Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation

[Joel Fernandes <joel@joelfernandes.org>]

On 9/6/2022 12:31 PM, Joel Fernandes wrote:
> 
> 
> On 9/6/2022 12:15 PM, Joel Fernandes wrote:
>>>> @@ -461,16 +521,29 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>>>  	// We need to use the bypass.
>>>>  	rcu_nocb_wait_contended(rdp);
>>>>  	rcu_nocb_bypass_lock(rdp);
>>>> +
>>>>  	ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
>>>>  	rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
>>>>  	rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
>>>> +
>>>> +	if (IS_ENABLED(CONFIG_RCU_LAZY) && lazy)
>>>> +		WRITE_ONCE(rdp->lazy_len, rdp->lazy_len + 1);
>>>> +
>>>>  	if (!ncbs) {
>>>>  		WRITE_ONCE(rdp->nocb_bypass_first, j);
>>>>  		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
>>>>  	}
>>>> +
>>>>  	rcu_nocb_bypass_unlock(rdp);
>>>>  	smp_mb(); /* Order enqueue before wake. */
>>>> -	if (ncbs) {
>>>> +
>>>> +	// We had CBs in the bypass list before. There is nothing else to do if:
>>>> +	// There were only non-lazy CBs before, in this case, the bypass timer
>>> Kind of misleading. I would replace "There were only non-lazy CBs before" with
>>> "There was at least one non-lazy CBs before".
>> I really mean "There were only non-lazy CBs ever queued in the bypass list
>> before". That's the bypass_is_lazy variable. So I did not fully understand your
>> suggested comment change.
>>
>>>> +	// or GP-thread will handle the CBs including any new lazy ones.
>>>> +	// Or, the new CB is lazy and the old bypass-CBs were also lazy. In this
>>>> +	// case the old lazy timer would have been setup. When that expires,
>>>> +	// the new lazy one will be handled.
>>>> +	if (ncbs && (!bypass_is_lazy || lazy)) {
>>>>  		local_irq_restore(flags);
>>>>  	} else {
>>>>  		// No-CBs GP kthread might be indefinitely asleep, if so, wake.
>>>> @@ -479,6 +552,10 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>>>>  			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
>>>>  					    TPS("FirstBQwake"));
>>>>  			__call_rcu_nocb_wake(rdp, true, flags);
>>>> +		} else if (bypass_is_lazy && !lazy) {
>>>> +			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
>>>> +					    TPS("FirstBQwakeLazy2Non"));
>>>> +			__call_rcu_nocb_wake(rdp, true, flags);
>>>
>>> Not sure we need this chunk. Since there are pending callbacks anyway,
>>> nocb_gp_wait() should be handling them and it will set the appropriate
>>> timer on the next loop.
>>
>> We do because those pending callbacks could be because of a bypass list flush
>> and not because there were pending CBs before, right? I do recall missed wake
>> ups of non-lazy CBs, and them having to wait for the full lazy timer duration
>> and slowing down synchronize_rcu() which is on the ChromeOS boot critical path!
>>
> 
> Just to add more details, consider the series of events:
> 
> 1. Only lazy CBs are ever queued. Timer is armed for multiple seconds.
> rcu_segcblist_pend_cbs remains false.
> 
> 2. First non-lazy CB triggers to code that does the bypyass rate-limit thing.
> 
> 3. By pass list is flushed because it is non-lazy CB and we need to start GP
> processing soon.

Correcting the events, #3 does not happen if we got here.

> 
> 4. Due to flush, rcu_segcblist_pend_cbs() is now true.

So rcu_segcblist_pend_cbs() cannot be true.

> 5. We reach this "else if" clause because bypass_is_lazy means only lazy CBs
> were ever buffered. We need to reprogram the timer or do an immediate wake up.
> That's the intention of __call_rcu_nocb_wake().
> 
> I really saw #1 and #2 trigger during boot up itself and cause a multi-second
> boot regression.

So may be this hunk is needed not needed any more and the boot regression is
fine. I can try to drop this hunk and run the tests again...

Thanks!

 - Joel