Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu batching

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Aug 09, 2019 at 01:42:17PM -0700, Paul E. McKenney wrote:
> > Also, I can go back to 500M if I just keep KFREE_DRAIN_JIFFIES at HZ/50. So I
> > am quite happy about that. I think I can declare that the "let list grow
> > indefinitely" design works quite well even with an insanely heavily loaded
> > case of every CPU in a 16CPU system with 500M memory, indefinitely doing
> > kfree_rcu()in a tight loop with appropriate cond_resched(). And I am like
> > thinking - wow how does this stuff even work at such insane scales :-D
> 
> A lot of work by a lot of people over a long period of time.  On their
> behalf, I thank you for the implied compliment.  So once this patch gets
> in, perhaps you will have complimented yourself as well.  ;-)
> 
> But more work is needed, and will continue to be as new workloads,
> compiler optimizations, and hardware appears.  And it would be good to
> try this on a really big system at some point.

Cool!

> > > > > o	Along with the above boot parameter, use "rcutree.use_softirq=0"
> > > > > 	to cause RCU to use kthreads instead of softirq.  (You might well
> > > > > 	find issues in priority setting as well, but might as well find
> > > > > 	them now if so!)
> > > > 
> > > > Doesn't think one actually reduce the priority of the core RCU work? softirq
> > > > will always have higher priority than any there. So wouldn't that have the
> > > > effect of not reclaiming things fast enough? (Or, in my case not scheduling
> > > > the rcu_work which does the reclaim).
> > > 
> > > For low kfree_rcu() loads, yes, it increases overhead due to the need
> > > for context switches instead of softirq running at the tail end of an
> > > interrupt.  But for high kfree_rcu() loads, it gets you realtime priority
> > > (in conjunction with "rcutree.kthread_prio=", that is).
> > 
> > I meant for high kfree_rcu() loads, a softirq context executing RCU callback
> > is still better from the point of view of the callback running because the
> > softirq will run above all else (higher than the highest priority task) so
> > use_softirq=0 would be a down grade from that perspective if something higher
> > than rcutree.kthread_prio is running on the CPU. So unless kthread_prio is
> > set to the highest prio, then softirq running would work better. Did I miss
> > something?
> 
> Under heavy load, softirq stops running at the tail end of interrupts and
> is instead run within the context of a per-CPU ksoftirqd kthread.  At normal
> SCHED_OTHER priority.

Ah, yes. Agreed!

> > > > > o	With any of the above, invoke rcu_momentary_dyntick_idle() along
> > > > > 	with cond_resched() in your kfree_rcu() loop.  This simulates
> > > > > 	a trip to userspace for nohz_full CPUs, so if this helps for
> > > > > 	non-nohz_full CPUs, adjustments to the kernel might be called for.
> > 
> > I did not try this yet. But I am thinking why would this help in nohz_idle
> > case? In nohz_idle we already have the tick active when CPU is idle. I guess
> > it is because there may be a long time that elapses before
> > rcu_data.rcu_need_heavy_qs == true ?
> 
> Under your heavy rcuperf load, none of the CPUs would ever be idle.  Nor
> would they every be in nohz_full userspace context, either.

Sorry I made a typo, I meant 'tick active when CPU is non-idle for NOHZ_IDLE
systems' above.

> In contrast, a heavy duty userspace-driven workload would transition to
> and from userspace for each kfree_rcu(), and that would increment the
> dyntick-idle count on each transition to and from userspace.  Adding the
> rcu_momentary_dyntick_idle() emulates a pair of such transitions.

But even if we're in kernel mode and not transitioning, I thought the FQS
loop (rcu_implicit_dynticks_qs() function) would set need_heavy_qs to true at
2 * jiffies_to_sched_qs.

Hmm, I forgot that jiffies_to_sched_qs can be quite large I guess. You're
right, we could call rcu_momentary_dyntick_idle() in advance before waiting
for FQS loop to do the setting of need_heavy_qs.

Or, am I missing something with the rcu_momentary_dyntick_idle() point you
made?

thanks,

 - Joel


> 
> 							Thanx, Paul
> 
> > > > Ok, will try it.
> > > > 
> > > > Save these bullet points for future reference! ;-)  thanks,
> > > 
> > > I guess this is helping me to prepare for Plumbers.  ;-)
> > 
> > :-)
> > 
> > thanks, Paul!
> > 
> >  - Joel
> > 
>