The inner loop in poll_idle() polls over the thread_info flags,
waiting to see if the thread has TIF_NEED_RESCHED set. The loop
exits once the condition is met, or if the poll time limit has
been exceeded.
To minimize the number of instructions executed in each iteration,
the time check is rate-limited. In addition, each loop iteration
executes cpu_relax() which on certain platforms provides a hint to
the pipeline that the loop busy-waits, allowing the processor to
reduce power consumption.
Switch over to tif_need_resched_relaxed_wait() instead, since that
provides exactly that.
However, given that when running in idle we want to minimize our power
consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
serves as an indicator that the platform supports an optimized version
of tif_need_resched_relaxed_wait() (via
smp_cond_load_acquire_timeout()).
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: linux-pm@vger.kernel.org
Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
Notes:
- use tif_need_resched_relaxed_wait() instead of
smp_cond_load_relaxed_timeout()
drivers/cpuidle/poll_state.c | 27 +++++----------------------
1 file changed, 5 insertions(+), 22 deletions(-)
diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
index c7524e4c522a..20136b3a08c2 100644
--- a/drivers/cpuidle/poll_state.c
+++ b/drivers/cpuidle/poll_state.c
@@ -6,41 +6,24 @@
#include <linux/cpuidle.h>
#include <linux/export.h>
#include <linux/irqflags.h>
-#include <linux/sched.h>
-#include <linux/sched/clock.h>
#include <linux/sched/idle.h>
#include <linux/sprintf.h>
#include <linux/types.h>
-#define POLL_IDLE_RELAX_COUNT 200
-
static int __cpuidle poll_idle(struct cpuidle_device *dev,
struct cpuidle_driver *drv, int index)
{
- u64 time_start;
-
- time_start = local_clock_noinstr();
-
dev->poll_time_limit = false;
raw_local_irq_enable();
if (!current_set_polling_and_test()) {
- unsigned int loop_count = 0;
- u64 limit;
+ s64 limit;
+ bool nr_set;
- limit = cpuidle_poll_time(drv, dev);
+ limit = (s64)cpuidle_poll_time(drv, dev);
- while (!need_resched()) {
- cpu_relax();
- if (loop_count++ < POLL_IDLE_RELAX_COUNT)
- continue;
-
- loop_count = 0;
- if (local_clock_noinstr() - time_start > limit) {
- dev->poll_time_limit = true;
- break;
- }
- }
+ nr_set = tif_need_resched_relaxed_wait(limit);
+ dev->poll_time_limit = !nr_set;
}
raw_local_irq_disable();
--
2.31.1Hi Ankur, kernel test robot noticed the following build warnings: [auto build test WARNING on bpf-next/master] [also build test WARNING on bpf/master linus/master v6.19-rc1 next-20251215] [cannot apply to arm64/for-next/core bpf-next/net arnd-asm-generic/master] [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/Ankur-Arora/asm-generic-barrier-Add-smp_cond_load_relaxed_timeout/20251215-130116 base: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master patch link: https://lore.kernel.org/r/20251215044919.460086-13-ankur.a.arora%40oracle.com patch subject: [PATCH v8 12/12] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait() config: x86_64-randconfig-005-20251215 (https://download.01.org/0day-ci/archive/20251216/202512160519.pKMkOXsP-lkp@intel.com/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/20251216/202512160519.pKMkOXsP-lkp@intel.com/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 <lkp@intel.com> | Closes: https://lore.kernel.org/oe-kbuild-all/202512160519.pKMkOXsP-lkp@intel.com/ All warnings (new ones prefixed by >>): >> vmlinux.o: warning: objtool: poll_idle+0x3b: call to tif_need_resched_relaxed_wait() leaves .noinstr.text section -- 0-DAY CI Kernel Test Service https://github.com/intel/lkp-tests/wiki
On Mon, Dec 15, 2025 at 5:55 AM Ankur Arora <ankur.a.arora@oracle.com> wrote:
>
> The inner loop in poll_idle() polls over the thread_info flags,
> waiting to see if the thread has TIF_NEED_RESCHED set. The loop
> exits once the condition is met, or if the poll time limit has
> been exceeded.
>
> To minimize the number of instructions executed in each iteration,
> the time check is rate-limited. In addition, each loop iteration
> executes cpu_relax() which on certain platforms provides a hint to
> the pipeline that the loop busy-waits, allowing the processor to
> reduce power consumption.
>
> Switch over to tif_need_resched_relaxed_wait() instead, since that
> provides exactly that.
>
> However, given that when running in idle we want to minimize our power
> consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
> serves as an indicator that the platform supports an optimized version
> of tif_need_resched_relaxed_wait() (via
> smp_cond_load_acquire_timeout()).
