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SubjectRE: [PATCH 2/2] x86, acpi: Handle apic/x2apic entries in MADT in correct order
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-----Original Message-----
From: Lorenzo Pieralisi [mailto:lorenzo.pieralisi@arm.com]
Sent: Wednesday, September 9, 2015 3:56 PM

> On Wed, Sep 09, 2015 at 10:30:18AM +0100, Lukasz Anaczkowski wrote:
> > () it's hard to predict how cores and threads are enumerated

> So ? Why would the logical cpus order matters at all ? I guessed
> there are probeable properties that allows the kernel to build
> the affinity (ie topology list, shared caches, smt siblings, etc).
> Please explain, since I am confused, to me all you need is a list
> of existing physical ids, in whatever order they come, that's at least
> what we need on ARM.

Hi Lorenzo,

Sure, let me try to explain this better.

Proper (i.e. predictable way of CPU enumeration) matters for HPC software,
(this is where I come from) as there are workloads that have some assumptions
on CPU enumeration in order to keep cache hit ratio as high as possible.
E.g. in KNL cores share L2 caches, and if during enumeration logical cores do not
reflect physical cores, S/W can start affinitize threads to the same physical cores
causing great performance impact exactly due to L2 cache misses.
(e.g. s/w assumes that HT CPUs are separated by core count).

Now, those changes would not be required if someone who have written
APIC spec had reserved more than just 1 byte for CPU id :)
Unfortunately, it's the case for x86 APIC ID and once it turns out there's a need
to enumerate more than that, they added X2APIC spec which has 4 bytes for ID.
Even that would be also fine if there were just physical cores, but with HT, ACPI
clearly says, that first must be listed physical cores and only after that HT CPUs
(and that's why APIC/X2APIC subtables are interleaved).

When GIC spec was added, someone was smart enough to put 4 bytes from
the begging, so you don't need to care about it on ARM :)

> > () enumeration is inconsistent with how threads are enumerated on
> > other Intel Xeon processors

> And why does that matter ? Is it because userspace is making assumptions
> on the logical cpu enumeration scheme ? I am just asking, I would
> like to understand.

Yes, HPC software makes some assumptions about CPU enumeration (as mentioned
above) and having inconsistent enumeration between different x86 CPUs (Xeon vs Xeon Phi)
make such s/w basically not portable.

> > So, order in which MADT APIC/X2APIC handlers are passed is
> > reverse and both handlers are passed to be called during same MADT
> > table to walk to achieve correct CPU enumeration.

> Define "correct" please, you define the logical ordering you
> want to achieve, you do not define why that's more "correct"
> than the current implementation.

Ok, probably 'correct' word is not the best here :)
Does 'compatible' sound better?

Thanks,
Lukasz


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