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SubjectRe: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks
On Wed, 23 May 2018 08:57:34 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Tue, May 22, 2018 at 11:38:12PM -0700, Joel Fernandes wrote:
> > From: "Joel Fernandes (Google)" <joel@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 <rostedt@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 <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: byungchul.park@lge.com
> > Cc: kernel-team@android.com
> > Signed-off-by: Joel Fernandes (Google) <joel@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
> >

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