Re: [PATCH rcu 01/12] rcu: Decrease FQS scan wait time in case of callback overloading

Neeraj Upadhyay
2022-06-22T11:46:50Z

On 6/22/2022 3:49 AM, Paul E. McKenney wrote:
> On Tue, Jun 21, 2022 at 10:59:58AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> 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);...
>> }
>
> Good catch, thank you! How about the updated patch shown below?
>

Looks good to me.

Thanks
Neeraj

> Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 77de092c78f549b5c28075bfee9998a525d21f84
> Author: Paul E. McKenney <paulmck@kernel.org>
> Date: Tue Apr 12 15:08:14 2022 -0700
>
> rcu: Decrease FQS scan wait time in case of callback overloading
>
> 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.
>
> [ paulmck: Apply Neeraj Upadhyay feedback. ]
>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c25ba442044a6..52094e72866e5 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1983,7 +1983,12 @@ static noinline_for_stack void rcu_gp_fqs_loop(void)
> gf = RCU_GP_FLAG_OVLD;
> ret = 0;
> for (;;) {
> - if (!ret) {
> + if (rcu_state.cbovld) {
> + j = (j + 2) / 3;
> + if (j <= 0)
> + j = 1;
> + }
> + if (!ret || time_before(jiffies + j, rcu_state.jiffies_force_qs)) {
> WRITE_ONCE(rcu_state.jiffies_force_qs, jiffies + j);
> /*
> * jiffies_force_qs before RCU_GP_WAIT_FQS state