cc: kernel-t...@android.com > > Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org> > > --- > > kernel/rcu/update.c | 12 +++++++++++- > > 1 file changed, 11 insertions(+), 1 deletion(-) > > > > 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? -- Steve > > 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. > > > } > > > > /* > > -- > > 2.17.0.441.gb46fe60e1d-goog > >
