linux-kernel - Re: [PATCHv2 02/12] acpi/hmat: Parse and report heterogeneous memory

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Date:   Tue, 11 Dec 2018 13:44:51 -0700
From:   Keith Busch <keith.busch@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCHv2 02/12] acpi/hmat: Parse and report heterogeneous memory

On Tue, Dec 11, 2018 at 12:29:45PM -0800, Dan Williams wrote:
> On Tue, Dec 11, 2018 at 8:58 AM Keith Busch <keith.busch@...el.com> wrote:
> > +static int __init
> > +acpi_parse_cache(union acpi_subtable_headers *header, const unsigned long end)
> > +{
> > +       struct acpi_hmat_cache *cache = (void *)header;
> > +       u32 attrs;
> > +
> > +       attrs = cache->cache_attributes;
> > +       if (((attrs & ACPI_HMAT_CACHE_ASSOCIATIVITY) >> 8) ==
> > +                                               ACPI_HMAT_CA_DIRECT_MAPPED)
> > +               set_bit(cache->memory_PD, node_side_cached);
> 
> I'm not sure I see a use case for 'node_side_cached'. Instead I need
> to know if a cache intercepts a "System RAM" resource, because a cache
> in front of a reserved address range would not be impacted by page
> allocator randomization. Or, are you saying have memblock generically
> describes this capability and move the responsibility of acting on
> that data to a higher level?

The "node_side_cached" array isn't intended to be used directly. It's
just holding the PXM's that HMAT says have a side cache so we know which
PXM's have that attribute before parsing SRAT's memory affinity.

The intention was that this is just another attribute of a memory range
similiar to hotpluggable. Whoever needs to use it may query it from
the memblock, if that makes sense.

> The other detail to consider is the cache ratio size, but that would
> be a follow on feature. The use case is to automatically determine the
> ratio to pass to numa_emulation:
> 
>     cc9aec03e58f x86/numa_emulation: Introduce uniform split capability

Will look into that.
 
> > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > index aee299a6aa76..a24c918a4496 100644
> > --- a/include/linux/memblock.h
> > +++ b/include/linux/memblock.h
> > @@ -44,6 +44,7 @@ enum memblock_flags {
> >         MEMBLOCK_HOTPLUG        = 0x1,  /* hotpluggable region */
> >         MEMBLOCK_MIRROR         = 0x2,  /* mirrored region */
> >         MEMBLOCK_NOMAP          = 0x4,  /* don't add to kernel direct mapping */
> > +       MEMBLOCK_SIDECACHED     = 0x8,  /* System side caches memory access */
> 
> I'm concerned that we may be stretching memblock past its intended use
> case especially for just this randomization case. For example, I think
> memblock_find_in_range() gets confused in the presence of
> MEMBLOCK_SIDECACHED memblocks.

Ok, I see. Is there a better structure or interface that you may recommend
for your use case to identify which memory ranges contain this attribute?