Re: [PATCH v4] rcu/tree: Reduce wake up for synchronize_rcu() common case

[Paul E. McKenney]

On Wed, Mar 20, 2024 at 03:36:13PM +0100, Uladzislau Rezki wrote:
> On Tue, Mar 19, 2024 at 02:54:57PM -0400, Joel Fernandes (Google) wrote:
> > 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>
> > ---
> > v1->v2: Rebase on paul/dev
> > v2->v3: Additional optimization for wait_tail->next == NULL case.
> > v3->v4: Apply clean ups from Vlad. Tested rcutorture all scenarios.
> > ---
> >  kernel/rcu/tree.c | 35 ++++++++++++++++++++++++++++++-----
> >  kernel/rcu/tree.h |  1 +
> >  2 files changed, 31 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 9fbb5ab57c84..f3193670fe42 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 +1706,34 @@ 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 except putting the second last
> > +	 * wait head if no inflight-workers. If there are in-flight workers,
> > +	 * they will remove the last wait head.
> > +	 *
> > +	 * Note that the ACQUIRE orders atomic access with list manipulation.
> > +	 */
> > +	if (wait_tail->next && wait_tail->next->next == NULL &&
> > +	    rcu_sr_is_wait_head(wait_tail->next) &&
> > +	    !atomic_read_acquire(&rcu_state.srs_cleanups_pending)) {
> > +		rcu_sr_put_wait_head(wait_tail->next);
> > +		wait_tail->next = NULL;
> > +	}
> > +
> > +	/* 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);
> >  
> >  	/*
> >  	 * 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);
> > +	if (wait_tail->next) {
> > +		atomic_inc(&rcu_state.srs_cleanups_pending);
> > +		if (!queue_work(sync_wq, &rcu_state.srs_cleanup_work))
> > +			atomic_dec(&rcu_state.srs_cleanups_pending);
> > +	}
> >  }
> >  
> >  /*
> > 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
> > 
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>

Queued and pushed, thank you both!

							Thanx, Paul