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SubjectRE: [PATCH v2] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty
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On Wed, Jun 08, 2022 at 01:01:31AM +0000, Zhang, Qiang1 wrote:
> On Wed, Jun 08, 2022 at 12:41:28AM +0000, Zhang, Qiang1 wrote:
> > On Tue, Jun 07, 2022 at 03:50:57PM +0800, Zqiang wrote:
> > > Currently, If the 'rcu_nocb_poll' bootargs is enable, all rcuog
> > > kthreads enter polling mode. however, due to only insert CPU's rdp
> > > which belong to rcu_nocb_mask to 'nocb_head_rdp' list or all CPU's
> > > rdp served by rcuog kthread have been de-offloaded, these cause
> > > the 'nocb_head_rdp' list served by rcuog kthread is empty, when
> > > the 'nocb_head_rdp' is empty, the rcuog kthread in polling mode
> > > not actually do anything. fix it by exiting polling mode when the
> > > 'nocb_head_rdp'list is empty, otherwise entering polling mode.
> > >
> > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > >
> > >This looks a bit more plausible, but what have you done to test this?
> >
> > Yes , I have only tested as follows and I added two trace events.
> >
> > + trace_rcu_nocb_wake(rcu_state.name, cpu,
> > + TPS("EnterNoPoll"));
> >
> > rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
> > + trace_rcu_nocb_wake(rcu_state.name, cpu,
> > + TPS("ExitNoPoll"));
> >
> > runqemu kvm slirp nographic qemuparams="-m 2048 -smp 4"
> > bootparams="rcu_nocbs=2,3 rcutree.dump_tree=1 rcu_nocb_poll
> > rcutorture.nocbs_nthreads=4 rcutorture.test_boost=0" -d

Hi Paul, The test trace log is as follows:

rcuog/0-19 [003] ..... 169.400494: rcu_nocb_wake: rcu_preempt 0 EnterNoPoll
rcuog/0-19 [003] ..... 172.592072: rcu_nocb_wake: rcu_preempt 0 ExitNoPoll
rcuog/2-36 [002] ..... 173.553717: rcu_nocb_wake: rcu_preempt 2 EnterNoPoll
rcuog/2-36 [002] ..... 173.750084: rcu_nocb_wake: rcu_preempt 2 ExitNoPoll

Thanks
Zqiang

>
> >To exercise your change, could you please also add an appropriate value for the rcutorture.nocbs_nthreads kernel boot parameter? Without that boot parameter, your kernel will not actually offload or deoffload any CPUs.
>
> I have set ' rcutorture.nocbs_nthreads=4 ' in bootargs.
>
>I see it now, apologies for my confusion!
>
> Thanx, Paul
>
> >An alternative approach is to run rcutorture scenario TREE01.
>
> I will also use TREE01 for testing
>
> Thanks
> Zqiang
>
> >
> > Thanx, Paul
>
> > Thanks
> > Zqiang
> >
> > >
> > > Thanx, Paul
> > >
> > > ---
> > > v1->v2:
> > > Move rcu_nocb_poll flags check from rdp_offload_toggle() to
> > > rcu_nocb_rdp_offload/deoffload(), avoid unnecessary setting of
> > > rdp_gp->nocb_gp_sleep flags, because when rcu_nocb_poll is set
> > > the rdp_gp->nocb_gp_sleep is not used.
> > >
> > > kernel/rcu/tree_nocb.h | 16 ++++++++++------
> > > 1 file changed, 10 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index
> > > fa8e4f82e60c..2a52c9abc681 100644
> > > --- a/kernel/rcu/tree_nocb.h
> > > +++ b/kernel/rcu/tree_nocb.h
> > > @@ -698,10 +698,14 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
> > > TPS("WakeBypassIsDeferred"));
> > > }
> > > if (rcu_nocb_poll) {
> > > - /* Polling, so trace if first poll in the series. */
> > > - if (gotcbs)
> > > - trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> > > - schedule_timeout_idle(1);
> > > + if (list_empty(&my_rdp->nocb_head_rdp)) {
> > > + rcu_wait(READ_ONCE(my_rdp->nocb_toggling_rdp));
> > > + } else {
> > > + /* Polling, so trace if first poll in the series. */
> > > + if (gotcbs)
> > > + trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
> > > + schedule_timeout_idle(1);
> > > + }
> > > } else if (!needwait_gp) {
> > > /* Wait for callbacks to appear. */
> > > trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep")); @@
> > > -1030,7
> > > +1034,7 @@ static long rcu_nocb_rdp_deoffload(void *arg)
> > >
> > > mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
> > > if (rdp_gp->nocb_gp_kthread) {
> > > - if (wake_gp)
> > > + if (wake_gp || rcu_nocb_poll)
> > > wake_up_process(rdp_gp->nocb_gp_kthread);
> > >
> > > /*
> > > @@ -1152,7 +1156,7 @@ static long rcu_nocb_rdp_offload(void *arg)
> > > * rcu_nocb_unlock() rcu_nocb_unlock()
> > > */
> > > wake_gp = rdp_offload_toggle(rdp, true, flags);
> > > - if (wake_gp)
> > > + if (wake_gp || rcu_nocb_poll)
> > > wake_up_process(rdp_gp->nocb_gp_kthread);
> > > swait_event_exclusive(rdp->nocb_state_wq,
> > > rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)
> > > &&
> > > --
> > > 2.25.1
> > >

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