linux-kernel - Re: [PATCH] arm64: PCI: Remove node-local allocations when initialising host controller

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Message-ID: <871sb83pqq.fsf@e105922-lin.cambridge.arm.com>
Date:   Thu, 09 Aug 2018 09:31:41 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Jeremy Linton <jeremy.linton@....com>,
        linux-arm <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Jiang Liu <jiang.liu@...ux.intel.com>,
        linux-pci@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-acpi@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] arm64: PCI: Remove node-local allocations when initialising host controller

Bjorn Helgaas <helgaas@...nel.org> writes:

> On Wed, Aug 08, 2018 at 03:44:03PM +0100, Punit Agrawal wrote:
>> Bjorn Helgaas <bhelgaas@...gle.com> writes:
>> > On Thu, Aug 2, 2018 at 9:33 AM Lorenzo Pieralisi
>> > <lorenzo.pieralisi@....com> wrote:
>> >> On Wed, Aug 01, 2018 at 02:38:51PM -0500, Jeremy Linton wrote:
>> >>
>> >> Jiang Liu does not work on the kernel anymore so we won't know
>> >> anytime soon the reasoning behind commit 965cd0e4a5e5
>> >>
>> >> > On 08/01/2018 12:31 PM, Punit Agrawal wrote:
>> >> > >Memory for host controller data structures is allocated local to the
>> >> > >node to which the controller is associated with. This has been the
>> >> > >behaviour since support for ACPI was added in
>> >> > >commit 0cb0786bac15 ("ARM64: PCI: Support ACPI-based PCI host controller").
>> >> >
>> >> > Which was apparently influenced by:
>> >> >
>> >> > 965cd0e4a5e5 x86, PCI, ACPI: Use kmalloc_node() to optimize for performance
>> >> >
>> >> > Was there an actual use-case behind that change?
>> >> >
>> >> > I think this fixes the immediate boot problem, but if there is any
>> >> > perf advantage it seems wise to keep it... Particularly since x86
>> >> > seems to be doing the node sanitation in pci_acpi_root_get_node().
>> >>
>> >> I am struggling to see the perf advantage of allocating a struct
>> >> that the PCI controller will never read/write from a NUMA node that
>> >> is local to the PCI controller, happy to be corrected if there is
>> >> a sound rationale behind that.
>> >
>> > If there is no reason to use kzalloc_node() here, we shouldn't use it.
>> >
>> > But we should use it (or not use it) consistently across arches.  I do
>> > not believe there is an arch-specific reason to be different.
>> > Currently, pci_acpi_scan_root() uses kzalloc_node() on x86 and arm64,
>> > but kzalloc() on ia64.  They all ought to be the same.
>> 
>> From my understanding, arm64 use of kzalloc_node() was derived from the
>> x86 version. Maybe somebody familiar with behaviour on x86 can provide
>> input here.
>
> If you want to remove use of kzalloc_node(), I'm fine with that as
> long as you do it for x86 at the same time (maybe separate patches,
> but at least in the same series).
>
> I don't see any evidence in 965cd0e4a5e5 ("x86, PCI, ACPI: Use
> kmalloc_node() to optimize for performance") that it actually improves
> performance, so I'd be inclined to just use kzalloc().

Thanks for confirming.

I'm happy to add a patch updating x86 use of kzalloc_node() as
well. I'll post something once the merge window closes.

>
> Bjorn