>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> Cc: linux-pm@vger.kernel.org
> Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>
> Notes:
> - use tif_need_resched_relaxed_wait() instead of
> smp_cond_load_relaxed_timeout()
>
> drivers/cpuidle/poll_state.c | 27 +++++----------------------
> 1 file changed, 5 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index c7524e4c522a..20136b3a08c2 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -6,41 +6,24 @@
> #include <linux/cpuidle.h>
> #include <linux/export.h>
> #include <linux/irqflags.h>
> -#include <linux/sched.h>
> -#include <linux/sched/clock.h>
> #include <linux/sched/idle.h>
> #include <linux/sprintf.h>
> #include <linux/types.h>
>
> -#define POLL_IDLE_RELAX_COUNT 200
> -
> static int __cpuidle poll_idle(struct cpuidle_device *dev,
> struct cpuidle_driver *drv, int index)
> {
> - u64 time_start;
> -
> - time_start = local_clock_noinstr();
> -
> dev->poll_time_limit = false;
>
> raw_local_irq_enable();
> if (!current_set_polling_and_test()) {
> - unsigned int loop_count = 0;
> - u64 limit;
> + s64 limit;
> + bool nr_set;
It doesn't look like the nr_set variable is really needed.
>
> - limit = cpuidle_poll_time(drv, dev);
> + limit = (s64)cpuidle_poll_time(drv, dev);
Is the explicit cast needed to suppress a warning? If not, I'd drop it.
>
> - while (!need_resched()) {
> - cpu_relax();
> - if (loop_count++ < POLL_IDLE_RELAX_COUNT)
> - continue;
> -
> - loop_count = 0;
> - if (local_clock_noinstr() - time_start > limit) {
> - dev->poll_time_limit = true;
> - break;
> - }
> - }
> + nr_set = tif_need_resched_relaxed_wait(limit);
> + dev->poll_time_limit = !nr_set;
This can be
dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);
> }
> raw_local_irq_disable();
>
> --
Rafael J. Wysocki <rafael@kernel.org> writes:
> On Mon, Dec 15, 2025 at 5:55 AM Ankur Arora <ankur.a.arora@oracle.com> wrote:
>>
>> The inner loop in poll_idle() polls over the thread_info flags,
>> waiting to see if the thread has TIF_NEED_RESCHED set. The loop
>> exits once the condition is met, or if the poll time limit has
>> been exceeded.
>>
>> To minimize the number of instructions executed in each iteration,
>> the time check is rate-limited. In addition, each loop iteration
>> executes cpu_relax() which on certain platforms provides a hint to
>> the pipeline that the loop busy-waits, allowing the processor to
>> reduce power consumption.
>>
>> Switch over to tif_need_resched_relaxed_wait() instead, since that
>> provides exactly that.
>>
>> However, given that when running in idle we want to minimize our power
>> consumption, continue to depend on CONFIG_ARCH_HAS_CPU_RELAX as that
>> serves as an indicator that the platform supports an optimized version
>> of tif_need_resched_relaxed_wait() (via
>> smp_cond_load_acquire_timeout()).
>>
>> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
>> Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
>> Cc: linux-pm@vger.kernel.org
>> Suggested-by: "Rafael J. Wysocki" <rafael@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>
>> Notes:
>> - use tif_need_resched_relaxed_wait() instead of
>> smp_cond_load_relaxed_timeout()
>>
>> drivers/cpuidle/poll_state.c | 27 +++++----------------------
>> 1 file changed, 5 insertions(+), 22 deletions(-)
>>
>> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
>> index c7524e4c522a..20136b3a08c2 100644
>> --- a/drivers/cpuidle/poll_state.c
>> +++ b/drivers/cpuidle/poll_state.c
>> @@ -6,41 +6,24 @@
>> #include <linux/cpuidle.h>
>> #include <linux/export.h>
>> #include <linux/irqflags.h>
>> -#include <linux/sched.h>
>> -#include <linux/sched/clock.h>
>> #include <linux/sched/idle.h>
>> #include <linux/sprintf.h>
>> #include <linux/types.h>
>>
>> -#define POLL_IDLE_RELAX_COUNT 200
>> -
>> static int __cpuidle poll_idle(struct cpuidle_device *dev,
>> struct cpuidle_driver *drv, int index)
>> {
>> - u64 time_start;
>> -
>> - time_start = local_clock_noinstr();
>> -
>> dev->poll_time_limit = false;
>>
>> raw_local_irq_enable();
>> if (!current_set_polling_and_test()) {
>> - unsigned int loop_count = 0;
>> - u64 limit;
>> + s64 limit;
>> + bool nr_set;
>
> It doesn't look like the nr_set variable is really needed.
>
>>
>> - limit = cpuidle_poll_time(drv, dev);
>> + limit = (s64)cpuidle_poll_time(drv, dev);
>
> Is the explicit cast needed to suppress a warning? If not, I'd drop it.
Ack.
>>
>> - while (!need_resched()) {
>> - cpu_relax();
>> - if (loop_count++ < POLL_IDLE_RELAX_COUNT)
>> - continue;
>> -
>> - loop_count = 0;
>> - if (local_clock_noinstr() - time_start > limit) {
>> - dev->poll_time_limit = true;
>> - break;
>> - }
>> - }
>> + nr_set = tif_need_resched_relaxed_wait(limit);
>> + dev->poll_time_limit = !nr_set;
>
> This can be
>
> dev->poll_time_limit = !tif_need_resched_relaxed_wait(limit);
Yeah that looks pretty clear.
Thanks!
--
ankur
© 2016 - 2025 Red Hat, Inc.