PM: QoS: Introduce boot parameter pm_qos_resume_latency_us

PATCH Superseded Aaron Tomlin 2 participants 3 messages Mar 7 – Mar 8

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Aaron Tomlin

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2026-03-07 20:07 UTC #0 lore raw

PATCH v3 1/1 PM: QoS: Introduce boot parameter pm_qos_resume_latency_us

Users currently lack a mechanism to define granular, per-CPU PM QoS resume latency constraints during the early boot phase.

While the idle=poll boot parameter exists, it enforces a global override, forcing all CPUs in the system to "poll". This global approach is not suitable for asymmetric workloads where strict latency guarantees are required only on specific critical CPUs, while housekeeping or non-critical CPUs should be allowed to enter deeper idle states to save energy.

Additionally, the existing sysfs interface (/sys/devices/system/cpu/cpuN/power/pm_qos_resume_latency_us) becomes available only after userspace initialisation. This is too late to prevent deep C-state entry during the early kernel boot phase, which may be required for debugging early boot hangs related to C-state transitions or for workloads requiring strict latency guarantees immediately upon system start.

This patch introduces the pm_qos_resume_latency_us kernel boot parameter, which allows users to specify distinct resume latency constraints for specific CPU ranges.

Syntax: pm_qos_resume_latency_us=range:value,range:value...

Unlike the sysfs interface which accepts the special string "n/a" to remove a constraint, this boot parameter strictly requires integer values. The special value "n/a" is not supported; the integer 0 must be used to represent a 0 us latency constraint (polling).

For example:

	"pm_qos_resume_latency_us=0:0,1-15:20"

Forces CPU 0 to poll on idle; constrains CPUs 1-15 to not enter a sleep state that takes longer than 20 us to wake up. All other CPUs will have the default (no resume latency) applied.

Implementation Details:

- The parameter string is captured via __setup() and parsed in an early_initcall() to ensure suitable memory allocators are available.

- Constraints are stored in a read-only linked list.

- The constraints are queried and applied in register_cpu(). This ensures the latency requirement is active immediately upon CPU registration, effectively acting as a "birth" constraint before the cpuidle governor takes over.

- The parsing logic enforces a "First Match Wins" policy: if a CPU falls into multiple specified ranges, the latency value from the first matching entry is used.

- The constraints persist across CPU hotplug events.

Signed-off-by Aaron Tomlin <[email protected]>

---
Changes in v2 [1]:
- Add pr_fmt() to standardise log prefixes (Zhongqiu Han)
- Drop <asm/setup.h> by duplicating the command line with kstrdup() (Zhongqiu Han)
- Fix init_pm_qos_resume_latency_us_setup() error path to return -ENOMEM (Zhongqiu Han)

Changes since v1 [2]:
- Removed boot_option_idle_override == IDLE_POLL check
- Decoupled implementation from CONFIG_CPU_IDLE
- Added kernel-parameters.txt documentation
- Renamed internal setup functions for consistency

[1]: https://lore.kernel.org/lkml/[email protected]/
[2]: https://lore.kernel.org/lkml/[email protected]/

 .../admin-guide/kernel-parameters.txt         |  23 +++
 drivers/base/cpu.c                            |   5 +-
 include/linux/pm_qos.h                        |   5 +
 kernel/power/qos.c                            | 148 ++++++++++++++++++
 4 files changed, 179 insertions(+), 2 deletions(-)

Documentation/admin-guide/kernel-parameters.txt +23 -0

index 55ffc0f8858a..ef1f70de7d47 100644
@@ -2251,6 +2251,29 @@ Kernel parameters
 	icn=		[HW,ISDN]
 			Format: <io>[,<membase>[,<icn_id>[,<icn_id2>]]]
 
+	pm_qos_resume_latency_us=	[KNL,EARLY]
+			Format: <cpu-list>:<value>[,<cpu-list>:<value>...]
+
+			Establish per-CPU resume latency constraints. These constraints
+			are applied immediately upon CPU registration and persist
+			across CPU hotplug events.
+
+			For example:
+				"pm_qos_resume_latency_us=0:0,1-15:20"
+
+			This restricts CPU 0 to a 0us resume latency (effectively
+			forcing polling) and limits CPUs 1-15 to C-states with a
+			maximum exit latency of 20us. All other CPUs remain
+			unconstrained by this parameter.
+
+			Unlike the sysfs interface, which accepts the string "n/a" to
+			remove a constraint, this boot parameter strictly requires
+			integer values. To specify a 0us latency constraint (polling),
+			the integer 0 must be used.
+
+			NOTE: The parsing logic enforces a "First Match Wins" policy.
+			If a CPU is included in multiple specified ranges, the latency
+			value from the first matching entry takes precedence.
 
 	idle=		[X86,EARLY]
 			Format: idle=poll, idle=halt, idle=nomwait

drivers/base/cpu.c +3 -2

index 875abdc9942e..e522cf6e7312 100644
@@ -416,6 +416,7 @@ EXPORT_SYMBOL_GPL(cpu_subsys);
 int register_cpu(struct cpu *cpu, int num)
 {
 	int error;
+	s32 resume_latency;
 
 	cpu->node_id = cpu_to_node(num);
 	memset(&cpu->dev, 0x00, sizeof(struct device));

