Re: [PATCH RESEND] perf/marvell_cn10k: Add MPAM support for TAD PMU

[James Morse <james.morse@arm.com>]

Hi Tanmay,

On 27/06/2022 14:18, Tanmay Jagdale wrote:
>> On 2022-06-24 13:14, Will Deacon wrote:
>>> On Sat, May 28, 2022 at 12:26:47AM +0530, Tanmay Jagdale wrote:
>>>> The TAD PMU supports following counters that can be filtered by MPAM
>>>> partition id.
>>>>      - (0x1a) tad_alloc_dtg : Allocations to DTG.
>>>>      - (0x1b) tad_alloc_ltg : Allocations to LTG.
>>>>      - (0x1c) tad_alloc_any : Total allocations to DTG/LTG.
>>>>      - (0x1d) tad_hit_dtg   : DTG hits.
>>>>      - (0x1e) tad_hit_ltg   : LTG hits.
>>>>      - (0x1f) tad_hit_any   : Hit in LTG/DTG.
>>>>      - (0x20) tad_tag_rd    : Total tag reads.
>>>>
>>>> Add a new 'partid' attribute of 16-bits to get the partition id
>>>> passed from perf tool. This value would be stored in config1 field
>>>> of perf_event_attr structure.
>>>>
>>>> Example:
>>>> perf stat -e tad/tad_alloc_any,partid=0x12/ <program>
>>>>
>>>> - Drop read of TAD_PRF since we don't have to preserve any
>>>>    bit fields and always write an updated value.
>>>> - Update register offsets of TAD_PRF and TAD_PFC.
>>>
>>> It would be great if you could document some of this under
>>> Documentation/admin-guide/perf like many of the other PMU drivers have
>>> done.
>>
>> Especially documenting how the user obtains the required partid value to
>> pass.

> We created MPAM partitions using the resctrl filesystem interface.
> Example:
>         $ cd /sys/fs/resctrl
>         $ mkdir p1
>         $ echo "L3:0=f" > p1/schemata (configure 4 L3 cache ways)
>         $ mkdir p2
>         $ echo "L3:1=ff0" > p2/schemata (configure 8 L3 cache ways)
> 
> Here directory name 'p1' creates a MPAM partid 0x1 and 'p2' creates
> 0x2 and so on.

You can't rely on this.

See the KNOWN_ISSUES file in the the mpam tree: PARTID 0 should be reserved for unknown
hardware. In fact any number of PARTID may be reserved for in-kernel users. You can't
guess what the offset might be from user-space.


> Right now, there is no file which exposes the partid to userspace.
> We must rely on the sequential order in which we create partitions
> via resctrl and use that to derive the partid.

If you dig in the MPAM tree you'll find how I intend to solve this for exposing the MPAM
counters via perf. But this is a user-space visible change to resctrl, so it will need to
wait until all the refactoring is done and the bulk of the MPAM driver is upstream.


Thanks,

James