[RFC] fpga: dfl: fme: Fix cpu hotplug code

[kajoljain]

On 6/29/21 12:10 AM, Moritz Fischer wrote:
> On Mon, Jun 28, 2021 at 12:45:46PM +0530, Kajol Jain wrote:
>> Commit 724142f8c42a ("fpga: dfl: fme: add performance
>> reporting support") added performance reporting support
>> for FPGA management engine via perf.
>>
>> It also added cpu hotplug feature but it didn't add
>> pmu migration call in cpu offline function.
>> This can create an issue incase the current designated
>> cpu being used to collect fme pmu data got offline,
>> as based on current code we are not migrating fme pmu to
>> new target cpu. Because of that perf will still try to
>> fetch data from that offline cpu and hence we will not
>> get counter data.
>>
>> Patch fixed this issue by adding pmu_migrate_context call
>> in fme_perf_offline_cpu function.
>>
>> Fixes: 724142f8c42a ("fpga: dfl: fme: add performance reporting support")
>> Signed-off-by: Kajol Jain <kjain at linux.ibm.com>
> 
> You might want to Cc: stable at vger.kernel.org if it fixes an actual bug.

Hi Moritz,
  I already send patch out without RFC tag yesterday.
Link to the patch: https://lkml.org/lkml/2021/6/28/275

I will cc stable at vger.kernel.org there as suggested by you.

Thanks,
Kajol Jain

>> ---
>>  drivers/fpga/dfl-fme-perf.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> ---
>> - This fix patch is not tested (as I don't have required environment).
>>   But issue mentioned in the commit msg can be re-created, by starting any
>>   fme_perf event and while its still running, offline current designated
>>   cpu pointed by cpumask file. Since current code didn't migrating pmu,
>>   perf gonna try getting counts from that offlined cpu and hence we will
>>   not get event data.
>> ---
>> diff --git a/drivers/fpga/dfl-fme-perf.c b/drivers/fpga/dfl-fme-perf.c
>> index 4299145ef347..b9a54583e505 100644
>> --- a/drivers/fpga/dfl-fme-perf.c
>> +++ b/drivers/fpga/dfl-fme-perf.c
>> @@ -953,6 +953,10 @@ static int fme_perf_offline_cpu(unsigned int cpu, struct hlist_node *node)
>>  		return 0;
>>  
>>  	priv->cpu = target;
>> +
>> +	/* Migrate fme_perf pmu events to the new target cpu */
>> +	perf_pmu_migrate_context(&priv->pmu, cpu, target);
>> +
>>  	return 0;
>>  }
>>  
>> -- 
>> 2.31.1
>>
> - Moritz
>