Re: [PATCH v2 rcu/dev 1/2] rcu/tree: Reduce wake up for synchronize_rcu() common case

[Joel Fernandes]

> On Mar 18, 2024, at 2:58 PM, Uladzislau Rezki <urezki@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
>> ---
>> 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.
> 
>>    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)) {
>> +        wait_tail->next = NULL;
>> +        rcu_sr_put_wait_head(rcu);
>> +        smp_store_release(&rcu_state.srs_done_tail, wait_tail);
>> +        return;
>> +    }
>> +
>> +    /* Concurrent sr_normal_gp_cleanup work might observe this update. */
>> +    smp_store_release(&rcu_state.srs_done_tail, wait_tail);
>> +
>>    /*
>>     * We schedule a work in order to perform a final processing
>>     * of outstanding users(if still left) and releasing wait-heads
>>     * added by rcu_sr_normal_gp_init() call.
>>     */
>> -    queue_work(sync_wq, &rcu_state.srs_cleanup_work);
>> +    atomic_inc(&rcu_state.srs_cleanups_pending);
>> +    if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work)) {
>> +        atomic_dec(&rcu_state.srs_cleanups_pending);
>> +    }
>> }
> No need an extra "{}" pair.

I do prefer it for readability but I am ok with dropping it.

Thanks!

 - Joel 


> 
>> 
>> /*
>> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
>> index bae7925c497f..affcb92a358c 100644
>> --- a/kernel/rcu/tree.h
>> +++ b/kernel/rcu/tree.h
>> @@ -420,6 +420,7 @@ struct rcu_state {
>>    struct llist_node *srs_done_tail; /* ready for GP users. */
>>    struct sr_wait_node srs_wait_nodes[SR_NORMAL_GP_WAIT_HEAD_MAX];
>>    struct work_struct srs_cleanup_work;
>> +    atomic_t srs_cleanups_pending; /* srs inflight worker cleanups. */
>> };
>> 
>> /* Values for rcu_state structure's gp_flags field. */
>> --
>> 2.34.1
>>