linux-kernel - RE: [RFC PATCH] x86/CPU/AMD: Bring back Compute Unit ID

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Date:   Wed, 1 Feb 2017 21:37:02 +0000
From:   "Ghannam, Yazen" <Yazen.Ghannam@....com>
To:     Borislav Petkov <bp@...en8.de>, x86-ml <x86@...nel.org>
CC:     Yves Dionne <yves.dionne@...il.com>,
        Brice Goglin <Brice.Goglin@...ia.fr>,
        Peter Zijlstra <peterz@...radead.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH] x86/CPU/AMD: Bring back Compute Unit ID

> -----Original Message-----
> From: Borislav Petkov [mailto:bp@...en8.de]
> Sent: Wednesday, February 1, 2017 3:03 PM
> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> index 548da5a8013e..f06fa338076b 100644
> --- a/arch/x86/kernel/smpboot.c
> +++ b/arch/x86/kernel/smpboot.c
> @@ -433,9 +433,13 @@ static bool match_smt(struct cpuinfo_x86 *c, struct
> cpuinfo_x86 *o)
>  		int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
> 
>  		if (c->phys_proc_id == o->phys_proc_id &&
> -		    per_cpu(cpu_llc_id, cpu1) == per_cpu(cpu_llc_id, cpu2) &&
> -		    c->cpu_core_id == o->cpu_core_id)
> -			return topology_sane(c, o, "smt");
> +		    per_cpu(cpu_llc_id, cpu1) == per_cpu(cpu_llc_id, cpu2)) {
> +			if (c->cpu_core_id == o->cpu_core_id)
> +				return topology_sane(c, o, "smt");
> +
> +			if (c->cu_id == o->cu_id)
> +				return topology_sane(c, o, "smt");
> +		}
> 

This hunk won't work for SMT enabled systems. It'll cause all threads under
an LLC to be considered SMT siblings. For example, threads 0 &2 will have
different cpu_core_id, so the first check will fail. But it'll match on the
second check since cu_id will be initialized to 0.

To get around this we can set cu_id for all TOPOEXT systems, and update
cpu_core_id, etc. for SMT enabled systems. This way we can just change
cpu_core_id to cu_id in match_smt().

I tested this patch,  with the above changes, on a Fam17h SMT enabled
system. I'll test with SMT disabled and also on a fully-loaded Fam15h
system soon.

Thanks,
Yazen