linux-kernel - Re: [PATCH v2 2/5] perf: arm_cspmu: Support shared interrupts

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Date:   Fri, 2 Jun 2023 00:04:42 -0700 (PDT)
From:   Ilkka Koskinen <ilkka@...amperecomputing.com>
To:     Robin Murphy <robin.murphy@....com>
cc:     Ilkka Koskinen <ilkka@...amperecomputing.com>,
        Jonathan Corbet <corbet@....net>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Besar Wicaksono <bwicaksono@...dia.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 2/5] perf: arm_cspmu: Support shared interrupts


Hi Robin,

On Thu, 1 Jun 2023, Robin Murphy wrote:
> On 2023-06-01 04:01, Ilkka Koskinen wrote:
>> Some of the PMUs may share the interrupt. Support them by
>> setting IRQF_SHARED
>
> This has the usual problem of allowing any PMU instance to move the IRQ 
> affinity to a different CPU without also migrating all the other PMU 
> contexts, and thus breaking perf core's assumptions of mutual exclusion.

I see, I wasn't aware of such an assumption. Sounds like there isn't 
necessarily an easy and clean solution for the shared interrupt case. I 
drop the patch and get back on the issue if we come up with something 
reasonable later.

Cheers, Ilkka

>
> Thanks,
> Robin.
>
>> Signed-off-by: Ilkka Koskinen <ilkka@...amperecomputing.com>
>> ---
>>   drivers/perf/arm_cspmu/arm_cspmu.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/perf/arm_cspmu/arm_cspmu.c 
>> b/drivers/perf/arm_cspmu/arm_cspmu.c
>> index 88547a2b73e6..cc5204d1b5fb 100644
>> --- a/drivers/perf/arm_cspmu/arm_cspmu.c
>> +++ b/drivers/perf/arm_cspmu/arm_cspmu.c
>> @@ -1067,8 +1067,8 @@ static int arm_cspmu_request_irq(struct arm_cspmu 
>> *cspmu)
>>   		return irq;
>>     	ret = devm_request_irq(dev, irq, arm_cspmu_handle_irq,
>> -			       IRQF_NOBALANCING | IRQF_NO_THREAD, 
>> dev_name(dev),
>> -			       cspmu);
>> +			       IRQF_NOBALANCING | IRQF_NO_THREAD | 
>> IRQF_SHARED,
>> +			       dev_name(dev), cspmu);
>>   	if (ret) {
>>   		dev_err(dev, "Could not request IRQ %d\n", irq);
>>   		return ret;
>