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From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Lina Iyer <ilina@codeaurora.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	"Raju P . L . S . S . S . N" <rplsssn@codeaurora.org>,
	Amit Kucheria <amit.kucheria@linaro.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Niklas Cassel <niklas.cassel@linaro.org>,
	Tony Lindgren <tony@atomide.com>,
	Kevin Hilman <khilman@kernel.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Souvik Chakravarty <souvik.chakravarty@arm.com>,
	Linux PM <linux-pm@vger.kernel.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 14/18] drivers: firmware: psci: Manage runtime PM in the idle path for CPUs
Date: Fri, 19 Jul 2019 11:02:16 +0100
Message-ID: <20190719100216.GA8587@e121166-lin.cambridge.arm.com> (raw)
In-Reply-To: <CAPDyKFrxBdZfskyp2HOb5YykkAqkBzRfW4-LLbcj1DAaL65XpA@mail.gmail.com>

On Thu, Jul 18, 2019 at 11:49:11PM +0200, Ulf Hansson wrote:
> On Thu, 18 Jul 2019 at 19:41, Lina Iyer <ilina@codeaurora.org> wrote:
> >
> > On Thu, Jul 18 2019 at 10:55 -0600, Ulf Hansson wrote:
> > >On Thu, 18 Jul 2019 at 15:31, Lorenzo Pieralisi
> > ><lorenzo.pieralisi@arm.com> wrote:
> > >>
> > >> On Thu, Jul 18, 2019 at 12:35:07PM +0200, Ulf Hansson wrote:
> > >> > On Tue, 16 Jul 2019 at 17:53, Lorenzo Pieralisi
> > >> > <lorenzo.pieralisi@arm.com> wrote:
> > >> > >
> > >> > > On Mon, May 13, 2019 at 09:22:56PM +0200, Ulf Hansson wrote:
> > >> > > > When the hierarchical CPU topology layout is used in DT, let's allow the
> > >> > > > CPU to be power managed through its PM domain, via deploying runtime PM
> > >> > > > support.
> > >> > > >
> > >> > > > To know for which idle states runtime PM reference counting is needed,
> > >> > > > let's store the index of deepest idle state for the CPU, in a per CPU
> > >> > > > variable. This allows psci_cpu_suspend_enter() to compare this index with
> > >> > > > the requested idle state index and then act accordingly.
> > >> > >
> > >> > > I do not see why a system with two CPU CPUidle states, say CPU retention
> > >> > > and CPU shutdown, should not be calling runtime PM on CPU retention
> > >> > > entry.
> > >> >
> > >> > If the CPU idle governor did select the CPU retention for the CPU, it
> > >> > was probably because the target residency for the CPU shutdown state
> > >> > could not be met.
> > >>
> > >> The kernel does not know what those cpu states represent, so, this is an
> > >> assumption you are making and it must be made clear that this code works
> > >> as long as your assumption is valid.
> > >>
> > >> If eg a "cluster" retention state has lower target_residency than
> > >> the deepest CPU idle state this assumption is wrong.
> > >
> > >Good point, you are right. I try to find a place to document this assumption.
> > >
> > >>
> > >> And CPUidle and genPD governor decisions are not synced anyway so,
> > >> again, this is an assumption, not a certainty.
> > >>
> > >> > In this case, there is no point in allowing any other deeper idle
> > >> > states for cluster/package/system, since those have even greater
> > >> > residencies, hence calling runtime PM doesn't make sense.
> > >>
> > >> On the systems you are testing on.
> > >
> > >So what you are saying typically means, that if all CPUs in the same
> > >cluster have entered the CPU retention state, on some system the
> > >cluster may also put into a cluster retention state (assuming the
> > >target residency is met)?
> > >
> > >Do you know of any systems that has these characteristics?
> > >
> > Many QCOM SoCs can do that. But with the hardware improving, the
> > power-performance benefits skew the results in favor of powering off
> > the cluster than keeping the CPU and cluster in retention.
> >
> > Kevin H and I thought of this problem earlier on. But that is a second
> > level problem to solve and definitely to be thought of after we have the
> > support for the deepest states in the kernel. We left that out for a
> > later date. The idea would have been to setup the allowable state(s) in
> > the DT for CPU and cluster state definitions and have the genpd take
> > that into consideration when deciding the idle state for the domain.
> 
> Thanks for confirming.
> 
> This more or less means we need to improve the hierarchical support in
> genpd to support more levels, such that it makes sense to have a genpd
> governor assigned at more than one level. This doesn't work well
> today. As I also have stated, this is on my todo list for genpd.
> 
> However, I also agree with your standpoint, that let's start simple to
> enable the deepest state as a start with, then we can improve things
> on top.

How to solve this in the kernel I don't know but please do make sure
that the DT bindings allow you to describe what's needed, once they are
merged you won't be able to change them and I won't bodge the code to
make things fit, so if anything let's focus on getting them right as a
matter of priority to get this done please.

