Re: [RFC] perf/amd/ibs: Move ibs pmus under perf_sw_context

[Ravi Bangoria <ravi.bangoria@amd.com>]

On 15-Nov-21 5:37 PM, Peter Zijlstra wrote:
> On Mon, Nov 15, 2021 at 05:31:21PM +0530, Ravi Bangoria wrote:
>>
>>
>> On 15-Nov-21 4:47 PM, Peter Zijlstra wrote:
>>> On Mon, Nov 15, 2021 at 03:18:38PM +0530, Ravi Bangoria wrote:
>>>> Ideally, a pmu which is present in each hw thread belongs to
>>>> perf_hw_context, but perf_hw_context has limitation of allowing only
>>>> one pmu (a core pmu) and thus other hw pmus need to use either sw or
>>>> invalid context which limits pmu functionalities.
>>>>
>>>> This is not a new problem. It has been raised in the past, for example,
>>>> Arm big.LITTLE (same for Intel ADL) and s390 had this issue:
>>>>
>>>>   Arm:  https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20160425175837.GB3141%40leverpostej&data=04%7C01%7Cravi.bangoria%40amd.com%7C8b30ce7e52c0414dc65e08d9a8307149%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637725748302455206%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=59fepiNH9du3ZR0owjPcquQ4YLc%2B8hE74H%2BpdnQggQY%3D&reserved=0
>>>>   s390: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20160606082124.GA30154%40twins.programming.kicks-ass.net&data=04%7C01%7Cravi.bangoria%40amd.com%7C8b30ce7e52c0414dc65e08d9a8307149%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637725748302455206%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=cSEGVgRbClDXeGJmEuGr1P8YXSIvhx%2FxbQjmdydQiuU%3D&reserved=0
>>>>
>>>> Arm big.LITTLE (followed by Intel ADL) solved this by allowing multiple
>>>> (heterogeneous) pmus inside perf_hw_context. It makes sense as they are
>>>> still registering single pmu for each hw thread.
>>>>
>>>> s390 solved it by moving 2nd hw pmu to perf_sw_context, though that 2nd
>>>> hw pmu is count mode only, i.e. no sampling.
>>>>
>>>> AMD IBS also has similar problem. IBS pmu is present in each hw thread.
>>>> But because of perf_hw_context restriction, currently it belongs to
>>>> perf_invalid_context and thus important functionalities like per-task
>>>> profiling is not possible with IBS pmu. Moving it to perf_sw_context
>>>> will:
>>>>  - allow per-task monitoring
>>>>  - allow cgroup wise profiling
>>>>  - allow grouping of IBS with other pmu events
>>>>  - disallow multiplexing
>>>>
>>>> Please let me know if I missed any major benefit or drawback of
>>>> perf_sw_context. I'm also not sure how easy it would be to lift
>>>> perf_hw_context restriction and start allowing more pmus in it.
>>>>
>>>> Suggestions?
>>>
>>> Same as I do every time this comes up... this patch is still lingering
>>> and wanting TLC:
>>>
>>>   https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Flkml%2F20181010104559.GO5728%40hirez.programming.kicks-ass.net%2F&data=04%7C01%7Cravi.bangoria%40amd.com%7C8b30ce7e52c0414dc65e08d9a8307149%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637725748302455206%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&sdata=sn7QOMMjrzsbBsvdEAPttouujDel0JuDptIFMHkm3Q0%3D&reserved=0
>>
>> Thanks for the pointer Peter. I have looked at the patch and it is quite complex,
>> altering the very way perf event scheduling works.
>>
>> I don't dispute that is the right 'fix' for the issue, but do you think adding a
>> new perf context can help alleviate some of the issues in the interim?
> 
> And take away the motivation for people to do the right thing? How does
> that work out in my favour?
> 

:-) Understood Peter. I will take a stab at making your patch upstream ready.

Ravi