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Wysocki writes: > On Wed, Oct 29, 2025 at 5:42=E2=80=AFAM Ankur Arora wrote: >> >> >> Rafael J. Wysocki writes: >> >> > On Tue, Oct 28, 2025 at 6:32=E2=80=AFAM Ankur Arora 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 done only intermittently (once every >> >> POLL_IDLE_RELAX_COUNT iterations). 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. >> >> >> >> This is close to what smp_cond_load_relaxed_timeout() provides. So, >> >> restructure the loop and fold the loop condition and the timeout chec= k >> >> in smp_cond_load_relaxed_timeout(). >> > >> > Well, it is close, but is it close enough? >> >> I guess that's the question. >> >> >> Cc: "Rafael J. Wysocki" >> >> Cc: Daniel Lezcano >> >> Signed-off-by: Ankur Arora >> >> --- >> >> drivers/cpuidle/poll_state.c | 29 ++++++++--------------------- >> >> 1 file changed, 8 insertions(+), 21 deletions(-) >> >> >> >> diff --git a/drivers/cpuidle/poll_state.c b/drivers/cpuidle/poll_stat= e.c >> >> index 9b6d90a72601..dc7f4b424fec 100644 >> >> --- a/drivers/cpuidle/poll_state.c >> >> +++ b/drivers/cpuidle/poll_state.c >> >> @@ -8,35 +8,22 @@ >> >> #include >> >> #include >> >> >> >> -#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 =3D local_clock_noinstr(); >> >> + u64 time_end; >> >> + u32 flags =3D 0; >> >> >> >> dev->poll_time_limit =3D false; >> >> >> >> + time_end =3D local_clock_noinstr() + cpuidle_poll_time(drv, d= ev); >> > >> > Is there any particular reason for doing this unconditionally? If >> > not, then it looks like an arbitrary unrelated change to me. >> >> Agreed. Will fix. >> >> >> + >> >> raw_local_irq_enable(); >> >> if (!current_set_polling_and_test()) { >> >> - unsigned int loop_count =3D 0; >> >> - u64 limit; >> >> - >> >> - limit =3D cpuidle_poll_time(drv, dev); >> >> - >> >> - while (!need_resched()) { >> >> - cpu_relax(); >> >> - if (loop_count++ < POLL_IDLE_RELAX_COUNT) >> >> - continue; >> >> - >> >> - loop_count =3D 0; >> >> - if (local_clock_noinstr() - time_start > limi= t) { >> >> - dev->poll_time_limit =3D true; >> >> - break; >> >> - } >> >> - } >> >> + flags =3D smp_cond_load_relaxed_timeout(¤t_thre= ad_info()->flags, >> >> + (VAL & _TIF_NEE= D_RESCHED), >> >> + (local_clock_no= instr() >=3D time_end)); >> > >> > So my understanding of this is that it reduces duplication with some >> > other places doing similar things. Fair enough. >> > >> > However, since there is "timeout" in the name, I'd expect it to take >> > the timeout as an argument. >> >> The early versions did have a timeout but that complicated the >> implementation significantly. And the current users poll_idle(), >> rqspinlock don't need a precise timeout. >> >> smp_cond_load_relaxed_timed(), smp_cond_load_relaxed_timecheck()? >> >> The problem with all suffixes I can think of is that it makes the >> interface itself nonobvious. >> >> Possibly something with the sense of bail out might work. > > It basically has two conditions, one of which is checked in every step > of the internal loop and the other one is checked every > SMP_TIMEOUT_POLL_COUNT steps of it. That isn't particularly > straightforward IMV. Right. And that's similar to what poll_idle(). > Honestly, I prefer the existing code. It is much easier to follow and > I don't see why the new code would be better. Sorry. I don't think there's any problem with the current code. However, I'd like to add support for poll_idle() on arm64 (and maybe other platforms) where instead of spinning in a cpu_relax() loop, you wait on a cacheline. And that's what using something like smp_cond_load_relaxed_timeout() would enable. Something like the series here: https://lore.kernel.org/lkml/87wmaljd81.fsf@oracle.com/ (Sorry, should have mentioned this in the commit message.) -- ankur