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Message-ID: <8f221ab6-6d34-4c3b-a6a7-6c1de405000a@joelfernandes.org>
Date: Tue, 19 Mar 2024 13:29:49 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: Uladzislau Rezki <urezki@...il.com>
Cc: linux-kernel@...r.kernel.org, frederic@...nel.org, boqun.feng@...il.com,
neeraj.iitr10@...il.com, rcu@...r.kernel.org, rostedt@...dmis.org,
"Paul E. McKenney" <paulmck@...nel.org>,
Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
Josh Triplett <josh@...htriplett.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for
synchronize_rcu() common case
On 3/19/2024 1:26 PM, Uladzislau Rezki wrote:
> On Tue, Mar 19, 2024 at 12:11:28PM -0400, Joel Fernandes wrote:
>> On Tue, Mar 19, 2024 at 12:02 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>>>
>>> On Tue, Mar 19, 2024 at 03:48:46PM +0100, Uladzislau Rezki wrote:
>>>> On Tue, Mar 19, 2024 at 10:29:59AM -0400, Joel Fernandes wrote:
>>>>>
>>>>>
>>>>>> On Mar 19, 2024, at 5:53 AM, Uladzislau Rezki <urezki@...il.com> wrote:
>>>>>>
>>>>>> On Mon, Mar 18, 2024 at 05:05:31PM -0400, Joel Fernandes wrote:
>>>>>>>
>>>>>>>
>>>>>>>>> On Mar 18, 2024, at 2:58 PM, Uladzislau Rezki <urezki@...il.com> wrote:
>>>>>>>>
>>>>>>>> Hello, Joel!
>>>>>>>>
>>>>>>>> Sorry for late checking, see below few comments:
>>>>>>>>
>>>>>>>>> In the synchronize_rcu() common case, we will have less than
>>>>>>>>> SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
>>>>>>>>> is pointless just to free the last injected wait head since at that point,
>>>>>>>>> all the users have already been awakened.
>>>>>>>>>
>>>>>>>>> Introduce a new counter to track this and prevent the wakeup in the
>>>>>>>>> common case.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>>>>>>>>> ---
>>>>>>>>> Rebased on paul/dev of today.
>>>>>>>>>
>>>>>>>>> kernel/rcu/tree.c | 36 +++++++++++++++++++++++++++++++-----
>>>>>>>>> kernel/rcu/tree.h | 1 +
>>>>>>>>> 2 files changed, 32 insertions(+), 5 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>>>>>>> index 9fbb5ab57c84..bd29fe3c76bf 100644
>>>>>>>>> --- a/kernel/rcu/tree.c
>>>>>>>>> +++ b/kernel/rcu/tree.c
>>>>>>>>> @@ -96,6 +96,7 @@ static struct rcu_state rcu_state = {
>>>>>>>>> .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
>>>>>>>>> .srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
>>>>>>>>> rcu_sr_normal_gp_cleanup_work),
>>>>>>>>> + .srs_cleanups_pending = ATOMIC_INIT(0),
>>>>>>>>> };
>>>>>>>>>
>>>>>>>>> /* Dump rcu_node combining tree at boot to verify correct setup. */
>>>>>>>>> @@ -1642,8 +1643,11 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>>>>>>>>> * the done tail list manipulations are protected here.
>>>>>>>>> */
>>>>>>>>> done = smp_load_acquire(&rcu_state.srs_done_tail);
>>>>>>>>> - if (!done)
>>>>>>>>> + if (!done) {
>>>>>>>>> + /* See comments below. */
>>>>>>>>> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
>>>>>>>>> return;
>>>>>>>>> + }
>>>>>>>>>
>>>>>>>>> WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
>>>>>>>>> head = done->next;
>>>>>>>>> @@ -1666,6 +1670,9 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>>>>>>>>>
>>>>>>>>> rcu_sr_put_wait_head(rcu);
>>>>>>>>> }
>>>>>>>>> +
>>>>>>>>> + /* Order list manipulations with atomic access. */
>>>>>>>>> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> /*
>>>>>>>>> @@ -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.
If you want to add that and resend my patch with the above diff, that would also
be fine. Or I can do that, let me know. Thanks!
- Joel
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