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

[Joel Fernandes <joel@joelfernandes.org>]

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@gmail.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@gmail.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@joelfernandes.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;
+
 	/*
 	 * 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);
-	ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
 
-	if (rcu && rcu_sr_is_wait_head(rcu) && rcu->next == NULL &&
+	if (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;
@@ -1724,6 +1727,7 @@ static void rcu_sr_normal_gp_cleanup(void)
 	}
 
 	/* Concurrent sr_normal_gp_cleanup work might observe this update. */
+	ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);
 	smp_store_release(&rcu_state.srs_done_tail, wait_tail);
 
 	/*