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SubjectRe: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks
On Wed, May 23, 2018 at 12:45:31PM -0400, Steven Rostedt wrote:
> 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.

Very good, thank you!

> > > 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?

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.

Thanx, Paul

> -- 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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