Re: [PATCH] rcu: Latch normal synchronize_rcu() path on flood

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Jan 15, 2026 at 10:53:47PM +0530, Shrikanth Hegde wrote:
> +samir
> 
> On 1/15/26 12:04 AM, Uladzislau Rezki (Sony) wrote:
> > Currently, rcu_normal_wake_from_gp is only enabled by default
> > on small systems(<= 16 CPUs) or when a user explicitly set it
> > enabled.
> > 
> > This patch introduces an adaptive latching mechanism:
> >   * Tracks the number of in-flight synchronize_rcu() requests
> >     using a new atomic_t counter(rcu_sr_normal_count);
> > 
> 
> is this atomic variable getting updated by multiple CPUs at the
> same time? We had seen in past such updates tend to be very costly.
> 
The threshold we maintain is 64 users in a queue. So first 64
parallel users create contention but it is mitigated right away
by switching to another path. Latching normal path due to flood.

> >   * If the count exceeds RCU_SR_NORMAL_LATCH_THR(64), it sets
> >     the rcu_sr_normal_latched, reverting new requests onto the
> >     scaled wait_rcu_gp() path;
> > 
> >   * The latch is cleared only when the pending requests are fully
> >     drained(nr == 0);
> > 
> >   * Enables rcu_normal_wake_from_gp by default for all systems,
> >     relying on this dynamic throttling instead of static CPU
> >     limits.
> > 
> > Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > ---
> >   kernel/rcu/tree.c | 37 ++++++++++++++++++++++++++-----------
> >   1 file changed, 26 insertions(+), 11 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 293bbd9ac3f4..c42d480d6e0b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1631,17 +1631,21 @@ static void rcu_sr_put_wait_head(struct llist_node *node)
> >   	atomic_set_release(&sr_wn->inuse, 0);
> >   }
> > -/* Enable rcu_normal_wake_from_gp automatically on small systems. */
> > -#define WAKE_FROM_GP_CPU_THRESHOLD 16
> > -
> > -static int rcu_normal_wake_from_gp = -1;
> > +static int rcu_normal_wake_from_gp = 1;
> >   module_param(rcu_normal_wake_from_gp, int, 0644);
> >   static struct workqueue_struct *sync_wq;
> > +#define RCU_SR_NORMAL_LATCH_THR 64
> > +
> > +/* Number of in-flight synchronize_rcu() calls queued on srs_next. */
> > +static atomic_long_t rcu_sr_normal_count;
> > +static atomic_t rcu_sr_normal_latched;
> > +
> >   static void rcu_sr_normal_complete(struct llist_node *node)
> >   {
> >   	struct rcu_synchronize *rs = container_of(
> >   		(struct rcu_head *) node, struct rcu_synchronize, head);
> > +	long nr;
> >   	WARN_ONCE(IS_ENABLED(CONFIG_PROVE_RCU) &&
> >   		!poll_state_synchronize_rcu_full(&rs->oldstate),
> > @@ -1649,6 +1653,15 @@ static void rcu_sr_normal_complete(struct llist_node *node)
> >   	/* Finally. */
> >   	complete(&rs->completion);
> > +	nr = atomic_long_dec_return(&rcu_sr_normal_count);
> > +	WARN_ON_ONCE(nr < 0);
> > +
> > +	/*
> > +	 * Unlatch: switch back to normal path when fully
> > +	 * drained and if it has been latched.
> > +	 */
> > +	if (nr == 0)
> > +		(void)atomic_cmpxchg(&rcu_sr_normal_latched, 1, 0);
> >   }
> >   static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> > @@ -1794,7 +1807,14 @@ static bool rcu_sr_normal_gp_init(void)
> >   static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> >   {
> > +	long nr;
> > +
> >   	llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
> > +	nr = atomic_long_inc_return(&rcu_sr_normal_count);
> > +
> > +	/* Latch: only when flooded and if unlatched. */
> > +	if (nr >= RCU_SR_NORMAL_LATCH_THR)
> > +		(void)atomic_cmpxchg(&rcu_sr_normal_latched, 0, 1);
> >   }
> >   /*
> > @@ -3268,7 +3288,8 @@ static void synchronize_rcu_normal(void)
> >   	trace_rcu_sr_normal(rcu_state.name, &rs.head, TPS("request"));
> > -	if (READ_ONCE(rcu_normal_wake_from_gp) < 1) {
> > +	if (READ_ONCE(rcu_normal_wake_from_gp) < 1 ||
> > +			atomic_read(&rcu_sr_normal_latched)) {
> >   		wait_rcu_gp(call_rcu_hurry);
> >   		goto trace_complete_out;
> >   	}
> > @@ -4892,12 +4913,6 @@ void __init rcu_init(void)
> >   	sync_wq = alloc_workqueue("sync_wq", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
> >   	WARN_ON(!sync_wq);
> > -	/* Respect if explicitly disabled via a boot parameter. */
> > -	if (rcu_normal_wake_from_gp < 0) {
> > -		if (num_possible_cpus() <= WAKE_FROM_GP_CPU_THRESHOLD)
> > -			rcu_normal_wake_from_gp = 1;
> > -	}
> > -
> >   	/* Fill in default value for rcutree.qovld boot parameter. */
> >   	/* -After- the rcu_node ->lock fields are initialized! */
> >   	if (qovld < 0)
> 
> 
> Samir,
> Could you please give this patch a try on 1000+ cpu system?
> 
> Specifically test time taken for SMT1 to SMT8 and SMT8 to SMT1 switching
> time.
> 
> 
> Uladzislau, Is there any specific testing(other than above) you are looking for?
> 
Yep, your test cases which are important for you. You also run it on
~400 CPUs system according to previous conversation in first topic.

--
Uladzislau Rezki