On Mon, Jan 12, 2026 at 03:13:33PM +0530, Vishal Chourasia wrote: > Performance data on a PPC64 system with 400 CPUs: > >+ ppc64_cpu --smt=1 (SMT8 to SMT1) >Before: real 1m14.792s >After: real 0m03.205s # ~23x improvement > >+ ppc64_cpu --smt=8 (SMT1 to SMT8) >Before: real 2m27.695s >After: real 0m02.510s # ~58x improvement > >Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b
On Mon, Jan 19, 2026 at 12:18:35AM -0500, Joel Fernandes wrote: > Hi Vishal, Samir, > > Thanks for the testing on your large CPU count system. > > Considering the SMT=on performance is still terrible, before we expedite > RCU, could we try the approach Peter suggested (avoiding repeated > lock/unlock)? I wrote a patch below. > > git://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git > tag: cpuhp-bulk-optimize-rfc-v1 > > I tested it lightly on rcutorture hotplug test and it passes. Please share > any performance results, thanks. > > Also I'd like to use expediting of RCU as a last resort TBH, we should > optimize the outer operations that require RCU in the first place such as > Peter's suggestion since that will improve the overall efficiency of the > code. And if/when expediting RCU, Peter's other suggestion to not do it in > cpus_write_lock() and instead do it from cpuhp_smt_enable() also makes sense > to me. > > ---8<----------------------- > > From: Joel Fernandes <[email protected]> > Subject: [PATCH] cpuhp: Optimize batch SMT enable by reducing lock acquiring > > Bulk CPU hotplug operations such as enabling SMT across all cores > require hotplugging multiple CPUs. The current implementation takes > cpus_write_lock() for each individual CPU causing multiple slow grace > period requests. > > Therefore introduce cpu_up_locked() that assumes the caller already > holds cpus_write_lock(). The cpuhp_smt_enable() function is updated to > hold the lock once around the entire loop rather than for each CPU. > > Link: > https://lore.kernel.org/all/[email protected]/ > Suggested-by: Peter Zijlstra <[email protected]> > Signed-off-by: Joel Fernandes <[email protected]> > --- > kernel/cpu.c | 40 +++++++++++++++++++++++++--------------- > 1 file changed, 25 insertions(+), 15 deletions(-) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 8df2d773fe3b..4ce7deb236d7 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -1623,34 +1623,31 @@ void cpuhp_online_idle(enum cpuhp_state state) > complete_ap_thread(st, true); > } > -/* Requires cpu_add_remove_lock to be held */ > -static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state > target) > +/* Requires cpu_add_remove_lock and cpus_write_lock to be held. */ > +static int cpu_up_locked(unsigned int cpu, int tasks_frozen, > + enum cpuhp_state target) > { > struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu); > struct task_struct *idle; > int ret = 0; > - cpus_write_lock(); > + lockdep_assert_cpus_held(); > - if (!cpu_present(cpu)) { > - ret = -EINVAL; > - goto out; > - } > + if (!cpu_present(cpu)) > + return -EINVAL; > /* > * The caller of cpu_up() might have raced with another > * caller. Nothing to do. > */ > if (st->state >= target) > - goto out; > + return 0; > if (st->state == CPUHP_OFFLINE) { > /* Let it fail before we try to bring the cpu up */ > idle = idle_thread_get(cpu); > - if (IS_ERR(idle)) { > - ret = PTR_ERR(idle); > - goto out; > - } > + if (IS_ERR(idle)) > + return PTR_ERR(idle); > /* > * Reset stale stack state from the last time this CPU was > online. > @@ -1673,7 +1670,7 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, > enum cpuhp_state target) > * return the error code.. > */ > if (ret) > - goto out; > + return ret; > } > /* > @@ -1683,7 +1680,16 @@ static int _cpu_up(unsigned int cpu, int tasks_frozen, > enum cpuhp_state target) > */ > target = min((int)target, CPUHP_BRINGUP_CPU); > ret = cpuhp_up_callbacks(cpu, st, target); > -out: > + return ret; > +} > + > +/* Requires cpu_add_remove_lock to be held */ > +static int _cpu_up(unsigned int cpu, int tasks_frozen, enum cpuhp_state > target) > +{ > + int ret; > + > + cpus_write_lock(); > + ret = cpu_up_locked(cpu, tasks_frozen, target); > cpus_write_unlock(); > arch_smt_update(); > return ret; > @@ -2715,6 +2721,8 @@ int cpuhp_smt_enable(void) > int cpu, ret = 0; > cpu_maps_update_begin(); > + /* Hold cpus_write_lock() for entire batch operation. */ > + cpus_write_lock(); > cpu_smt_control = CPU_SMT_ENABLED; > for_each_present_cpu(cpu) { > /* Skip online CPUs and CPUs on offline nodes */ > @@ -2722,12 +2730,14 @@ int cpuhp_smt_enable(void) > continue; > if (!cpu_smt_thread_allowed(cpu) || > !topology_is_core_online(cpu)) > continue; > - ret = _cpu_up(cpu, 0, CPUHP_ONLINE); > + ret = cpu_up_locked(cpu, 0, CPUHP_ONLINE); > if (ret) > break; > /* See comment in cpuhp_smt_disable() */ > cpuhp_online_cpu_device(cpu); > } > + cpus_write_unlock(); > + arch_smt_update(); > cpu_maps_update_done(); > return ret; > } > -- > 2.34.1 > Hi Joel, I tested above patch on 400 CPU machine that I had originally posted the numbers for. # time echo 1 > /sys/devices/system/cpu/smt/control real 1m27.133s # Base real 1m25.859s # With patch # time echo 8 > /sys/devices/system/cpu/smt/control real 1m0.682s # Base real 1m3.423s # With patch
