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) Tested rcutorture TREE and SRCU configurations. [apply paulmck feedack on moving logic to rcu_gp_init()] 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 8293bae1dec1..0c7710caf041 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; } -- 2.34.1
