linux-kernel - Re: [tip: sched/core] sched: Add cluster scheduler level for x86

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Date:   Wed, 20 Oct 2021 22:36:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>, x86@...nel.org
Subject: Re: [tip: sched/core] sched: Add cluster scheduler level for x86

On Wed, Oct 20, 2021 at 10:25:42PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 20, 2021 at 03:08:41PM -0500, Tom Lendacky wrote:
> > On 10/20/21 2:51 PM, Peter Zijlstra wrote:
> > > On Wed, Oct 20, 2021 at 08:12:51AM -0500, Tom Lendacky wrote:
> > > > On 10/15/21 4:44 AM, tip-bot2 for Tim Chen wrote:
> > > > > The following commit has been merged into the sched/core branch of tip:
> > 
> > > 
> > > If it does boot, what does something like:
> > > 
> > >    for i in /sys/devices/system/cpu/cpu*/topology/*{_id,_list}; do echo -n "${i}: " ; cat $i; done
> > > 
> > > produce?
> > 
> > The output is about 160K in size, I'll email it to you off-list.
> 
> /sys/devices/system/cpu/cpu0/topology/cluster_cpus_list: 0
> /sys/devices/system/cpu/cpu0/topology/core_cpus_list: 0,128
> 
> /sys/devices/system/cpu/cpu128/topology/cluster_cpus_list: 128
> /sys/devices/system/cpu/cpu128/topology/core_cpus_list: 0,128
> 
> So for some reason that thing thinks each SMT thread has it's own L2,
> which seems rather unlikely. Or SMT has started to mean something
> radically different than it used to be :-)
> 
> Let me continue trying to make sense of cacheinfo.c

OK, I think I see what's happening.

AFAICT cacheinfo.c does *NOT* set l2c_id on AMD/Hygon hardware, this
means it's set to BAD_APICID.

This then results in match_l2c() to never match. And as a direct
consequence set_cpu_sibling_map() will generate cpu_l2c_shared_mask with
just the one CPU set.

And we have the above result and things come unstuck if we assume:
  SMT <= L2 <= LLC

Now, the big question, how to fix this... Does AMD have means of
actually setting l2c_id or should we fall back to using match_smt() for
l2c_id == BAD_APICID ?