@@ -436,8 +437,8 @@ int register_cpu(struct cpu *cpu, int num)
 
 	per_cpu(cpu_sys_devices, num) = &cpu->dev;
 	register_cpu_under_node(num, cpu_to_node(num));
-	dev_pm_qos_expose_latency_limit(&cpu->dev,
-					PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
+	resume_latency = pm_qos_get_boot_cpu_latency_limit(num);
+	dev_pm_qos_expose_latency_limit(&cpu->dev, resume_latency);
 	set_cpu_enabled(num, true);
 
 	return 0;

include/linux/pm_qos.h +5 -0

index 6cea4455f867..556a7dff1419 100644
@@ -174,6 +174,7 @@ static inline s32 cpu_wakeup_latency_qos_limit(void)
 #ifdef CONFIG_PM
 enum pm_qos_flags_status __dev_pm_qos_flags(struct device *dev, s32 mask);
 enum pm_qos_flags_status dev_pm_qos_flags(struct device *dev, s32 mask);
+s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu);
 s32 __dev_pm_qos_resume_latency(struct device *dev);
 s32 dev_pm_qos_read_value(struct device *dev, enum dev_pm_qos_req_type type);
 int dev_pm_qos_add_request(struct device *dev, struct dev_pm_qos_request *req,

@@ -218,6 +219,10 @@ static inline s32 dev_pm_qos_raw_resume_latency(struct device *dev)
 		pm_qos_read_value(&dev->power.qos->resume_latency);
 }
 #else
+static inline s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)
+{
+	return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
+}
 static inline enum pm_qos_flags_status __dev_pm_qos_flags(struct device *dev,
 							  s32 mask)
 			{ return PM_QOS_FLAGS_UNDEFINED; }

kernel/power/qos.c +148 -0

166 lines not on this page · raw message

Signature · 1 line

2.51.0

kernel test robot

build bot

2026-03-08 01:03 UTC #1 lore raw

kernel test robot noticed the following build errors:

Hi Aaron,

kernel test robot noticed the following build errors:

[auto build test ERROR on driver-core/driver-core-testing]
[also build test ERROR on driver-core/driver-core-next driver-core/driver-core-linus rafael-pm/linux-next rafael-pm/bleeding-edge akpm-mm/mm-everything linus/master v7.0-rc2 next-20260306] [If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Aaron-Tomlin/PM-QoS-Introduce-boot-parameter-pm_qos_resume_latency_us/20260308-040909
base:   driver-core/driver-core-testing
patch link:    https://lore.kernel.org/r/20260307200736.4192234-1-atomlin%40atomlin.com
patch subject: [PATCH v3] PM: QoS: Introduce boot parameter pm_qos_resume_latency_us
config: alpha-allnoconfig (https://download.01.org/0day-ci/archive/20260308/[email protected]/config)
compiler: alpha-linux-gcc (GCC) 15.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260308/[email protected]/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <[email protected]>
| Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/

All errors (new ones prefixed by >>):

>> kernel/power/qos.c:344:5: error: redefinition of 'pm_qos_get_boot_cpu_latency_limit'

344 | s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from kernel/power/qos.c:25:
include/linux/pm_qos.h:222:19: note: previous definition of 'pm_qos_get_boot_cpu_latency_limit' with type 's32(unsigned int)' {aka 'int(unsigned int)'}
222 | static inline s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

vim +/pm_qos_get_boot_cpu_latency_limit +344 kernel/power/qos.c

332
333 /**
334 * pm_qos_get_boot_cpu_latency_limit - Get boot-time latency limit for a CPU.
335 * @cpu: Logical CPU number to check.
336 *
337 * Checks the read-only boot-time constraints list to see if a specific
338 * PM QoS latency override was requested for this CPU via the kernel
339 * command line.
340 *
341 * Return: The latency limit in microseconds if a constraint exists,
342 * or PM_QOS_RESUME_LATENCY_NO_CONSTRAINT if no boot override applies.
343 */

 > 344	s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)

345 {
346 struct pm_qos_boot_entry *entry;
347
348 if (list_empty(&pm_qos_boot_list))
349 return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
350
351 list_for_each_entry(entry, &pm_qos_boot_list, node) {
352 if (cpumask_test_cpu(cpu, &entry->mask))
353 return entry->latency;
354 }
355
356 return PM_QOS_RESUME_LATENCY_NO_CONSTRAINT;
357 }
358 EXPORT_SYMBOL_GPL(pm_qos_get_boot_cpu_latency_limit);
359

Signature · 2 lines

0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

kernel test robot

build bot

2026-03-08 04:26 UTC #2 lore raw

In reply to Aaron Tomlin

kernel test robot noticed the following build errors:

Hi Aaron,

kernel test robot noticed the following build errors:

[auto build test ERROR on driver-core/driver-core-testing]
[also build test ERROR on driver-core/driver-core-next driver-core/driver-core-linus rafael-pm/linux-next rafael-pm/bleeding-edge akpm-mm/mm-everything linus/master v6.16-rc1 next-20260306] [If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Aaron-Tomlin/PM-QoS-Introduce-boot-parameter-pm_qos_resume_latency_us/20260308-040909
base:   driver-core/driver-core-testing
patch link:    https://lore.kernel.org/r/20260307200736.4192234-1-atomlin%40atomlin.com
patch subject: [PATCH v3] PM: QoS: Introduce boot parameter pm_qos_resume_latency_us
config: i386-allnoconfig-bpf (https://download.01.org/0day-ci/archive/20260308/[email protected]/config)
compiler: gcc-14 (Debian 14.2.0-19) 14.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260308/[email protected]/reproduce)

All errors (new ones prefixed by >>):

>> kernel/power/qos.c:344:5: error: redefinition of 'pm_qos_get_boot_cpu_latency_limit'

344 | s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from kernel/power/qos.c:25:
./include/linux/pm_qos.h:222:19: note: previous definition of 'pm_qos_get_boot_cpu_latency_limit' with type 's32(unsigned int)' {aka 'int(unsigned int)'}
222 | static inline s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

vim +/pm_qos_get_boot_cpu_latency_limit +344 kernel/power/qos.c

 > 344	s32 pm_qos_get_boot_cpu_latency_limit(unsigned int cpu)

Signature · 2 lines

0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki