linux-kernel - Re: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks

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Date:   Wed, 23 May 2018 17:51:19 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org, Peter Zilstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 1/4] rcu: Speed up calling of RCU tasks callbacks

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" <paulmck@...ux.vnet.ibm.com> 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<stdio.h>

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