linux-kernel - Re: [PATCH v4 2/2] soc: qcom: add l2 cache perf events driver

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Date:   Fri, 16 Sep 2016 15:51:58 -0400
From:   Neil Leeder <nleeder@...eaurora.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Mark Langsdorf <mlangsdo@...hat.com>,
        Mark Salter <msalter@...hat.com>, Jon Masters <jcm@...hat.com>,
        Timur Tabi <timur@...eaurora.org>, cov@...eaurora.org,
        nleeder@...eaurora.org
Subject: Re: [PATCH v4 2/2] soc: qcom: add l2 cache perf events driver


On 9/16/2016 12:40 PM, Mark Rutland wrote:
> On Fri, Sep 16, 2016 at 11:33:39AM -0400, Neil Leeder wrote:
[...]
>> On 9/1/2016 12:30 PM, Mark Rutland wrote:
>>> On Tue, Aug 30, 2016 at 01:01:33PM -0400, Neil Leeder wrote:
>>>> +	/* Don't allow groups with mixed PMUs, except for s/w events */
>>>> +	if (event->group_leader->pmu != event->pmu &&
>>>> +	    !is_software_event(event->group_leader)) {
>>>> +		dev_warn(&l2cache_pmu->pdev->dev,
>>>> +			 "Can't create mixed PMU group\n");
>>>> +		return -EINVAL;
>>>> +	}
>>>> +
>>>> +	list_for_each_entry(sibling, &event->group_leader->sibling_list,
>>>> +			    group_entry)
>>>> +		if (sibling->pmu != event->pmu &&
>>>> +		    !is_software_event(sibling)) {
>>>> +			dev_warn(&l2cache_pmu->pdev->dev,
>>>> +				 "Can't create mixed PMU group\n");
>>>> +			return -EINVAL;
>>>> +		}
>>>> +
>>>> +	hwc->idx = -1;
>>>> +	hwc->config_base = event->attr.config;
>>>> +
>>>> +	/*
>>>> +	 * Ensure all events are on the same cpu so all events are in the
>>>> +	 * same cpu context, to avoid races on pmu_enable etc.
>>>> +	 */
>>>> +	slice = get_hml2_pmu(event->cpu);
>>>> +	event->cpu = slice->on_cpu;
>>>
>>> This could put an event on a different CPU to its group siblings, which
>>> is broken.
>>
>> This is the same logic as in arm-ccn.c:arm_ccn_pmu_event_init(), where there
>> is a single CPU designated as the CPU to be used for all events.
>>
>> All events for this slice are forced to slice->on_cpu which is the CPU
>> set in the cpumask for this slice.
> 
> The CCN is a little different. For the CCN, a single CPU is designated
> to handle *all* events.
> 
> For this driver, a CPU is designated per-slice, judging by the existence
> of hml2_pmu::on_cpu (unless that's superfluous). We've only verified
> that the events are all for this PMU, not the same slice, and thus each
> event->cpu may differ.
> 

I see. So I can add a check that the group_leader event must be on the
same slice, and thus on the same CPU.

>> I'm not sure how this can put an event on a different CPU to its group
>> siblings?
> 
> In practice today, we'll try to schedule the event on it's group
> leader's CPU, but accounting and subsequent manipulation could go wrong.
> 
> Thanks,
> Mark.
> 

Neil
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