Re: [PATCH 2/2] cpufreq: Update CPU capacity reduction in store_scaling_max_freq()
From: Lukasz Luba
Date: Mon Oct 10 2022 - 05:31:21 EST
On 10/10/22 10:15, Vincent Guittot wrote:
On Mon, 10 Oct 2022 at 11:02, Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
On 10/10/22 06:39, Viresh Kumar wrote:
Would be good to always CC Scheduler maintainers for such a patch.
Agree, I'll do that.
On 30-09-22, 10:48, Lukasz Luba wrote:
When the new max frequency value is stored, the task scheduler must
know about it. The scheduler uses the CPUs capacity information in the
task placement. Use the existing mechanism which provides information
about reduced CPU capacity to the scheduler due to thermal capping.
Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
---
drivers/cpufreq/cpufreq.c | 18 +++++++++++++++++-
1 file changed, 17 insertions(+), 1 deletion(-)
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 1f8b93f42c76..205d9ea9c023 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -27,6 +27,7 @@
#include <linux/slab.h>
#include <linux/suspend.h>
#include <linux/syscore_ops.h>
+#include <linux/thermal.h>
#include <linux/tick.h>
#include <linux/units.h>
#include <trace/events/power.h>
@@ -718,6 +719,8 @@ static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
static ssize_t store_scaling_max_freq
(struct cpufreq_policy *policy, const char *buf, size_t count)
{
+ unsigned int frequency;
+ struct cpumask *cpus;
unsigned long val;
int ret;
@@ -726,7 +729,20 @@ static ssize_t store_scaling_max_freq
return -EINVAL;
ret = freq_qos_update_request(policy->max_freq_req, val);
- return ret >= 0 ? count : ret;
+ if (ret >= 0) {
+ /*
+ * Make sure that the task scheduler sees these CPUs
+ * capacity reduction. Use the thermal pressure mechanism
+ * to propagate this information to the scheduler.
+ */
+ cpus = policy->related_cpus;
No need of this, just use related_cpus directly.
+ frequency = __resolve_freq(policy, val, CPUFREQ_RELATION_HE);
+ arch_update_thermal_pressure(cpus, frequency);
I wonder if using the thermal-pressure API here is the right thing to
do. It is a change coming from User, which may or may not be
thermal-related.
Yes, I thought the same. Thermal-pressure name might be not the
best for covering this use case. I have been thinking about this
thermal pressure mechanism for a while, since there are other
use cases like PowerCap DTPM which also reduces CPU capacity
because of power policy from user-space. We don't notify
the scheduler about it. There might be also an issue with virtual
guest OS and how that kernel 'sees' the capacity of CPUs.
We might try to use this 'thermal-pressure' in the guest kernel
to notify about available CPU capacity (just a proposal, not
even an RFC, since we are missing requirements, but issues where
discussed on LPC 2022 on ChromeOS+Android_guest)
The User space setting scaling_max_freq is a long scale event and it
should be considered as a new running environnement instead of a
transient event. I would suggest updating the EM is and capacity orig
of the system in this case. Similarly, we rebuild sched_domain with a
cpu hotplug. scaling_max_freq interface should not be used to do any
kind of dynamic scaling.
I tend to agree, but the EM capacity would be only used in part of EAS
code. The whole fair.c view to the capacity_of() (RT + DL + irq +
thermal_pressure) would be still wrong in other parts, e.g.
select_idle_sibling() and load balance.
When we get this powerhint we might be already in overutilied state
so EAS is disabled. IMO other mechanisms in the task scheduler
should be also aware of that capacity reduction.