barrier: Add smp_cond_load_*_timeout()

[kernel-patches-daemon-bpf-rc]

Pull request for series with
subject: barrier: Add smp_cond_load_*_timeout()
version: 7
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679

[kernel-patches-daemon-bpf-rc]

Upstream branch: 48a97ff
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-review-bot]

commit 33ad33820ece5fb116e40213b1214560e03cfbdc
Author: Ankur Arora <[email protected]>

cpuidle/poll_state: Poll via smp_cond_load_relaxed_timeout()

The commit replaces the manual polling loop with
smp_cond_load_relaxed_timeout() to simplify the code while maintaining
the same polling behavior with intermittent timeout checks.

> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_state.c
> index 9b6d90a72..dc7f4b424 100644
> --- a/drivers/cpuidle/poll_state.c
> +++ b/drivers/cpuidle/poll_state.c
> @@ -8,35 +8,22 @@
>  #include <linux/sched/clock.h>
>  #include <linux/sched/idle.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();
> +	u64 time_end;
> +	u32 flags = 0;
>
>  	dev->poll_time_limit = false;
>
> +	time_end = local_clock_noinstr() + cpuidle_poll_time(drv, dev);
> +
>  	raw_local_irq_enable();
>  	if (!current_set_polling_and_test()) {
> -		unsigned int loop_count = 0;
> -		u64 limit;
> -
> -		limit = 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;
> -			}
> -		}
> +		flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
> +						      (VAL & _TIF_NEED_RESCHED),
> +						      (local_clock_noinstr() >= time_end));
> +		dev->poll_time_limit = !(flags & _TIF_NEED_RESCHED);
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Can this incorrectly clear poll_time_limit when a timeout occurs?

The old code set poll_time_limit = true specifically when the timeout
condition caused the loop exit. The new code bases poll_time_limit on
the final flag state after smp_cond_load_relaxed_timeout() returns.

Looking at smp_cond_load_relaxed_timeout() in
include/asm-generic/barrier.h, when the timeout expression becomes true,
it performs an additional READ_ONCE() before returning:

    if (time_check_expr) {
        VAL = READ_ONCE(*__PTR);
        break;
    }

This creates a window where _TIF_NEED_RESCHED could be set after the
timeout is detected but before the final read. In this case,
poll_time_limit would be set to false even though the loop exited due
to timeout, not because work became available.

The flag affects idle governor behavior in three places:

1. drivers/cpuidle/governors/haltpoll.c: If poll_time_limit is true,
   switch from polling to halt state

2. drivers/cpuidle/governors/menu.c: If poll_time_limit is true, use
   next_timer_ns instead of actual residency for idle prediction

3. drivers/cpuidle/governors/teo.c: If poll_time_limit is true, mark
   the wakeup as artificial rather than genuine

If poll_time_limit is incorrectly false after a timeout, the governors
would treat a timeout as a genuine wakeup with available work, affecting
their idle state selection heuristics.

>  	}
>  	raw_local_irq_disable();
>

---

AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

In-Reply-To-Subject: cpuidle/poll_state: Poll via smp_cond_load_relaxed_timeout()
CI run summary: https://github.com/kernel-patches/bpf-rc/actions/runs/18584379078

[kernel-patches-daemon-bpf-rc]

Forwarding comment 3414086411 via email
In-Reply-To: [email protected]
Patch: https://patchwork.kernel.org/project/netdevbpf/patch/[email protected]/

[kernel-patches-daemon-bpf-rc]

Upstream branch: 48a97ff
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

Upstream branch: 50de48a
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

Upstream branch: c67f4ae
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

Upstream branch: 7361c86
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

Upstream branch: 04a8995
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

Upstream branch: 04a8995
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679
version: 7

[kernel-patches-daemon-bpf-rc]

At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=1012679 irrelevant now. Closing PR.