linux-kernel - Re: [PATCH v3] x86,sched: allow topologies where NUMA nodes share an LLC

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Date:   Thu, 29 Mar 2018 15:57:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Alison Schofield <alison.schofield@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        David Rientjes <rientjes@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        Prarit Bhargava <prarit@...hat.com>, brice.goglin@...il.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] x86,sched: allow topologies where NUMA nodes share an
 LLC

On Thu, Mar 29, 2018 at 06:45:12AM -0700, Dave Hansen wrote:
> On 03/29/2018 06:16 AM, Thomas Gleixner wrote:
> >> This is OK at least on the hardware we are immediately concerned about
> >> because the LLC sharing happens at both the slice and at the package
> >> level, which are also NUMA boundaries.
> > So that addresses the scheduler interaction, but it still leaves the
> > information in the sysfs files unchanged. See cpu/intel_cacheinfo.c.  There
> > are applications which use that information so it should be correct.
> 
> Were you thinking of shared_cpu_list/map? 

Yes, the fact that those are changed and the Changelog doesn't mention
them is a giant fail all by itself.

> The information in there is
> correct for core->off-package access.  It is not correct for
> core->on-package access, unless that access is perfectly interleaved
> across both package "slices".

The fact that 'simple' measures like cache/cpu are now completely broken
is a problem.