[PATCH v14 12/15] cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()

[bot+bpf-ci at kernel.org]

>     cpuidle/poll_state: Wait for need-resched via tif_need_resched_relaxed_wait()
>
>     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, since we want to minimize power consumption in idle, building
>     of cpuidle/poll_state.c continues 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()).

This isn't a bug, but the changelog names smp_cond_load_acquire_timeout() as
the backing primitive, while the code path looks like it uses the relaxed
variant:

include/linux/sched/idle.h:tif_need_resched_relaxed_wait() {
    return tif_bitset_relaxed_wait(TIF_NEED_RESCHED, timeout_ns);
}

include/linux/sched/idle.h:tif_bitset_relaxed_wait() {
    flags = smp_cond_load_relaxed_timeout(&current_thread_info()->flags,
                                          (VAL & BIT(tif)),
                                          local_clock_noinstr(),
                                          timeout_ns);
    ...
}

Should this reference smp_cond_load_relaxed_timeout() instead?


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