Re: [PATCH 2/6] drivers base/arch_topology: frequency-invariant load-tracking support

From: Saravana Kannan
Date: Tue Jun 20 2017 - 20:31:35 EST

On 06/19/2017 11:17 PM, Viresh Kumar wrote:
On Thu, Jun 8, 2017 at 1:25 PM, Dietmar Eggemann
<dietmar.eggemann@xxxxxxx> wrote:

diff --git a/drivers/base/arch_topology.c b/drivers/base/arch_topology.c

static int __init register_cpufreq_notifier(void)
+ int ret;
* on ACPI-based systems we need to use the default cpu capacity
* until we have the necessary code to parse the cpu capacity, so
@@ -225,8 +265,14 @@ static int __init register_cpufreq_notifier(void)

cpumask_copy(cpus_to_visit, cpu_possible_mask);

- return cpufreq_register_notifier(&init_cpu_capacity_notifier,
+ ret = cpufreq_register_notifier(&init_cpu_capacity_notifier,

Wanted to make sure that we all understand the constraints this is going to add
for the ARM64 platforms.

With the introduction of this transition notifier, we would not be able to use
the fast-switch path in the schedutil governor. I am not sure if there are any
ARM platforms that can actually use the fast-switch path in future or not
though. The requirement of fast-switch path is that the freq can be changed
without sleeping in the hot-path.

So, will we ever want fast-switching for ARM platforms ?

I don't think we should go down a path that'll prevent ARM platform from switching over to fast-switching in the future.

Having said that, I'm not sure I fully agree with the decision to completely disable notifiers in the fast-switching case. How many of the current users of notifiers truly need support for sleeping in the notifier? Why not make all the transition notifiers atomic? Or at least add atomic transition notifiers that can be registered for separately if the client doesn't need the ability to sleep?

Most of the clients don't seem like ones that'll need to sleep.

There are a bunch of generic off-tree drivers (can't upstream them yet because it depends on the bus scaling framework) that also depend on CPUfreq transition notifiers that are going to stop working if fast switching becomes available in the future. So, this decision to disallow transition notifiers is painful for other reasons too.


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