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From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
	linux-pm@vger.kernel.org,  loongarch@lists.linux.dev,
	linux-acpi@vger.kernel.org,  linux-arch@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	 linux-arm-kernel@lists.infradead.org,
	linux-riscv@lists.infradead.org,  kvmarm@lists.linux.dev,
	x86@kernel.org,  acpica-devel@lists.linuxfoundation.org,
	linux-csky@vger.kernel.org,  linux-doc@vger.kernel.org,
	linux-ia64@vger.kernel.org,  linux-parisc@vger.kernel.org,
	Salil Mehta <salil.mehta@huawei.com>,
	 Jean-Philippe Brucker <jean-philippe@linaro.org>,
	jianyong.wu@arm.com, justin.he@arm.com,
	 James Morse <james.morse@arm.com>
Subject: Re: [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info()
Date: Wed, 21 Feb 2024 13:04:43 +0100	[thread overview]
Message-ID: <CAJZ5v0hu32UCLPO6txptfn1DxCNqdYc+Ls-yNa09LdzhroyddQ@mail.gmail.com> (raw)
In-Reply-To: <CAJZ5v0i0c3bg8E9yuRk00VAEW5isZ4N-mbnhRuTR8aiFLXo1_A@mail.gmail.com>

On Tue, Feb 20, 2024 at 8:59 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Tue, Feb 20, 2024 at 5:24 PM Jonathan Cameron
> <Jonathan.Cameron@huawei.com> wrote:
> >
> > On Tue, 20 Feb 2024 15:13:58 +0000
> > "Russell King (Oracle)" <linux@armlinux.org.uk> wrote:
> >
> > > On Tue, Feb 20, 2024 at 11:27:15AM +0000, Russell King (Oracle) wrote:
> > > > On Thu, Feb 15, 2024 at 08:22:29PM +0100, Rafael J. Wysocki wrote:
> > > > > On Wed, Jan 31, 2024 at 5:50 PM Russell King <rmk+kernel@armlinux.org.uk> wrote:
> > > > > > diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> > > > > > index cf7c1cca69dd..a68c475cdea5 100644
> > > > > > --- a/drivers/acpi/acpi_processor.c
> > > > > > +++ b/drivers/acpi/acpi_processor.c
> > > > > > @@ -314,6 +314,18 @@ static int acpi_processor_get_info(struct acpi_device *device)
> > > > > >                         cpufreq_add_device("acpi-cpufreq");
> > > > > >         }
> > > > > >
> > > > > > +       /*
> > > > > > +        * Register CPUs that are present. get_cpu_device() is used to skip
> > > > > > +        * duplicate CPU descriptions from firmware.
> > > > > > +        */
> > > > > > +       if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id) &&
> > > > > > +           !get_cpu_device(pr->id)) {
> > > > > > +               int ret = arch_register_cpu(pr->id);
> > > > > > +
> > > > > > +               if (ret)
> > > > > > +                       return ret;
> > > > > > +       }
> > > > > > +
> > > > > >         /*
> > > > > >          *  Extra Processor objects may be enumerated on MP systems with
> > > > > >          *  less than the max # of CPUs. They should be ignored _iff
> > > > >
> > > > > This is interesting, because right below there is the following code:
> > > > >
> > > > >     if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
> > > > >         int ret = acpi_processor_hotadd_init(pr);
> > > > >
> > > > >         if (ret)
> > > > >             return ret;
> > > > >     }
> > > > >
> > > > > and acpi_processor_hotadd_init() essentially calls arch_register_cpu()
> > > > > with some extra things around it (more about that below).
> > > > >
> > > > > I do realize that acpi_processor_hotadd_init() is defined under
> > > > > CONFIG_ACPI_HOTPLUG_CPU, so for the sake of the argument let's
> > > > > consider an architecture where CONFIG_ACPI_HOTPLUG_CPU is set.
> > > > >
> > > > > So why are the two conditionals that almost contradict each other both
> > > > > needed?  It looks like the new code could be combined with
> > > > > acpi_processor_hotadd_init() to do the right thing in all cases.
> > > > >
> > > > > Now, acpi_processor_hotadd_init() does some extra things that look
> > > > > like they should be done by the new code too.
> > > > >
> > > > > 1. It checks invalid_phys_cpuid() which appears to be a good idea to me.
> > > > >
> > > > > 2. It uses locking around arch_register_cpu() which doesn't seem
> > > > > unreasonable either.
> > > > >
> > > > > 3. It calls acpi_map_cpu() and I'm not sure why this is not done by
> > > > > the new code.
> > > > >
> > > > > The only thing that can be dropped from it is the _STA check AFAICS,
> > > > > because acpi_processor_add() won't even be called if the CPU is not
> > > > > present (and not enabled after the first patch).
> > > > >
> > > > > So why does the code not do 1 - 3 above?
> > > >
> > > > Honestly, I'm out of my depth with this and can't answer your
