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

[Huang Rui]

On Thu, Jan 21, 2016 at 10:02:57AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 21, 2016 at 03:04:38PM +0800, Huang Rui wrote:
> > I just quickly looked at about the spinlock on -rt mode. Because
> > realtime linux kernel provides two kinds of spinlock, the original
> > spinlock_t will be replaced the one which is able to sleep, actually,
> > like mutex. And another one (you mentioned here, raw_spinlock_t) can
> > keep on non-sleep behavior, that is the real spinlock.
> > 
> > And my lock here also will be nested under perf_event_context::lock,
> > right?
> 
> Yep.
> 
> > > I have a lockdep patch somewhere that checks these ordering things; I
> > > should rebase and post that again.
> > > 
> > 
> > Can you CC me when you post that patch next time?
> 
> Sure.
> 
> > > One should not use GFP_ATOMIC if at all possible, also no, -rt cannot do
> > > _any_ allocations from this site.
> > > 
> > 
> > OK, that's because allocation might sleep when IRQ disabled. That's
> > incorrect.
> 
> Right.
> 
> Its related to the above, the allocator locks are spinlock_t and as a
> consequence of them becoming a blocking lock, spin_lock_irq() will also
> no longer disable IRQs.
> 
> The CPU_STARTING notifier however will still be called with IRQs
> disabled because it is CPU bringup.
> 
> So on -rt even GFP_ATOMIC will no longer work here.
> 

OK, thanks to clarify it.

> > I draft an update diff that based on original patch, please take a
> > look.
> > 
> > 8<--------------------------------------------------------------------------
> > 
> > diff --git a/arch/x86/kernel/cpu/perf_event_amd_power.c b/arch/x86/kernel/cpu/perf_event_amd_power.c
> > index 69ef234..e71d993 100644
> > --- a/arch/x86/kernel/cpu/perf_event_amd_power.c
> > +++ b/arch/x86/kernel/cpu/perf_event_amd_power.c
> > @@ -46,10 +46,17 @@ static unsigned int cu_num;
> >  static u64 max_cu_acc_power;
> >  
> >  struct power_pmu {
> > -	spinlock_t		lock;
> > +	raw_spinlock_t		lock;
> >  	struct list_head	active_list;
> >  	struct pmu		*pmu; /* pointer to power_pmu_class */
> >  	local64_t		cpu_sw_pwr_ptsc;
> > +	/*
> > +	 * These two cpumasks is used for avoiding the allocations on
> > +	 * CPU_STARTING phase. Because power_cpu_prepare will be
> > +	 * called on IRQs disabled status.
> > +	 */
> > +	cpumask_var_t		mask;
> > +	cpumask_var_t		tmp_mask;
> >  };
> >  
> >  static struct pmu pmu_class;
> > @@ -126,9 +133,9 @@ static void pmu_event_start(struct perf_event *event, int mode)
> >  	struct power_pmu *pmu = __this_cpu_read(amd_power_pmu);
> >  	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&pmu->lock, flags);
> > +	raw_spin_lock_irqsave(&pmu->lock, flags);
> >  	__pmu_event_start(pmu, event);
> > -	spin_unlock_irqrestore(&pmu->lock, flags);
> > +	raw_spin_unlock_irqrestore(&pmu->lock, flags);
> >  }
> >  
> >  static void pmu_event_stop(struct perf_event *event, int mode)
> > @@ -137,7 +144,7 @@ static void pmu_event_stop(struct perf_event *event, int mode)
> >  	struct hw_perf_event *hwc = &event->hw;
> >  	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&pmu->lock, flags);
> > +	raw_spin_lock_irqsave(&pmu->lock, flags);
> >  
> >  	/* mark event as deactivated and stopped */
> >  	if (!(hwc->state & PERF_HES_STOPPED)) {
> > @@ -155,7 +162,7 @@ static void pmu_event_stop(struct perf_event *event, int mode)
> >  		hwc->state |= PERF_HES_UPTODATE;
> >  	}
> >  
> > -	spin_unlock_irqrestore(&pmu->lock, flags);
> > +	raw_spin_unlock_irqrestore(&pmu->lock, flags);
> >  }
> >  
> >  static int pmu_event_add(struct perf_event *event, int mode)
> > @@ -164,14 +171,14 @@ static int pmu_event_add(struct perf_event *event, int mode)
> >  	struct hw_perf_event *hwc = &event->hw;
> >  	unsigned long flags;
> >  
> > -	spin_lock_irqsave(&pmu->lock, flags);
> > +	raw_spin_lock_irqsave(&pmu->lock, flags);
> >  
> >  	hwc->state = PERF_HES_UPTODATE | PERF_HES_STOPPED;
> >  
> >  	if (mode & PERF_EF_START)
> >  		__pmu_event_start(pmu, event);
> >  
> > -	spin_unlock_irqrestore(&pmu->lock, flags);
> > +	raw_spin_unlock_irqrestore(&pmu->lock, flags);
> >  
> >  	return 0;
> >  }
> 
> So for these 4 {start,stop,add,del} you can drop the irqsave/irqrestore
> thing as its guaranteed that IRQs will be disabled.
> 

OK, I will remove the lock.

> > +	cpumask_clear(pmu->mask);
> > +	cpumask_clear(pmu->tmp_mask);
> >  
> >  	for (i = 0; i < cores_per_cu; i++)
> > +		cpumask_set_cpu(i, pmu->mask);
> >  
> > +	cpumask_shift_left(pmu->mask, pmu->mask, cu * cores_per_cu);
> 
> Couldn't you simply use topology_sibling_cpumask(cpu) instead?
> 

Looks like we couldn't. That's because cores number per cu (compute
unit) is got by CPUID 0x8000001e EBX. That relies on the CPU hardware.

> >  
> >  static int power_cpu_init(int cpu)
> >  {
> > +	struct power_pmu *pmu = per_cpu(amd_power_pmu, cpu);
> > +	int i, cu;
> >  
> > +	if (pmu)
> > +		return 0;
> >  
> > +	cu = cpu / cores_per_cu;
> >  
> >  	for (i = 0; i < cores_per_cu; i++)
> > +		cpumask_set_cpu(i, pmu->mask);
> >  
> > +	cpumask_shift_left(pmu->mask, pmu->mask, cu * cores_per_cu);
> 
> topology_sibling_cpumask(cpu) again?
> 

Ditto.

Thanks,
Rui