On Wed, May 20, 2026 at 04:43:18PM +0200, Frederic Weisbecker wrote:
> Le Tue, May 19, 2026 at 09:45:23PM +0200, Uladzislau Rezki (Sony) a écrit :
> > Currently, rcu_normal_wake_from_gp is only enabled by default
> > on small systems(<= 16 CPUs) or when a user explicitly set it
> > enabled.
> >
> > Introduce an adaptive latching mechanism:
> > * Track the number of in-flight synchronize_rcu() requests
> > using a new rcu_sr_normal_count counter;
> >
> > * If the count reaches/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.
> >
> > Testing(synthetic flood workload):
> > * Kernel version: 6.19.0-rc6
> > * Number of CPUs: 1536
> > * 60K concurrent synchronize_rcu() calls
> >
> > Perf(cycles, system-wide):
> > total cycles: 932020263832
> > rcu_sr_normal_add_req(): 2650282811 cycles(~0.28%)
> >
> > Perf report excerpt:
> > 0.01% 0.01% sync_test/... [k] rcu_sr_normal_add_req
> >
> > Measured overhead of rcu_sr_normal_add_req() remained ~0.28%
> > of total CPU cycles in this synthetic stress test.
> >
> > Tested-by: Samir M <[email protected]>
> > Suggested-by: Joel Fernandes <[email protected]>
> > Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
> > ---
> > .../admin-guide/kernel-parameters.txt | 10 ++--
> > kernel/rcu/tree.c | 52 ++++++++++++++-----
> > 2 files changed, 44 insertions(+), 18 deletions(-)
> >
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt
> > b/Documentation/admin-guide/kernel-parameters.txt
> > index 4d0f545fb3ec..d5db2e85d551 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -5862,13 +5862,13 @@ Kernel parameters
> > use a call_rcu[_hurry]() path. Please note, this is for
> > a
> > normal grace period.
> >
> > - How to enable it:
> > + How to disable it:
> >
> > - echo 1 >
> > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> > - or pass a boot parameter
> > "rcutree.rcu_normal_wake_from_gp=1"
> > + echo 0 >
> > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
> > + or pass a boot parameter
> > "rcutree.rcu_normal_wake_from_gp=0"
> >
> > - Default is 1 if num_possible_cpus() <= 16 and it is not
> > explicitly
> > - disabled by the boot parameter passing 0.
> > + Default is 1 if it is not explicitly disabled by the
> > boot parameter
> > + passing 0.
> >
> > rcuscale.gp_async= [KNL]
> > Measure performance of asynchronous
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 09f0cef5014c..94274330d1db 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1632,17 +1632,21 @@ 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 int rcu_sr_normal_latched; /* 0/1 */
> > +
> > 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),
> > @@ -1650,6 +1654,15 @@ 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)cmpxchg(&rcu_sr_normal_latched, 1, 0);
>
> Given that it's already ordered by the llist add / del and the
> atomic_long_inc/dec_return, there should be no chance for bad
> things happening such as negative returned dec.
>
> So it could be cmpxchg_relaxed(). But anyway, just an optimization.
>
> In any case,
>
> Reviewed-by: Frederic Weisbecker <[email protected]>
>
Hello, Frederic!
I change it accordingly, please check!
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 94274330d1db..2c76b59f75de 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1655,14 +1655,13 @@ 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)cmpxchg(&rcu_sr_normal_latched, 1, 0);
+ (void)cmpxchg_relaxed(&rcu_sr_normal_latched, 1, 0);
}
static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
@@ -1823,7 +1822,7 @@ static void rcu_sr_normal_add_req(struct rcu_synchronize
*rs)
* because it only selects between the fast and fallback paths.
*/
if (nr == RCU_SR_NORMAL_LATCH_THR)
- (void)cmpxchg(&rcu_sr_normal_latched, 0, 1);
+ (void)cmpxchg_relaxed(&rcu_sr_normal_latched, 0, 1);
/* Publish for the GP kthread/worker. */
llist_add((struct llist_node *) &rs->head, &rcu_state.srs_next);
Sounds good?
--
Uladzislau Rezki
