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
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation
Date: Tue, 6 Sep 2022 12:43:52 -0400 [thread overview]
Message-ID: <4f6061f0-0de7-2916-dc6e-9f5af9b944c0@joelfernandes.org> (raw)
In-Reply-To: <da45d265-52f9-6314-7fcd-ea71e2bf4cec@joelfernandes.org>
On 9/6/2022 12:38 PM, Joel Fernandes wrote:
>
>
> 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...
Ah, now I know why I got confused. I *used* to flush the bypass list before when
!lazy CBs showed up. Paul suggested this is overkill. In this old overkill
method, I was missing a wake up which was likely causing the boot regression.
Forcing a wake up fixed that. Now in v5 I make it such that I don't do the flush
on a !lazy rate-limit.
I am sorry for the confusion. Either way, in my defense this is just an extra
bit of code that I have to delete. This code is hard. I have mostly relied on a
test-driven development. But now thanks to this review and I am learning the
code more and more...
Thanks,
- Joel
next prev parent reply other threads:[~2022-09-06 16:57 UTC|newest]
Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-01 22:17 [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 01/18] mm/slub: perform free consistency checks before call_rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 02/18] mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head Joel Fernandes (Google)
2022-09-02 9:26 ` Vlastimil Babka
2022-09-02 9:30 ` Vlastimil Babka
2022-09-02 15:09 ` Joel Fernandes
2022-09-03 13:53 ` Paul E. McKenney
2022-09-01 22:17 ` [PATCH v5 03/18] rcu/tree: Use READ_ONCE() for lockless read of rnp->qsmask Joel Fernandes (Google)
2022-09-06 22:26 ` Boqun Feng
2022-09-06 22:31 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 04/18] rcu: Fix late wakeup when flush of bypass cblist happens Joel Fernandes (Google)
2022-09-02 11:35 ` Frederic Weisbecker
2022-09-02 23:58 ` Joel Fernandes
2022-09-03 15:10 ` Paul E. McKenney
2022-09-04 21:13 ` Frederic Weisbecker
2022-09-03 14:04 ` Paul E. McKenney
2022-09-03 14:05 ` Joel Fernandes
2022-09-06 3:07 ` Joel Fernandes
2022-09-06 9:48 ` Frederic Weisbecker
2022-09-07 2:43 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 05/18] rcu: Move trace_rcu_callback() before bypassing Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API implementation Joel Fernandes (Google)
2022-09-02 15:21 ` Frederic Weisbecker
2022-09-02 23:09 ` Joel Fernandes
2022-09-05 12:59 ` Frederic Weisbecker
2022-09-05 20:18 ` Joel Fernandes
2022-09-05 20:32 ` Joel Fernandes
2022-09-06 8:55 ` Paul E. McKenney
2022-09-06 16:16 ` Joel Fernandes
2022-09-06 17:05 ` Paul E. McKenney
2022-09-03 22:00 ` Joel Fernandes
2022-09-04 21:01 ` Frederic Weisbecker
2022-09-05 20:20 ` Joel Fernandes
2022-09-06 3:05 ` Joel Fernandes
2022-09-06 15:17 ` Frederic Weisbecker
2022-09-06 16:15 ` Joel Fernandes
2022-09-06 16:31 ` Joel Fernandes
2022-09-06 16:38 ` Joel Fernandes
2022-09-06 16:43 ` Joel Fernandes [this message]
2022-09-06 19:11 ` Frederic Weisbecker
2022-09-07 2:56 ` Joel Fernandes
2022-09-07 9:56 ` Frederic Weisbecker
2022-09-07 10:03 ` Frederic Weisbecker
2022-09-07 14:01 ` Joel Fernandes
2022-09-07 0:06 ` Joel Fernandes
2022-09-07 9:40 ` Frederic Weisbecker
2022-09-07 13:44 ` Joel Fernandes
2022-09-07 15:38 ` Frederic Weisbecker
2022-09-07 15:39 ` Joel Fernandes
2022-09-21 23:52 ` Joel Fernandes
2022-09-06 18:16 ` Uladzislau Rezki
2022-09-06 18:21 ` Joel Fernandes
2022-09-07 8:52 ` Uladzislau Rezki
2022-09-07 15:23 ` Joel Fernandes
2022-09-03 14:03 ` Paul E. McKenney
2022-09-03 14:05 ` Joel Fernandes
2022-09-01 22:17 ` [PATCH v5 07/18] rcu: shrinker for lazy rcu Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 08/18] rcu: Add per-CB tracing for queuing, flush and invocation Joel Fernandes (Google)
2022-09-02 16:48 ` kernel test robot
2022-09-03 12:39 ` Paul E. McKenney
2022-09-03 14:07 ` Joel Fernandes
2022-09-02 19:01 ` kernel test robot
2022-09-01 22:17 ` [PATCH v5 09/18] rcuscale: Add laziness and kfree tests Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 10/18] rcutorture: Add test code for call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 11/18] fs: Move call_rcu() to call_rcu_lazy() in some paths Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 12/18] cred: Move call_rcu() to call_rcu_lazy() Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 13/18] security: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 14/18] net/core: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 15/18] kernel: Move various core kernel usages " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 16/18] lib: Move call_rcu() " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 17/18] i915: " Joel Fernandes (Google)
2022-09-01 22:17 ` [PATCH v5 18/18] fork: Move thread_stack_free_rcu() " Joel Fernandes (Google)
2022-09-03 15:22 ` [PATCH v5 00/18] Implement call_rcu_lazy() and miscellaneous fixes Paul E. McKenney
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