Messages in this thread | | | Date | Tue, 21 Jun 2022 10:59:58 +0530 | Subject | Re: [PATCH rcu 01/12] rcu: Decrease FQS scan wait time in case of callback overloading | From | Neeraj Upadhyay <> |
| |
On 6/21/2022 3:50 AM, Paul E. McKenney wrote: > The force-quiesce-state loop function rcu_gp_fqs_loop() checks for > callback overloading and does an immediate initial scan for idle CPUs > if so. However, subsequent rescans will be carried out at as leisurely a > rate as they always are, as specified by the rcutree.jiffies_till_next_fqs > module parameter. It might be tempting to just continue immediately > rescanning, but this turns the RCU grace-period kthread into a CPU hog. > It might also be tempting to reduce the time between rescans to a single > jiffy, but this can be problematic on larger systems. > > This commit therefore divides the normal time between rescans by three, > rounding up. Thus a small system running at HZ=1000 that is suffering > from callback overload will wait only one jiffy instead of the normal > three between rescans. > > Signed-off-by: Paul E. McKenney <paulmck@kernel.org> > --- > kernel/rcu/tree.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index c25ba442044a6..c19d5926886fb 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -1993,6 +1993,11 @@ static noinline_for_stack void rcu_gp_fqs_loop(void) > WRITE_ONCE(rcu_state.jiffies_kick_kthreads, > jiffies + (j ? 3 * j : 2)); > } > + if (rcu_state.cbovld) { > + j = (j + 2) / 3; > + if (j <= 0) > + j = 1; > + }
We update 'j' here, after setting rcu_state.jiffies_force_qs
WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j)
So, we return from swait_event_idle_timeout_exclusive after 1/3 time duration.
swait_event_idle_timeout_exclusive(rcu_state.gp_wq, rcu_gp_fqs_check_wake(&gf), j);
This can result in !timer_after check to return false and we will enter the 'else' (stray signal block) code?
This might not matter for first 2 fqs loop iterations, where RCU_GP_FLAG_OVLD is set in 'gf', but subsequent iterations won't benefit from this patch?
if (!time_after(rcu_state.jiffies_force_qs, jiffies) || (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) { ... } else { /* Deal with stray signal. */ }
So, do we need to move this calculation above the 'if' block which sets rcu_state.jiffies_force_qs? if (!ret) {
WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j);... }
Thanks Neeraj
> trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq, > TPS("fqswait")); > WRITE_ONCE(rcu_state.gp_state, RCU_GP_WAIT_FQS);
| |