> > > > questions - and I really don't want to try fiddling with this code
> > > > because it's just too icky (even in its current form in mainline)
> > > > to be understandable to anyone who hasn't gained a detailed knowledge
> > > > of this code.
> > > >
> > > > It's going to require a lot of analysis - how acpi_map_cpuid() behaves
> > > > in all circumstances, what this means for invalid_logical_cpuid() and
> > > > invalid_phys_cpuid(), what paths will be taken in each case. This code
> > > > is already just too hairy for someone who isn't an experienced ACPI
> > > > hacker to be able to follow and I don't see an obvious way to make it
> > > > more readable.
> > > >
> > > > James' additions make it even more complex and less readable.
> > >
> > > As an illustration of the problems I'm having here, I was just writing
> > > a reply to this with a suggestion of transforming this code ultimately
> > > to:
> > >
> > >       if (!get_cpu_device(pr->id)) {
> > >               int ret;
> > >
> > >               if (!invalid_logical_cpuid(pr->id) && cpu_present(pr->id))
> > >                       ret = acpi_processor_make_enabled(pr);
> > >               else
> > >                       ret = acpi_processor_make_present(pr);
> > >
> > >               if (ret)
> > >                       return ret;
> > >       }
> > >
> > > (acpi_processor_make_present() would be acpi_processor_hotadd_init()
> > > and acpi_processor_make_enabled() would be arch_register_cpu() at this
> > > point.)
> > >
> > > Then I realised that's a bad idea - because we really need to check
> > > that pr->id is valid before calling get_cpu_device() on it, so this
> > > won't work. That leaves us with:
> > >
> > >       int ret;
> > >
> > >       if (invalid_logical_cpuid(pr->id) || !cpu_present(pr->id)) {
> > >               /* x86 et.al. path */
> > >               ret = acpi_processor_make_present(pr);
> > >       } else if (!get_cpu_device(pr->id)) {
> > >               /* Arm64 path */
> > >               ret = acpi_processor_make_enabled(pr);
> > >       } else {
> > >               ret = 0;
> > >       }
> > >
> > >       if (ret)
> > >               return ret;
> > >
> > > Now, the next transformation would be to move !get_cpu_device(pr->id)
> > > into acpi_processor_make_enabled() which would eliminate one of those
> > > if() legs.
> > >
> > > Now, if we want to somehow make the call to arch_regster_cpu() common
> > > in these two paths, the next question is what are the _precise_
> > > semantics of acpi_map_cpu(), particularly with respect to it
> > > modifying pr->id. Is it guaranteed to always give the same result
> > > for the same processor described in ACPI? What acpi_map_cpu() anyway,
> > > I can find no documentation for it.
> > >
> > > Then there's the question whether calling acpi_unmap_cpu() should be
> > > done on the failure path if arch_register_cpu() fails, which is done
> > > for the x86 path but not the Arm64 path. Should it be done for the
> > > Arm64 path? I've no idea, but as Arm64 doesn't implement either of
> > > these two functions, I guess they could be stubbed out and thus be
> > > no-ops - but then we open a hole where if pr->id is invalid, we
> > > end up passing that invalid value to arch_register_cpu() which I'm
> > > quite sure will explode with a negative CPU number.
> > >
> > > So, to my mind, what you're effectively asking for is a total rewrite
> > > of all the code in and called by acpi_processor_get_info()... and that
> > > is not something I am willing to do (because it's too far outside of
> > > my knowledge area.)
> > >
> > > As I said in my reply to patch 1, I think your comments on patch 2
> > > make Arm64 vcpu hotplug unachievable in a reasonable time frame, and
> > > certainly outside the bounds of what I can do to progress this.
> > >
> > > So, at this point I'm going to stand down from further participation
> > > with this patch set as I believe I've reached the limit of what I can
> > > do to progress it.
> > >
> >
> > Thanks for your hard work on this Russell - we have moved forwards.
> >
> > Short of anyone else stepping up I'll pick this up with
> > the help of some my colleagues. As such I'm keen on getting patch
> > 1 upstream ASAP so that we can exclude the need for some of the
> > other workarounds from earlier versions of this series (the ones
> > dropped before now).
>
> Applied (as 6.9 material).

