Re: [PATCH] powerpc/perf: Fix for core/nest imc call trace on cpuhotplug

[Anju T Sudhakar]

Hi Santosh,


On Thursday 05 October 2017 03:20 PM, Santosh Sivaraj wrote:
> * Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx> wrote (on 2017-10-04 06:50:52 +0000):
>
> > Nest/core pmu units are enabled only when it is used. A reference count is
> > maintained for the events which uses the nest/core pmu units. Currently in
> > *_imc_counters_release function a WARN() is used for notification of any
> > underflow of ref count.
> >                                                                                  
> > The case where event ref count hit a negative value is, when perf session is
> > started, followed by offlining of all cpus in a given core.
> > i.e. in cpuhotplug offline path ppc_core_imc_cpu_offline() function set the
> > ref->count to zero, if the current cpu which is about to offline is the last
> > cpu in a given core and make an OPAL call to disable the engine in that core.
> > And on perf session termination, perf->destroy (core_imc_counters_release) will
> > first decrement the ref->count for this core and based on the ref->count value
> > an opal call is made to disable the core-imc engine.
> > Now, since cpuhotplug path already clears the ref->count for core and disabled
> > the engine, perf->destroy() decrementing again at event termination make it
> > negative which in turn fires the WARN_ON. The same happens for nest units.
> >                                                                                  
> > Add a check to see if the reference count is alreday zero, before decrementing
> > the count, so that the ref count will not hit a negative value.
> >                                                                                  
> > Signed-off-by: Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Santosh Sivaraj <santosh@xxxxxxxxxx>

Thanks for reviewing.

-Anju
> > ---
> >   arch/powerpc/perf/imc-pmu.c | 28 ++++++++++++++++++++++++++++
> >   1 file changed, 28 insertions(+)
> >
> > diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
> > index 9ccac86f3463..e3a1f65933b5 100644
> > --- a/arch/powerpc/perf/imc-pmu.c
> > +++ b/arch/powerpc/perf/imc-pmu.c
> > @@ -399,6 +399,20 @@ static void nest_imc_counters_release(struct perf_event *event)
> >   
> >   	/* Take the mutex lock for this node and then decrement the reference count */
> >   	mutex_lock(&ref->lock);
> > +	if (ref->refc == 0) {
> > +		/*
> > +		 * The scenario where this is true is, when perf session is
> > +		 * started, followed by offlining of all cpus in a given node.
> > +		 *
> > +		 * In the cpuhotplug offline path, ppc_nest_imc_cpu_offline()
> > +		 * function set the ref->count to zero, if the cpu which is
> > +		 * about to offline is the last cpu in a given node and make
> > +		 * an OPAL call to disable the engine in that node.
> > +		 *
> > +		 */
> > +		mutex_unlock(&ref->lock);
> > +		return;
> > +	}
> >   	ref->refc--;
> >   	if (ref->refc == 0) {
> >   		rc = opal_imc_counters_stop(OPAL_IMC_COUNTERS_NEST,
> > @@ -646,6 +660,20 @@ static void core_imc_counters_release(struct perf_event *event)
> >   		return;
> >   
> >   	mutex_lock(&ref->lock);
> > +	if (ref->refc == 0) {
> > +		/*
> > +		 * The scenario where this is true is, when perf session is
> > +		 * started, followed by offlining of all cpus in a given core.
> > +		 *
> > +		 * In the cpuhotplug offline path, ppc_core_imc_cpu_offline()
> > +		 * function set the ref->count to zero, if the cpu which is
> > +		 * about to offline is the last cpu in a given core and make
> > +		 * an OPAL call to disable the engine in that core.
> > +		 *
> > +		 */
> > +		mutex_unlock(&ref->lock);
> > +		return;
> > +	}
> >   	ref->refc--;
> >   	if (ref->refc == 0) {
> >   		rc = opal_imc_counters_stop(OPAL_IMC_COUNTERS_CORE,