Messages in this thread | | | Date | Mon, 6 Jun 2022 20:41:33 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu/nocb: Avoid polling when myrdp->nocb_head_rdp list is empty |
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On Mon, Jun 06, 2022 at 02:51:43PM +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>
Looks promising! A couple of questions below.
> --- > kernel/rcu/tree_nocb.h | 14 +++++++++----- > 1 file changed, 9 insertions(+), 5 deletions(-) > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index fa8e4f82e60c..c25c3e3dd22d 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)) {
Do we need to set rdp_gp->nocb_gp_sleep to true in here somewhere?
> + 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")); > @@ -980,7 +984,7 @@ static int rdp_offload_toggle(struct rcu_data *rdp, > raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags); > // Queue this rdp for add/del to/from the list to iterate on rcuog > WRITE_ONCE(rdp_gp->nocb_toggling_rdp, rdp); > - if (rdp_gp->nocb_gp_sleep) { > + if (rcu_nocb_poll || rdp_gp->nocb_gp_sleep) {
Is it the case that other than when (de-)offloading, the rcuog kthread is never awakened when rcu_nocb_poll is set? If this is not the case, we would have spurious wakeups. (I believe that rcu_nocb_poll suppresses those wakeups, but it would be good to check.)
> rdp_gp->nocb_gp_sleep = false; > wake_gp = true; > } > -- > 2.25.1 >
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