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

[Uladzislau Rezki]

On Tue, Mar 19, 2024 at 10:29:59AM -0400, Joel Fernandes wrote:
> 
> 
> > On Mar 19, 2024, at 5:53 AM, Uladzislau Rezki <urezki@xxxxxxxxx> 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@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.
> >>> 
> > 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.

>
> I will resend the patch and we can discuss during tomorrow call as well.
> 
Good :)

--
Uladzislau Rezki