> On Jan 11, 2026, at 5:13 AM, Samir M <[email protected]> wrote: > > >> On 30/12/25 6:11 am, Joel Fernandes wrote: >> The RCU grace period mechanism uses a two-phase FQS (Force Quiescent >> State) design where the first FQS saves dyntick-idle snapshots and >> the second FQS compares them. This results in long and unnecessary latency >> for synchronize_rcu() on idle systems (two FQS waits of ~3ms each with >> 1000HZ) whenever one FQS wait sufficed. >> >> Some investigations showed that the GP kthread's CPU is the holdout CPU >> a lot of times after the first FQS as - it cannot be detected as "idle" >> because it's actively running the FQS scan in the GP kthread. >> >> Therefore, at the end of rcu_gp_init(), immediately report a quiescent >> state for the GP kthread's CPU using rcu_qs() + rcu_report_qs_rdp(). The >> GP kthread cannot be in an RCU read-side critical section while running >> GP initialization, so this is safe and results in significant latency >> improvements. >> >> I benchmarked 100 synchronize_rcu() calls with 32 CPUs, 10 runs each >> showing significant latency improvements (default settings for fqs jiffies): >> >> Baseline (without fix): >> | Run | Mean | Min | Max | >> |-----|-----------|----------|-----------| >> | 1 | 10.088 ms | 9.989 ms | 18.848 ms | >> | 2 | 10.064 ms | 9.982 ms | 16.470 ms | >> | 3 | 10.051 ms | 9.988 ms | 15.113 ms | >> | 4 | 10.125 ms | 9.929 ms | 22.411 ms | >> | 5 | 8.695 ms | 5.996 ms | 15.471 ms | >> | 6 | 10.157 ms | 9.977 ms | 25.723 ms | >> | 7 | 10.102 ms | 9.990 ms | 20.224 ms | >> | 8 | 8.050 ms | 5.985 ms | 10.007 ms | >> | 9 | 10.059 ms | 9.978 ms | 15.934 ms | >> | 10 | 10.077 ms | 9.984 ms | 17.703 ms | >> >> With fix: >> | Run | Mean | Min | Max | >> |-----|----------|----------|-----------| >> | 1 | 6.027 ms | 5.915 ms | 8.589 ms | >> | 2 | 6.032 ms | 5.984 ms | 9.241 ms | >> | 3 | 6.010 ms | 5.986 ms | 7.004 ms | >> | 4 | 6.076 ms | 5.993 ms | 10.001 ms | >> | 5 | 6.084 ms | 5.893 ms | 10.250 ms | >> | 6 | 6.034 ms | 5.908 ms | 9.456 ms | >> | 7 | 6.051 ms | 5.993 ms | 10.000 ms | >> | 8 | 6.057 ms | 5.941 ms | 10.001 ms | >> | 9 | 6.016 ms | 5.927 ms | 7.540 ms | >> | 10 | 6.036 ms | 5.993 ms | 9.579 ms | >> >> Summary: >> - Mean latency: 9.75 ms -> 6.04 ms (38% improvement) >> - Max latency: 25.72 ms -> 10.25 ms (60% improvement) >> >> Additional bridge setup/teardown testing by Uladzislau Rezki on x86_64 >> with 64 CPUs (100 iterations of bridge add/configure/delete): >> >> real time >> 1 - default: 24.221s >> 2 - this patch: 20.754s (14% faster) >> 3 - this patch + wake_from_gp: 15.895s (34% faster) >> 4 - wake_from_gp only: 18.947s (22% faster) >> >> Per-synchronize_rcu() latency (in usec): >> 1 2 3 4 >> median: 37249.5 31540.5 15765 22480 >> min: 7881 7918 9803 7857 >> max: 63651 55639 31861 32040 >> >> This patch combined with rcu_normal_wake_from_gp reduces bridge >> setup/teardown time from 24 seconds to 16 seconds. >> >> Tested rcutorture TREE and SRCU configurations. >> >> Reviewed-by: Paul E. McKenney <[email protected]> >> Tested-by: Uladzislau Rezki (Sony) <[email protected]> >> Signed-off-by: Joel Fernandes <[email protected]> >> --- >> kernel/rcu/tree.c | 12 ++++++++++++ >> 1 file changed, 12 insertions(+) >> >> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c >> index 78c045a5ef03..b7c818cabe44 100644 >> --- a/kernel/rcu/tree.c >> +++ b/kernel/rcu/tree.c >> @@ -160,6 +160,7 @@ static void rcu_report_qs_rnp(unsigned long mask, struct >> rcu_node *rnp, >> unsigned long gps, unsigned long flags); >> static void invoke_rcu_core(void); >> static void rcu_report_exp_rdp(struct rcu_data *rdp); >> +static void rcu_report_qs_rdp(struct rcu_data *rdp); >> static void check_cb_ovld_locked(struct rcu_data *rdp, struct rcu_node >> *rnp); >> static bool rcu_rdp_is_offloaded(struct rcu_data *rdp); >> static bool rcu_rdp_cpu_online(struct rcu_data *rdp); >> @@ -1983,6 +1984,17 @@ static noinline_for_stack bool rcu_gp_init(void) >> if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD)) >> on_each_cpu(rcu_strict_gp_boundary, NULL, 0); >> + /* >> + * Immediately report QS for the GP kthread's CPU. The GP kthread >> + * cannot be in an RCU read-side critical section while running >> + * the FQS scan. This eliminates the need for a second FQS wait >> + * when all CPUs are idle. >> + */ >> + preempt_disable(); >> + rcu_qs(); >> + rcu_report_qs_rdp(this_cpu_ptr(&rcu_data)); >> + preempt_enable(); >> + >> return true; >> } >> > > Hi, > > > I verified this patch on ppc64 systems and observed consistent performance > improvements. > > The testing was conducted on Power LPARs using 20 cores (160 CPUs) with SMT > enabled and disabled. All tests were performed on the latest upstream kernel > (v6.19.0-rc3+), and the patch showed measurable improvements in both SMT > configurations. > > SMT Mode | With Patch (s) | Without Patch (s) | Improvement (%) > ———————————————————————————————————— > SMT ON. | 51.662 | 75.540 | 31.61% faster > SMT OFF | 44.246 | 59.933 | 26.18% faster > > Please add below tag: Tested-by: Samir M <[email protected]>
Thanks! Boqun, do you mind adding the tag to the applied patch? - Joel > > Thanks, > Samir >
