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

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Date:   Thu, 5 Oct 2017 15:21:47 +0530
From:   Anju T Sudhakar <anju@...ux.vnet.ibm.com>
To:     Santosh Sivaraj <santosh@...six.org>
Cc:     mpe@...erman.id.au, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, maddy@...ux.vnet.ibm.com,
        hemant@...ux.vnet.ibm.com
Subject: Re: [PATCH] powerpc/perf: Fix for core/nest imc call trace on
 cpuhotplug

Hi Santosh,


On Thursday 05 October 2017 03:20 PM, Santosh Sivaraj wrote:
> * Anju T Sudhakar <anju@...ux.vnet.ibm.com> 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@...ux.vnet.ibm.com>
> Reviewed-by: Santosh Sivaraj <santosh@...six.org>

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,