Le Wed, May 20, 2026 at 05:16:19PM +0200, Uladzislau Rezki a écrit : > 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);
Why dropping this? Otherwise yes, looks good! Thanks! > > /* > * 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 > -- Frederic Weisbecker SUSE Labs