And I'm going to drop it, because it is not correct.

The problem is that it is going to affect non-processor devices, but
let me comment on that patch itself.

  reply	other threads:[~2024-02-21 12:04 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2024-01-31 16:48 [RFC PATCH v4 00/15] ACPI/arm64: add support for virtual cpu hotplug Russell King (Oracle)
2024-01-31 16:49 ` [PATCH RFC v4 01/15] ACPI: Only enumerate enabled (or functional) processor devices Russell King
2024-01-31 17:25   ` Miguel Luis
2024-02-15 20:10   ` Rafael J. Wysocki
2024-02-19  9:45     ` Jonathan Cameron
2024-02-20 11:30     ` Russell King (Oracle)
2024-02-21 13:01   ` Rafael J. Wysocki
2024-01-31 16:49 ` [PATCH RFC v4 02/15] ACPI: processor: Register all CPUs from acpi_processor_get_info() Russell King
2024-02-15 19:22   ` Rafael J. Wysocki
2024-02-20 11:27     ` Russell King (Oracle)
2024-02-20 15:13       ` Russell King (Oracle)
2024-02-20 16:24         ` Jonathan Cameron
2024-02-20 19:59           ` Rafael J. Wysocki
2024-02-21 12:04             ` Rafael J. Wysocki [this message]
2024-02-20 20:59     ` Thomas Gleixner
2024-03-22 18:53     ` Jonathan Cameron
2024-04-10 12:43       ` Jonathan Cameron
2024-04-10 13:28         ` Rafael J. Wysocki
2024-04-10 13:50           ` Jonathan Cameron
2024-04-10 14:19             ` Rafael J. Wysocki
2024-04-10 15:58               ` Jonathan Cameron
2024-04-10 18:56             ` Russell King (Oracle)
2024-04-10 19:08               ` Rafael J. Wysocki
2024-04-10 21:07                 ` Jonathan Cameron
2024-01-31 16:49 ` [PATCH RFC v4 03/15] ACPI: Move acpi_bus_trim_one() before acpi_scan_hot_remove() Russell King
2024-01-31 16:49 ` [PATCH RFC v4 04/15] ACPI: Rename acpi_processor_hotadd_init and remove pre-processor guards Russell King
2024-01-31 16:50 ` [PATCH RFC v4 05/15] ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug Russell King
2024-01-31 16:50 ` [PATCH RFC v4 06/15] ACPI: convert acpi_processor_post_eject() to use IS_ENABLED() Russell King (Oracle)
2024-01-31 16:50 ` [PATCH RFC v4 07/15] ACPI: Check _STA present bit before making CPUs not present Russell King
2024-01-31 16:50 ` [PATCH RFC v4 08/15] ACPI: Warn when the present bit changes but the feature is not enabled Russell King
2024-01-31 16:50 ` [PATCH RFC v4 09/15] arm64: acpi: Move get_cpu_for_acpi_id() to a header Russell King
2024-01-31 16:50 ` [PATCH RFC v4 10/15] irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc() Russell King
2024-02-02 16:44   ` Jonathan Cameron
2024-01-31 16:50 ` [PATCH RFC v4 11/15] irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs Russell King
2024-02-02 16:47   ` Jonathan Cameron
2024-01-31 16:50 ` [PATCH RFC v4 12/15] arm64: psci: Ignore DENIED CPUs Russell King
2024-04-11 11:35   ` Jonathan Cameron
2024-04-11 13:25     ` Jonathan Cameron
2024-01-31 16:50 ` [PATCH RFC v4 13/15] ACPI: add support to (un)register CPUs based on the _STA enabled bit Russell King
2024-01-31 16:50 ` [PATCH RFC v4 14/15] arm64: document virtual CPU hotplug's expectations Russell King
2024-02-02 17:04   ` Jonathan Cameron
2024-01-31 16:50 ` [PATCH RFC v4 15/15] cpumask: Add enabled cpumask for present CPUs that can be brought online Russell King

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