| Date | Thu, 29 Nov 2018 09:42:56 -0500 | From | Konrad Rzeszutek Wilk <> | Subject | Re: [patch V2 08/28] sched/smt: Make sched_smt_present track topology |
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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?
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > Signed-off-by: Thomas Gleixner <tglx@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; > > >
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