Date: Tue, 29 Nov 2016 22:15:12 +0100
From: Borislav Petkov <bp@suse.de>
To: Brice Goglin <Brice.Goglin@inria.fr>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: AMD Bulldozer topology regression since 4.6
Message-ID: <20161129211512.uetbapsqja6wycnp@pd.tnic>
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On Tue, Nov 29, 2016 at 10:02:00PM +0100, Brice Goglin wrote:
> However thread_siblings isn't back to where it was in 4.5. Now we have a
> single bit in each thread_siblings mask. That's correct with respect to
> the sysfs topology documentation. In 4.5, there were two bits (one for
> each core of the compute unit), which was wrong (cores with different
> core_ids shouldn't appear in each other thread_siblings). I assumed that
> these processors had to break the sysfs topology documentation to expose
> the concept of "dual-core compute-unit" which somehow sits between
> hyperthreading and dual-core.

This is exactly the problem - there's never proper fitting of the
compute unit "ideology" in the whole topology view. So what we're doing
now is revert to the old strategy of keeping Bulldozer have core_id ==
thread_id. Basically, all CU threads are cores, as they're more powerful
than, say, SMT threads, but they still share an FPU.

So it is a different kind of sharing and it's kinda, well, different.
There's no other way to put it.

> I personally do not care much about this regression, not sure about
> other user-space tools?
> 
> Another minor related change: /proc/cpuinfo shows "cpu cores : 16"
> instead of "8".

Can you send me the whole thing? Offlist is fine too.

Thanks.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--