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

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Date:   Wed, 22 Jun 2022 17:16:39 +0530
From:   Neeraj Upadhyay <quic_neeraju@...cinc.com>
To:     <paulmck@...nel.org>
CC:     <rcu@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, <rostedt@...dmis.org>
Subject: Re: [PATCH rcu 01/12] rcu: Decrease FQS scan wait time in case of
 callback overloading



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@...nel.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@...nel.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@...nel.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