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Message-ID: <64f5770c-df37-8975-200d-7908de23fa73@joelfernandes.org>
Date:   Tue, 6 Sep 2022 12:31:06 -0400
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Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API
 implementation
Content-Language: en-US
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        rushikesh.s.kadam@intel.com, urezki@gmail.com,
        neeraj.iitr10@gmail.com, paulmck@kernel.org, rostedt@goodmis.org,
        vineeth@bitbyteword.org, boqun.feng@gmail.com
References: <20220901221720.1105021-1-joel@joelfernandes.org>
 <20220901221720.1105021-7-joel@joelfernandes.org>
 <20220902152132.GA115525@lothringen> <Yxa5Ch574cRZxRdo@google.com>
 <20220906151757.GA183806@lothringen>
 <cde6586e-ae61-5e85-3c9a-1ce7dd2464ed@joelfernandes.org>
In-Reply-To: <cde6586e-ae61-5e85-3c9a-1ce7dd2464ed@joelfernandes.org>
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Precedence: bulk


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