On Mon, Jul 01, 2019 at 08:47:30PM -0700, Paul E. McKenney wrote: > On Mon, Jul 01, 2019 at 12:04:13AM -0400, Joel Fernandes (Google) wrote: > > The t->rcu_read_unlock_special union's need_qs bit can be set by the > > scheduler tick (in rcu_flavor_sched_clock_irq) to indicate that help is > > needed from the rcu_read_unlock path. When this help arrives however, we > > can do better to speed up the quiescent state reporting which if > > rcu_read_unlock_special::need_qs is set might be quite urgent. Make use > > of this information in deciding when to do heavy-weight softirq raising > > where possible. > > > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > > Cute thought, but I am going to have to pass on this one. The reason > is that by the time that ->rcu_read_unlock_special.b.need_qs gets set, > the grace period is already one full second old. At that point, the > extra tick of waiting is down in the noise. > > Right now, we do the extra work if we really are blocking an expedited > grace period (the first two lines of the original condition) or we are > running on a nohz_full CPU (which might never execute a scheduling clock > tick, thus potentially delaying forever). And expedited grace periods > are supposed to complete in tens or maybe hundreds of microseconds, > assuming the RCU readers are being cooperative, which is a whole > different level of urgent. Makes sense, I agree the patch may not be that helpful right now. I mixed up the different levels or urgencies. No problem dropping it. > > Nevertheless, thank you for looking into this! My pleasure! Will keep them coming. - Joel > > Thanx, Paul > > > --- > > kernel/rcu/tree_plugin.h | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index c588ef98efd3..bff6410fac06 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -622,7 +622,8 @@ static void rcu_read_unlock_special(struct task_struct *t) > > t->rcu_read_unlock_special.b.exp_hint = false; > > exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) || > > (rdp->grpmask & rnp->expmask) || > > - tick_nohz_full_cpu(rdp->cpu); > > + tick_nohz_full_cpu(rdp->cpu) || > > + t->rcu_read_unlock_special.b.need_qs; > > // Need to defer quiescent state until everything is enabled. > > if (irqs_were_disabled && use_softirq && > > (in_interrupt() || > > -- > > 2.22.0.410.gd8fdbe21b5-goog > > >
