On Thu, Nov 29, 2018 at 09:42:56AM -0500, Konrad Rzeszutek Wilk wrote: > On Sun, Nov 25, 2018 at 07:33:36PM +0100, Thomas Gleixner wrote: > > Currently the 'sched_smt_present' static key is enabled when at CPU bringup > > SMT topology is observed, but it is never disabled. However there is demand > > to also disable the key when the topology changes such that there is no SMT > > present anymore. > > > > Implement this by making the key count the number of cores that have SMT > > enabled. > > > > In particular, the SMT topology bits are set before interrrupts are enabled > > and similarly, are cleared after interrupts are disabled for the last time > > and the CPU dies. > > I see that the number you used is '2', but I thought that there are some > CPUs out there (Knights Landing?) that could have four threads? > > Would it be better to have a generic function that would provide the > amount of threads the platform does expose - and use that instead > of a constant value?
Nevermind - this would work even with 4 threads as we would hit the number '2' before '4' and the key would be turned on/off properly. Sorry for the noise. Reviewed-by: Konrad Rzeszutek Wilk <konrad.w...@oracle.com> Thank you! > > > > > Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> > > Signed-off-by: Thomas Gleixner <t...@linutronix.de> > > > > --- > > kernel/sched/core.c | 19 +++++++++++-------- > > 1 file changed, 11 insertions(+), 8 deletions(-) > > > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -5738,15 +5738,10 @@ int sched_cpu_activate(unsigned int cpu) > > > > #ifdef CONFIG_SCHED_SMT > > /* > > - * The sched_smt_present static key needs to be evaluated on every > > - * hotplug event because at boot time SMT might be disabled when > > - * the number of booted CPUs is limited. > > - * > > - * If then later a sibling gets hotplugged, then the key would stay > > - * off and SMT scheduling would never be functional. > > + * When going up, increment the number of cores with SMT present. > > */ > > - if (cpumask_weight(cpu_smt_mask(cpu)) > 1) > > - static_branch_enable_cpuslocked(&sched_smt_present); > > + if (cpumask_weight(cpu_smt_mask(cpu)) == 2) > > + static_branch_inc_cpuslocked(&sched_smt_present); > > #endif > > set_cpu_active(cpu, true); > > > > @@ -5790,6 +5785,14 @@ int sched_cpu_deactivate(unsigned int cp > > */ > > synchronize_rcu_mult(call_rcu, call_rcu_sched); > > > > +#ifdef CONFIG_SCHED_SMT > > + /* > > + * When going down, decrement the number of cores with SMT present. > > + */ > > + if (cpumask_weight(cpu_smt_mask(cpu)) == 2) > > + static_branch_dec_cpuslocked(&sched_smt_present); > > +#endif > > + > > if (!sched_smp_initialized) > > return 0; > > > > > >
