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McKenney" , Joel Fernandes , Boqun Feng , RCU , LKML , Samir M Subject: Re: [PATCH -next v2 10/11] rcu: Latch normal synchronize_rcu() path on flood Message-ID: References: <20260519194524.158515-1-urezki@gmail.com> <20260519194524.158515-11-urezki@gmail.com> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: 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 > > Suggested-by: Joel Fernandes > > Signed-off-by: Uladzislau Rezki (Sony) > > --- > > .../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 > 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