Re: [PATCH v2 5/5] perf/x86/amd/power: Add AMD accumulated power reporting mechanism

Peter Zijlstra
2016-01-20T09:23:21Z

On Wed, Jan 20, 2016 at 12:48:24PM +0800, Huang Rui wrote:
> Hi Peter,
>
> Thanks so much to your comments.
>
> On Tue, Jan 19, 2016 at 01:12:50PM +0100, Peter Zijlstra wrote:
> > On Thu, Jan 14, 2016 at 10:50:08AM +0800, Huang Rui wrote:
> > > +struct power_pmu {
> > > + spinlock_t lock;
> >
> > This should be a raw_spinlock_t, as it'll be nested under other
> > raw_spinlock_t's.
> >
>
> Do you mean the following spinlock operations are in hardware
> interrupts disabled case, so I need use raw_spinlock_t instead, right?

mainline -rt

raw_spinlock_t spin-waits spin-waits
spinlock_t spin-waits blocks (rt-mutex)
struct mutex blocks blocks (rt-mutex)

since these functions are themselves called with raw_spinlock_t held
(perf_event_context::lock for example, but also rq::lock), any lock
nested inside them must also be raw_spinlock_t.

I have a lockdep patch somewhere that checks these ordering things; I
should rebase and post that again.

> Use raw_spin_lock_irqsave/raw_spin_unlock_irqrestore?

pmu::{start,stop,add,del} will be called with IRQs already disabled.

> > > +static int power_cpu_init(int cpu)
> > > +{
> > > + int i, cu, ret = 0;
> > > + cpumask_var_t mask, dummy_mask;
> > > +
> > > + cu = cpu / cores_per_cu;
> > > +
> > > + if (!zalloc_cpumask_var(&mask, GFP_KERNEL))
> > > + return -ENOMEM;
> > > +
> > > + if (!zalloc_cpumask_var(&dummy_mask, GFP_KERNEL)) {
> > > + ret = -ENOMEM;
> > > + goto out;
> > > + }
> > > +
> > > + for (i = 0; i < cores_per_cu; i++)
> > > + cpumask_set_cpu(i, mask);
> > > +
> > > + cpumask_shift_left(mask, mask, cu * cores_per_cu);
> > > +
> > > + if (!cpumask_and(dummy_mask, mask, &cpu_mask))
> > > + cpumask_set_cpu(cpu, &cpu_mask);
> > > +
> > > + free_cpumask_var(dummy_mask);
> > > +out:
> > > + free_cpumask_var(mask);
> > > +
> > > + return ret;
> > > +}
> >
> > > +static int power_cpu_notifier(struct notifier_block *self,
> > > + unsigned long action, void *hcpu)
> > > +{
> > > + unsigned int cpu = (long)hcpu;
> > > +
> > > + switch (action & ~CPU_TASKS_FROZEN) {
> > > + case CPU_UP_PREPARE:
> > > + if (power_cpu_prepare(cpu))
> > > + return NOTIFY_BAD;
> > > + break;
> > > + case CPU_STARTING:
> > > + if (power_cpu_init(cpu))
> > > + return NOTIFY_BAD;
> >
> > this is called with IRQs disabled, which makes those GFP_KERNEL allocs
> > above a pretty bad idea.
> >
>
> Right, so should I use GFP_ATOMIC to allocate cpumask here?

One should not use GFP_ATOMIC if at all possible, also no, -rt cannot do
_any_ allocations from this site.

> > Also, note that -rt cannot actually do _any_ allocations/frees from
> > STARTING.
> >
> > Please move the allocs/frees to PREPARE/ONLINE.
> >
>
> How about add two cpumask_var_t at power_pmu structure? Then allocate
> the two cpumask_var_t (pmu->mask, pmu->dummy_mask), and they can be
> also used on power_cpu_init.

That would work.