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Message-ID: <1f7821ce-c036-4824-bb22-6d171714babf@joelfernandes.org>
Date: Tue, 19 Mar 2024 14:52:43 -0400
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Subject: Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for
 synchronize_rcu() common case
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To: Uladzislau Rezki <urezki@gmail.com>
Cc: linux-kernel@vger.kernel.org, frederic@kernel.org, boqun.feng@gmail.com,
 neeraj.iitr10@gmail.com, rcu@vger.kernel.org, rostedt@goodmis.org,
 "Paul E. McKenney" <paulmck@kernel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
 Josh Triplett <josh@joshtriplett.org>,
 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
 Lai Jiangshan <jiangshanlai@gmail.com>, Zqiang <qiang.zhang1211@gmail.com>
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 <851d8642-807c-481a-91e9-8b744c24913d@joelfernandes.org>
 <Zfnbc83lU6t8nIao@pc636>
From: Joel Fernandes <joel@joelfernandes.org>
In-Reply-To: <Zfnbc83lU6t8nIao@pc636>
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On 3/19/2024 2:37 PM, Uladzislau Rezki wrote:
> On Tue, Mar 19, 2024 at 01:33:11PM -0400, Joel Fernandes wrote:

>>> On 3/19/2024 1:26 PM, Uladzislau Rezki wrote:

>>>>>>>>>>>> /*
>>>>>>>>>>>> @@ -1673,7 +1680,7 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>>>>>>>>>>>> */
>>>>>>>>>>>> static void rcu_sr_normal_gp_cleanup(void)
>>>>>>>>>>>> {
>>>>>>>>>>>> -    struct llist_node *wait_tail, *next, *rcu;
>>>>>>>>>>>> +    struct llist_node *wait_tail, *next = NULL, *rcu = NULL;
>>>>>>>>>>>>   int done = 0;
>>>>>>>>>>>>
>>>>>>>>>>>>   wait_tail = rcu_state.srs_wait_tail;
>>>>>>>>>>>> @@ -1699,16 +1706,35 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>>>>>>>>>>           break;
>>>>>>>>>>>>   }
>>>>>>>>>>>>
>>>>>>>>>>>> -    // concurrent sr_normal_gp_cleanup work might observe this update.
>>>>>>>>>>>> -    smp_store_release(&rcu_state.srs_done_tail, wait_tail);
>>>>>>>>>>>> +    /*
>>>>>>>>>>>> +     * Fast path, no more users to process. Remove the last wait head
>>>>>>>>>>>> +     * if no inflight-workers. If there are in-flight workers, let them
>>>>>>>>>>>> +     * remove the last wait head.
>>>>>>>>>>>> +     */
>>>>>>>>>>>> +    WARN_ON_ONCE(!rcu);
>>>>>>>>>>>>
>>>>>>>>>>> This assumption is not correct. An "rcu" can be NULL in fact.
>>>>>>>>>>
>>>>>>>>>> Hmm I could never trigger that. Are you saying that is true after Neeraj recent patch or something else?
>>>>>>>>>> Note, after Neeraj patch to handle the lack of heads availability, it could be true so I requested
>>>>>>>>>> him to rebase his patch on top of this one.
>>>>>>>>>>
>>>>>>>>>> However I will revisit my patch and look for if it could occur but please let me know if you knew of a sequence of events to make it NULL.
>>>>>>>>>>>
>>>>>>>>> I think we should agree on your patch first otherwise it becomes a bit
>>>>>>>>> messy or go with a Neeraj as first step and then work on youth. So, i
>>>>>>>>> reviewed this patch based on latest Paul's dev branch. I see that Neeraj
>>>>>>>>> needs further work.
>>>>>>>>
>>>>>>>> You are right. So the only change is to drop the warning and those braces. Agreed?
>>>>>>>>
>>>>>>> Let me check a bit. Looks like correct but just in case.
>>>>>>>
>>>>>>
>>>>>> Thanks. I was also considering improving it for the rcu == NULL case, as
>>>>>> below. I will test it more before re-sending.
>>>>>>
>>>>>> On top of my patch:
>>>>>>
>>>>>> ---8<-----------------------
>>>>>>
>>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>>>> index 0df659a878ee..a5ef844835d4 100644
>>>>>> --- a/kernel/rcu/tree.c
>>>>>> +++ b/kernel/rcu/tree.c
>>>>>> @@ -1706,15 +1706,18 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>>>>                         break;
>>>>>>         }
>>>>>>
>>>>>> +
>>>>>> +       /* Last head stays. No more processing to do. */
>>>>>> +       if (!rcu)
>>>>>> +               return;
>>>>>> +
>>>>>
>>>>> Ugh, should be "if (!wait_head->next)"  instead of "if (!rcu)".  But
>>>>> in any case, the original patch except the warning should hold.
>>>>> Still, I am testing the above diff now.
>>>>>
>>>>>  - Joel
>>>>>
>>>> Just in case, it is based on your patch:
>>>>
>>>> <snip>
>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>> index bd29fe3c76bf..98546afe7c21 100644
>>>> --- a/kernel/rcu/tree.c
>>>> +++ b/kernel/rcu/tree.c
>>>> @@ -1711,29 +1711,25 @@ static void rcu_sr_normal_gp_cleanup(void)
>>>>  	 * if no inflight-workers. If there are in-flight workers, let them
>>>>  	 * remove the last wait head.
>>>>  	 */
>>>> -	WARN_ON_ONCE(!rcu);
>>>> -	ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
>>>> -
>>>> -	if (rcu && rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
>>>> -		/* Order atomic access with list manipulation. */
>>>> -		!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
>>>> +	if (wait_tail->next && rcu_sr_is_wait_head(wait_tail->next) && !wait_tail->next->next &&
>>>> +			!atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
>>>
>>>
>>> Yes this also works. But also if wait_tail->next == NULL, then you do not need
>>> to queue worker for that case as well. I sent this as v3.
>>>
>> Sorry, I see you did add that later in the patch ;-). I think we have converged
>> on the final patch then, give or take the use of 'rcu' versus 'wait_tail->next'.
>>
> Just combine all parts into one place and resend :)

Yes sir ;)

 - Joel