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Subject: Re: [PATCH v11 8/8] perf: ARM DynamIQ Shared Unit PMU support
To:     Saravana Kannan <skannan@codeaurora.org>
Cc:     Mark Rutland <mark.rutland@arm.com>, robh@kernel.org,
        mathieu.poirier@linaro.org, peterz@infradead.org,
        sudeep.holla@arm.com, will.deacon@arm.com,
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        jonathan.cameron@huawei.com, frowand.list@gmail.com,
        leo.yan@linaro.org, linux-arm-kernel@lists.infradead.org
References: <20180102112533.13640-1-suzuki.poulose@arm.com>
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From:   Suzuki K Poulose <Suzuki.Poulose@arm.com>
Message-ID: <c1b68ed8-d0a9-7672-4f72-b804d54155cf@arm.com>
Date:   Mon, 19 Mar 2018 09:50:18 +0000
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On 07/03/18 21:36, Saravana Kannan wrote:
> On 03/07/2018 06:59 AM, Suzuki K Poulose wrote:
>>
>>
>> Hi Saravana,
>>
>> Sorry for the late response, I was out on vacation.
>>
>> On 05/03/18 22:10, Saravana Kannan wrote:
>>> On 03/05/2018 02:59 AM, Mark Rutland wrote:
>>>> On Fri, Mar 02, 2018 at 11:19:56AM -0800, Saravana Kannan wrote:
>>>>> On 03/02/2018 02:42 AM, Mark Rutland wrote:
>>>>>> It's important to note that the DSU PMU's event_init() ensures events
>>>>>> are affine to a single CPU, and the perf core code serializes
>>>>>> operations
>>>>>> on those events via the context lock.
>>>>>
>>>>> Ah, I see that now. Thanks!
>>>>>
>>>>>> Therefore, two CPUs *won't* try to access the registers
>>>>>> simultaneously.
>>>>>
>>>>> Right, and this driver seems to be going through a lot of work to
>>>>> make sure
>>>>> all events are read in one CPU.
>>>>>
>>>>> Do you even have an upstream target where there are multiple DSU's in a
>>>>> system?
>>>>
>>>> I have no idea, though I do beleive that it's possible for a system to
>>>> have multiple DSUs.
>>>>
>>>>> If not, we can simplify a ton of this code (no hotplug notifiers, no
>>>>> migrating PMUs, no SMP calls, etc) by just adding a spinlock and
>>>>> letting any
>>>>> CPU read these DSU counters.
>>>>
>>>> Regardless of whether we allow arbitrary CPUs to read the counters,
>>>> other logic still needs to be CPU affine, and we'll still need hotplug
>>>> notifiers and event migration.
>>>
>>> If you have to support multiple DSUs in a system, then the need is
>>> obvious. But if you don't have to support multiple DSU, it's not
>>> obvious to me on why you still need CPU affining or hotplug notifiers.
>>> Could you please provide me pointers for general understanding?
>>>
>>
>> We need to support multiple DSUs as such configurations are possible.
>>
>>>
>>>> I am not necessarily opposed to allowing read() calls from associated
>>>> CPUs, but as before, I'll leave that to Suzuki.
>>
>> I am fine with reading the registers from any of the associated CPUs.
>>
> 
> If that's the case, can you please use my patch?

The DSU driver is already mainline. Please feel free to send a patch with me in Cc.

> And if it looks good to you, give an Ack and ask Peter to pull it in as you'd be the user?

As mentioned above, when you send a patch, please do keep me in Cc, and will do the
needful.

Thanks
Suzuki