Thanks,
Lorenzo

  reply index

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-13 19:22 [PATCH 00/18] ARM/ARM64: Support hierarchical CPU arrangement for PSCI Ulf Hansson
2019-05-13 19:22 ` [PATCH 01/18] dt: psci: Update DT bindings to support hierarchical PSCI states Ulf Hansson
2019-07-19 11:29   ` Lorenzo Pieralisi
2019-05-13 19:22 ` [PATCH 02/18] of: base: Add of_get_cpu_state_node() to get idle states for a CPU node Ulf Hansson
2019-05-13 19:22 ` [PATCH 03/18] cpuidle: dt: Support hierarchical CPU idle states Ulf Hansson
2019-05-13 19:22 ` [PATCH 04/18] ARM/ARM64: cpuidle: Let back-end init ops take the driver as input Ulf Hansson
2019-06-07 15:00   ` Sudeep Holla
2019-06-10 10:20     ` Ulf Hansson
2019-05-13 19:22 ` [PATCH 05/18] drivers: firmware: psci: Simplify state node parsing Ulf Hansson
2019-06-07 15:01   ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 06/18] drivers: firmware: psci: Support hierarchical CPU idle states Ulf Hansson
2019-06-07 15:03   ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 07/18] drivers: firmware: psci: Prepare to use OS initiated suspend mode Ulf Hansson
2019-06-07 15:17   ` Sudeep Holla
2019-06-10 10:21     ` Ulf Hansson
2019-06-10 10:42       ` Sudeep Holla
2019-07-16 14:53   ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 08/18] drivers: firmware: psci: Prepare to support PM domains Ulf Hansson
2019-06-07 15:21   ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 09/18] drivers: firmware: psci: Add support for PM domains using genpd Ulf Hansson
2019-06-07 15:27   ` Sudeep Holla
2019-06-10 10:21     ` Ulf Hansson
2019-06-10 10:59       ` Sudeep Holla
2019-07-16 15:05   ` Sudeep Holla
2019-07-18 11:04     ` Ulf Hansson
2019-07-18 13:19       ` Sudeep Holla
2019-07-18 17:57         ` Lina Iyer
2019-07-19  9:45           ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 10/18] drivers: firmware: psci: Add hierarchical domain idle states converter Ulf Hansson
2019-07-09 15:31   ` Lorenzo Pieralisi
2019-07-16  8:45     ` Ulf Hansson
2019-07-16 14:51       ` Lorenzo Pieralisi
2019-07-18 11:43         ` Ulf Hansson
2019-07-18 13:36           ` Lorenzo Pieralisi
2019-05-13 19:22 ` [PATCH 11/18] drivers: firmware: psci: Introduce psci_dt_topology_init() Ulf Hansson
2019-05-13 19:22 ` [PATCH 12/18] drivers: firmware: psci: Add a helper to attach a CPU to its PM domain Ulf Hansson
2019-05-13 19:22 ` [PATCH 13/18] drivers: firmware: psci: Attach the CPU's device " Ulf Hansson
2019-05-13 19:22 ` [PATCH 14/18] drivers: firmware: psci: Manage runtime PM in the idle path for CPUs Ulf Hansson
2019-07-16 15:53   ` Lorenzo Pieralisi
2019-07-18 10:35     ` Ulf Hansson
2019-07-18 13:30       ` Lorenzo Pieralisi
2019-07-18 16:54         ` Ulf Hansson
2019-07-18 17:41           ` Lina Iyer
2019-07-18 21:49             ` Ulf Hansson
2019-07-19 10:02               ` Lorenzo Pieralisi [this message]
2019-05-13 19:22 ` [PATCH 15/18] drivers: firmware: psci: Support CPU hotplug for the hierarchical model Ulf Hansson
2019-06-07 15:31   ` Sudeep Holla
2019-06-10 10:21     ` Ulf Hansson
2019-06-10 11:02       ` Sudeep Holla
2019-05-13 19:22 ` [PATCH 16/18] arm64: kernel: Respect the hierarchical CPU topology in DT for PSCI Ulf Hansson
2019-05-13 19:22 ` [PATCH 17/18] arm64: dts: Convert to the hierarchical CPU topology layout for MSM8916 Ulf Hansson
2019-07-16 14:47   ` Sudeep Holla
2019-07-16 20:36     ` Lina Iyer
2019-07-17 17:18       ` Sudeep Holla
2019-05-13 19:23 ` [PATCH 18/18] arm64: dts: hikey: Convert to the hierarchical CPU topology layout Ulf Hansson
2019-07-16 14:47   ` Sudeep Holla
2019-07-18 10:48     ` Ulf Hansson
2019-07-18 13:11       ` Sudeep Holla
2019-05-14  8:08 ` [PATCH 00/18] ARM/ARM64: Support hierarchical CPU arrangement for PSCI Rafael J. Wysocki
2019-05-14  8:58   ` Ulf Hansson
2019-06-07 15:42     ` Sudeep Holla
2019-06-07 19:34       ` Bjorn Andersson
2019-06-10 10:32         ` Sudeep Holla
2019-06-10 15:54           ` Ulf Hansson
2019-06-10 17:16             ` Lorenzo Pieralisi
2019-06-10 18:57               ` Ulf Hansson
2019-06-18 11:56                 ` Ulf Hansson
2019-06-07 11:19 ` Ulf Hansson

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