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]> -- Frederic Weisbecker SUSE Labs
