Re: [PATCH] rcu/tree: Force quiescent state on callback overload

[Neeraj Upadhyay <neeraju@codeaurora.org>]

Hi Paul,

On 6/22/2020 8:43 AM, Paul E. McKenney wrote:
> On Mon, Jun 22, 2020 at 01:30:31AM +0530, Neeraj Upadhyay wrote:
>> Hi Paul,
>>
>> On 6/22/2020 1:20 AM, Paul E. McKenney wrote:
>>> On Mon, Jun 22, 2020 at 12:07:27AM +0530, Neeraj Upadhyay wrote:
>>>> On callback overload, we want to force quiescent state immediately,
>>>> for the first and second fqs. Enforce the same, by including
>>>> RCU_GP_FLAG_OVLD flag, in fqsstart check.
>>>>
>>>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>>>
>>> Good catch!
>>>
>>> But what did you do to verify that this change does the right thing?
>>>
>>> 						Thanx, Paul
>>>
>>
>> I haven't done a runtime verification of this code path; I posted this,
>> based on review of this code.
> 
> My concern is that under overload, the FQS scans would happen continuously
> rather than accelerating only the first such scan in a given grace period.
> This would of course result in a CPU-bound grace-period kthread, which
> users might not be all that happy with.
> 
> Or am I missing something subtle that prevents this?

Looks like under overload, only the first and second scans are accelerated?

     gf = 0;
     if (first_gp_fqs) {
          first_gp_fqs = false;
           gf = rcu_state.cbovld ? RCU_GP_FLAG_OVLD : 0;
     }


Thanks
Neeraj

> 
> But yes, it does look like the current mainline code fails to do the
> first scan immediately, so again, good catch!
> 
> 							Thanx, Paul
>

>> Thanks
>> Neeraj
>>
>>>> ---
>>>>    kernel/rcu/tree.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>> index d0988a1..6226bfb 100644
>>>> --- a/kernel/rcu/tree.c
>>>> +++ b/kernel/rcu/tree.c
>>>> @@ -1865,7 +1865,7 @@ static void rcu_gp_fqs_loop(void)
>>>>    			break;
>>>>    		/* If time for quiescent-state forcing, do it. */
>>>>    		if (!time_after(rcu_state.jiffies_force_qs, jiffies) ||
>>>> -		    (gf & RCU_GP_FLAG_FQS)) {
>>>> +		    (gf & (RCU_GP_FLAG_FQS | RCU_GP_FLAG_OVLD))) {
>>>>    			trace_rcu_grace_period(rcu_state.name, rcu_state.gp_seq,
>>>>    					       TPS("fqsstart"));
>>>>    			rcu_gp_fqs(first_gp_fqs);
>>>> -- 
>>>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
>>>> a Linux Foundation Collaborative Project
>>>>
>>
>> -- 
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>> the Code Aurora Forum, hosted by The Linux Foundation

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation