[Question] About rcu_nocb_poll bootargs

[Question] About rcu_nocb_poll bootargs

[Zhang, Qiang1]

Hi All

Currently, I have tested as follows:

runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4"  bootparams="isolcpus=2,3  nohz_full=2,3 rcu_nocbs=2,3 rcutree.dump_tree=1, rcu_nocb_poll" -d

rcu:     Offload RCU callbacks from CPUs: 2-3.
rcu:     Poll for callbacks from no-CBs CPUs.
rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 0: 1   (rcuog/0)
rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 2: 3   (rcuog/2)

but I find the rt-fifo rcuog/0 kthreads always doing polling, but due to the my_rdp->nocb_head_rdp always is empty,
this doesn't do the actual thing,  Is it possible that we do not polling when the nocb_head_rdp  linked list is empty, 
and polling when it is not empty.

Any thought?

Thanks
Zqiang

Re: [Question] About rcu_nocb_poll bootargs

[Paul E. McKenney]

On Thu, Jun 02, 2022 at 02:06:50AM +0000, Zhang, Qiang1 wrote:
> Hi All
> 
> Currently, I have tested as follows:
> 
> runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4"  bootparams="isolcpus=2,3  nohz_full=2,3 rcu_nocbs=2,3 rcutree.dump_tree=1, rcu_nocb_poll" -d
> 
> rcu:     Offload RCU callbacks from CPUs: 2-3.
> rcu:     Poll for callbacks from no-CBs CPUs.
> rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 0: 1   (rcuog/0)
> rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 2: 3   (rcuog/2)
> 
> but I find the rt-fifo rcuog/0 kthreads always doing polling, but due to the my_rdp->nocb_head_rdp always is empty,
> this doesn't do the actual thing,  Is it possible that we do not polling when the nocb_head_rdp  linked list is empty, 
> and polling when it is not empty.
> 
> Any thought?

If I understand your question correctly, you would like to avoid polling
when the rcuog kthread's CPU's rcu_data structure's nocb_head_rdp list
is empty.  This could happen only when all of the CPUs served by that
rcuog kthread have been de-offloaded, correct?

If so, this is not currently a user-visible issue, but it would be good
to fix it before offloading and de-offloading becomes user-visible.

If this is what you were thinking, what did you have in mind for a fix?

(We need to get Frederic's view in any case, of course.)

							Thanx, Paul

RE: [Question] About rcu_nocb_poll bootargs

[Zhang, Qiang1]

On Thu, Jun 02, 2022 at 02:06:50AM +0000, Zhang, Qiang1 wrote:
> Hi All
> 
> Currently, I have tested as follows:
> 
> runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4"  
> bootparams="isolcpus=2,3  nohz_full=2,3 rcu_nocbs=2,3 
> rcutree.dump_tree=1, rcu_nocb_poll" -d
> 
> rcu:     Offload RCU callbacks from CPUs: 2-3.
> rcu:     Poll for callbacks from no-CBs CPUs.
> rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 0: 1   (rcuog/0)
> rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 2: 3   (rcuog/2)
> 
> but I find the rt-fifo rcuog/0 kthreads always doing polling, but due 
> to the my_rdp->nocb_head_rdp always is empty, this doesn't do the 
> actual thing,  Is it possible that we do not polling when the nocb_head_rdp  linked list is empty, and polling when it is not empty.
> 
> Any thought?

>If I understand your question correctly, you would like to avoid polling when the rcuog kthread's CPU's rcu_data structure's nocb_head_rdp list is empty.  This could happen only when all of the CPUs served by that rcuog kthread have been de-offloaded, correct?

Yes, thanks Paul reply.  I'll be more careful with my wording next time.

>
>If so, this is not currently a user-visible issue, but it would be good to fix it before offloading and de-offloading becomes user-visible.
>
>If this is what you were thinking, what did you have in mind for a fix?

When rcu_nocb_poll is set and the rcuog kthread's CPU's rcu_data structure's nocb_head_rdp list is empty, 
treat rcuog kthread like non-polling mode and put them in the waitqueue(my_rdp->nocb_gp_wq) for waiting,
when nocb_head_rdp list is not empty(rcu_nocb_poll && !list_empty(nocb_head_rdp)),  re-entering periodic wake-up mode. I need yours opinion.

Any thought?

Thanks
Zqiang

>
>(We need to get Frederic's view in any case, of course.)
>
>							Thanx, Paul

Re: [Question] About rcu_nocb_poll bootargs

[Paul E. McKenney]

On Thu, Jun 02, 2022 at 05:45:18AM +0000, Zhang, Qiang1 wrote:
> 
> On Thu, Jun 02, 2022 at 02:06:50AM +0000, Zhang, Qiang1 wrote:
> > Hi All
> > 
> > Currently, I have tested as follows:
> > 
> > runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4"  
> > bootparams="isolcpus=2,3  nohz_full=2,3 rcu_nocbs=2,3 
> > rcutree.dump_tree=1, rcu_nocb_poll" -d
> > 
> > rcu:     Offload RCU callbacks from CPUs: 2-3.
> > rcu:     Poll for callbacks from no-CBs CPUs.
> > rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 0: 1   (rcuog/0)
> > rcu: rcu_organize_nocb_kthreads: No-CB GP kthread CPU 2: 3   (rcuog/2)
> > 
> > but I find the rt-fifo rcuog/0 kthreads always doing polling, but due 
> > to the my_rdp->nocb_head_rdp always is empty, this doesn't do the 
> > actual thing,  Is it possible that we do not polling when the nocb_head_rdp  linked list is empty, and polling when it is not empty.
> > 
> > Any thought?
> 
> >If I understand your question correctly, you would like to avoid polling when the rcuog kthread's CPU's rcu_data structure's nocb_head_rdp list is empty.  This could happen only when all of the CPUs served by that rcuog kthread have been de-offloaded, correct?
> 
> Yes, thanks Paul reply.  I'll be more careful with my wording next time.

不是问题！ 毕竟，情况可能会更糟。 我可能会尝试用中文或其他东西写作。

Et la langue dominante change avec le temps. Il n'y a pas si longtemps,
c'était le français plutôt que l'anglais. En tant qu'anglophone natif,
une partie de mon travail consiste à aider les locuteurs non natifs. Et
qui sait? Peut-être qu'un jour la langue dominante sera le chinois.

Yeah, Google Translate.  Why do you ask?  ;-)

> >If so, this is not currently a user-visible issue, but it would be good to fix it before offloading and de-offloading becomes user-visible.
> >
> >If this is what you were thinking, what did you have in mind for a fix?
> 
> When rcu_nocb_poll is set and the rcuog kthread's CPU's rcu_data structure's nocb_head_rdp list is empty, 
> treat rcuog kthread like non-polling mode and put them in the waitqueue(my_rdp->nocb_gp_wq) for waiting,
> when nocb_head_rdp list is not empty(rcu_nocb_poll && !list_empty(nocb_head_rdp)),  re-entering periodic wake-up mode. I need yours opinion.
> 
> Any thought?

This seems plausible to me!  This code is tricky, so it may take a time
or two to get it right, but what is life without a challenge?

							Thanx, Paul

> Thanks
> Zqiang
> 
> >
> >(We need to get Frederic's view in any case, of course.)
> >
> >							Thanx, Paul