[PATCH v3 0/2] cpuhp: Improve SMT switch time via lock batching and RCU expedition

[Vishal Chourasia <vishalc@linux.ibm.com>]

On large systems with high core counts, toggling SMT modes via sysfs
(/sys/devices/system/cpu/smt/control) incurs significant latency. For
instance, on ~2000 CPUs, switching SMT modes can take close to an hour
as the system hotplugs each hardware thread individually. This series
reduces this time to minutes.

Analysis of the hotplug path [1] identifies synchronize_rcu() as the
primary bottleneck. During a bulk SMT switch, the kernel repeatedly
enters RCU grace periods for each CPU being brought online or offline.

This series optimizes the SMT transition in two ways:

1. Lock Batching [1]: Instead of repeatedly acquiring and releasing the CPU
hotplug write lock for every individual CPU, we now hold cpus_write_lock
across the entire SMT toggle operation.

2. Expedite RCU grace period [2]: It utilizes rcu_expedite_gp() to force
expedite grace periods specifically for the duration of the SMT switch.
The trade-off is justified here to prevent the administrative task of
SMT switching from stalling for an unacceptable duration on large
systems. 

Changes since v1
Link: https://lore.kernel.org/all/20260112094332.66006-2-vishalc@linux.ibm.com/
Expedite system-wide synchronize_rcu only when SMT switch operation are
triggered via /sys/devices/system/cpu/smt/control interface.

Changes since v2
Link: https://lore.kernel.org/all/20260216121927.489062-2-vishalc@linux.ibm.com/
Move the declaration of rcu_[un]expedite_gp() to
include/linux/rcupdate.h. Thanks Shrikanth for sharing the fix and
kernel test robot for finding the issue. [3]

[1] https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com
[2] https://lore.kernel.org/all/20260113090153.GS830755@noisy.programming.kicks-ass.net/
[3] https://lore.kernel.org/all/202602170049.WQD7Wcuj-lkp@intel.com/

Vishal Chourasia (2):
  cpuhp: Optimize SMT switch operation by batching lock acquisition
  cpuhp: Expedite RCU grace periods during SMT operations

 include/linux/rcupdate.h |  8 +++++
 kernel/cpu.c             | 76 +++++++++++++++++++++++++++++-----------
 kernel/rcu/rcu.h         |  4 ---
 3 files changed, 64 insertions(+), 24 deletions(-)

-- 
2.53.0