linux-kernel - Re: Initial testing of MPAM patches

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Message-ID: <74ed130f-d191-d8a2-084c-ae096ca5a47d@arm.com>
Date:   Tue, 22 Aug 2023 14:23:42 +0100
From:   James Morse <james.morse@....com>
To:     Carl Worth <carl@...amperecomputing.com>,
        linux-kernel@...r.kernel.org
Cc:     "D. Scott Phillips" <scott@...amperecomputing.com>,
        Darren Hart <darren@...amperecomputing.com>,
        Amit Singh Tomar <amitsinght@...vell.com>,
        Akanksha Jain <Akanksha.Jain2@....com>
Subject: Re: Initial testing of MPAM patches

Hi Carl,

On 18/08/2023 17:15, Carl Worth wrote:
> James Morse <james.morse@....com> writes:
>> On 18/08/2023 15:13, Carl Worth wrote:

>>> 4. The current schemata allows for cache portion, but not cache capacity
>>
>> See KNOWN_ISSUES:
>> | Only features that match what resctrl already supports are supported.
>> | This is very deliberate.
> ...
>>>    Is this due to a limitation in mapping MPAM to the current resctrl
>>>    interface?
>>
>> It is. Getting feature parity with x86 is the critical path to getting this upstream.
>> Supporting other bits of MPAM can come next - we'd need a discussion with Intel about how
>> any changes should be done, so that they can support them too if they ever have similar
>> features.
>>
>> This conversation can't happen until we have some support upstream.
>
> Got it. This approach makes sense to me, and it's good for me to
> understand what limitations exist in the current implementation and why.

There is some noise around having a discussion about how we make resctrl work on wildly
different platforms, maybe at plumbers, but there is nothing solid yet.
Can I share your name as an interested party?


>>> 5. Linked-list corruption with missing cache entries in PPTT
>>>
>>>    At one point, I tried booting with the MPAM ACPI table populated
>>>    for my L3 cache, but without the necessary entries in the PPTT ACPI
>>>    table. The driver fell over with linked-list corruption, halting
>>>    Linux boot. I'll follow up this report with more details.
>>
>> This kind of thing won't have seen much testing. Any details you can
>> share would help!
>
> Yeah, I figured as much. Since I can replicate this I don't think it
> should be too hard for me to give in and root-cause this bug.
>
> Thanks again for the quick response. I'll do my next testing against
> more recent code, and I should be able to follow-up against some
> specific patches for the couple of bugs I identified above and that I'll
> look closer into.

Thanks,


> Beyond that, I hope to be able to provide some Reviewed-by and Tested-by
> soon.

Great! Could you do that against the most recent branch on kernel.org?


> I see that you've been going several rounds on the earlier portions of
> this patch set, (the parts that refactor resctrl to prepare for
> things). I trust that you've got that part of the process in hand?

Well, its been going on for years.


> Otherwise, let me know if there's anything I can do to help with there.

Any review or testing of those patches would help. The maintainer needs some indication
that people want this stuff, and it's not just me!

I CC you on the patches, but lately I've been getting 'inbox full' bounces from your mail
server.


> Again, I haven't been looking into details of those patches yet, just
> testing to ensure they work, (and so far, the generic parts of resctrl
> seem to be working just fine for me).

Good to know!


Thanks,

James
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