Expedite synchronize_rcu() during the cpuhp_smt_[enable|disable] path to accelerate the operation.
Bulk CPU hotplug operations—such as switching SMT modes across all cores—require hotplugging multiple CPUs in rapid succession. On large systems, this process takes significant time, increasing as the number of CPUs to hotplug during SMT switch grows, leading to substantial delays on high-core-count machines. Analysis [1] reveals that the majority of this time is spent waiting for synchronize_rcu(). SMT switch is a user-initiated administrative task, it should complete as quickly as possible. Performance data on a PPC64 system with 2048 CPUs: + ppc64_cpu --smt=1 (SMT8 to SMT1) Before: real 30m53.194s After: real 6m4.678s # ~5x improvement + ppc64_cpu --smt=8 (SMT1 to SMT8) Before: real 49m5.920s After: real 36m47.798s # ~1.3x improvement [1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.ca...@linux.vnet.ibm.com Signed-off-by: Vishal Chourasia <[email protected]> Tested-by: Samir M <[email protected]> --- include/linux/rcupdate.h | 3 +++ kernel/cpu.c | 4 ++++ 2 files changed, 7 insertions(+) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index c5b30054cd01..03c06cfb2b6d 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -1192,6 +1192,9 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f) extern int rcu_expedited; extern int rcu_normal; +extern void rcu_expedite_gp(void); +extern void rcu_unexpedite_gp(void); + DEFINE_LOCK_GUARD_0(rcu, do { rcu_read_lock(); diff --git a/kernel/cpu.c b/kernel/cpu.c index 8df2d773fe3b..a264d7170842 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -2669,6 +2669,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) int cpu, ret = 0; cpu_maps_update_begin(); + rcu_expedite_gp(); for_each_online_cpu(cpu) { if (topology_is_primary_thread(cpu)) continue; @@ -2698,6 +2699,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) } if (!ret) cpu_smt_control = ctrlval; + rcu_unexpedite_gp(); cpu_maps_update_done(); return ret; } @@ -2716,6 +2718,7 @@ int cpuhp_smt_enable(void) cpu_maps_update_begin(); cpu_smt_control = CPU_SMT_ENABLED; + rcu_expedite_gp(); for_each_present_cpu(cpu) { /* Skip online CPUs and CPUs on offline nodes */ if (cpu_online(cpu) || !node_online(cpu_to_node(cpu))) @@ -2728,6 +2731,7 @@ int cpuhp_smt_enable(void) /* See comment in cpuhp_smt_disable() */ cpuhp_online_cpu_device(cpu); } + rcu_unexpedite_gp(); cpu_maps_update_done(); return ret; } -- 2.52.0
