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.
4. Due to flush, rcu_segcblist_pend_cbs() is now 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.
The chunk is needed to handle this case. I indeed do not see a boot regression
any more. Did I miss something?
Thanks,
- Joel
