Re: [PATCH v5 1/1] cpuhp: Expedite RCU when toggling system-wide SMT mode

[Shrikanth Hegde]

On 6/15/26 8:35 PM, Paul E. McKenney wrote:

On Mon, Jun 15, 2026 at 09:55:16AM +0530, Vishal Chourasia wrote:

On large idle systems, changing system-wide SMT level using the sysfs
control interface still takes approximately 40-55 minutes. Changing SMT
levels is an user triggered operation generally done when systems are
fairly idle. This causes users to be blocked from doing other subsequent
work.

Analyzing the collected profile data during SMT level switch, showed
that CPU hotplug machinery was blocked on synchronize_rcu() calls.

Expedite RCU grace periods for the entire duration of triggered SMT
switch operation via the sysfs control interface. Individual CPU hotplug
operations via the online/offline interface are not affected.

On a PPC64 system with 400 CPUs:

   SMT8 to SMT1:
     before:  1m14s
     after:   3.2s   (~23x faster)

   SMT1 to SMT8:
     before:  2m27s
     after:   2.5s   (~58x faster)

On a large config system with 1920 CPUs, completion time improves from
~1 hour to 5-6 minutes.

Tested-by: Samir <sa...@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshe...@linux.ibm.com>
Signed-off-by: Vishal Chourasia <vish...@linux.ibm.com>

 From an RCU viewpoint:

Reviewed-by: Paul E. McKenney <paul...@kernel.org>

Thank you Paul for reviewing this.

I am guessing that you will be taking this up some other path, but if
you would like it to to via RCU, please let me know.

                                                        Thanx, Paul

I would prefer this to go via tip tree since main change is in cpu.c

Thomas, peter,
What do you guys suggest? can this go via tip tree?

Also, do you have any other review comments on this or test to be done?

---
  include/linux/rcupdate.h | 8 ++++++++
  kernel/cpu.c             | 4 ++++
  kernel/rcu/rcu.h         | 4 ----
  3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index bfa765132de8..b6bccf131f1c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1178,6 +1178,14 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, 
rcu_callback_t f)
  extern int rcu_expedited;
  extern int rcu_normal;

+#ifdef CONFIG_TINY_RCU

+static inline void rcu_expedite_gp(void) { }
+static inline void rcu_unexpedite_gp(void) { }
+#else
+void rcu_expedite_gp(void);
+void rcu_unexpedite_gp(void);
+#endif
+
  DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock())
  DECLARE_LOCK_GUARD_0_ATTRS(rcu, __acquires_shared(RCU), 
__releases_shared(RCU))

diff --git a/kernel/cpu.c b/kernel/cpu.c

index bc4f7a9ba64e..6351da9dffdc 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2658,6 +2658,7 @@ 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;
@@ -2687,6 +2688,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval)
        }
        if (!ret)
                cpu_smt_control = ctrlval;
+       rcu_unexpedite_gp();
        cpu_maps_update_done();
        return ret;
  }
@@ -2704,6 +2706,7 @@ int cpuhp_smt_enable(void)
        int cpu, ret = 0;

cpu_maps_update_begin();

+       rcu_expedite_gp();
        cpu_smt_control = CPU_SMT_ENABLED;
        for_each_present_cpu(cpu) {
                /* Skip online CPUs and CPUs on offline nodes */
@@ -2717,6 +2720,7 @@ 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;
  }
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index fa6d30ce73d1..d6a63ee60bc0 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -521,8 +521,6 @@ do {                                                        
                \
  static inline bool rcu_gp_is_normal(void) { return true; }
  static inline bool rcu_gp_is_expedited(void) { return false; }
  static inline bool rcu_async_should_hurry(void) { return false; }
-static inline void rcu_expedite_gp(void) { }
-static inline void rcu_unexpedite_gp(void) { }
  static inline void rcu_async_hurry(void) { }
  static inline void rcu_async_relax(void) { }
  static inline bool rcu_cpu_online(int cpu) { return true; }
@@ -530,8 +528,6 @@ static inline bool rcu_cpu_online(int cpu) { return true; }
  bool rcu_gp_is_normal(void);     /* Internal RCU use. */
  bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
  bool rcu_async_should_hurry(void);  /* Internal RCU use. */
-void rcu_expedite_gp(void);
-void rcu_unexpedite_gp(void);
  void rcu_async_hurry(void);
  void rcu_async_relax(void);
  void rcupdate_announce_bootup_oddness(void);
--
2.54.0