On Fri, Aug 09, 2019 at 05:36:43PM -0400, Joel Fernandes wrote: > 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? The trick is that rcu_momentary_dyntick_idle() directly increments the CPU's dyntick counter, so that the next FQS loop will note that the CPU passed through a quiescent state. No need for need_heavy_qs in this case. Thanx, Paul > 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 > > > > > >
