Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

Rik van Riel
2018-03-25T20:16:06Z

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On Thu, 2018-03-22 at 18:09 +0100, Rafael J. Wysocki wrote:
>
> +++ linux-pm/drivers/cpuidle/poll_state.c
> @@ -10,6 +10,7 @@
> #include <linux/sched/idle.h>
>
> #define POLL_IDLE_TIME_LIMIT (TICK_NSEC / 16)
> +#define POLL_IDLE_COUNT 1000
>
> static int __cpuidle poll_idle(struct cpuidle_device *dev,
> struct cpuidle_driver *drv, int
> index)
> @@ -18,9 +19,14 @@ static int __cpuidle poll_idle(struct cp
>
> local_irq_enable();
> if (!current_set_polling_and_test()) {
> + unsigned int loop_count = 0;
> +
> while (!need_resched()) {
> cpu_relax();
> + if (loop_count++ < POLL_IDLE_COUNT)
> + continue;
>
> + loop_count = 0;
> if (local_clock() - time_start >
> POLL_IDLE_TIME_LIMIT)
> break;
> }

OK, I am still seeing a performance
degradation with the above, though
not throughout the entire workload.

It appears that making the idle loop
do anything besides cpu_relax() for
a significant amount of time slows
things down.

I plan to try two more things:

1) Disable polling on SMT systems, with
the idea that putting one thread to
sleep with monitor/mwait in C1 will
allow the other thread to run faster.

2) Insert more cpu_relax() calls into the
main loop, so the CPU core spends more
of its time in cpu_relax() and less
time doing other things:

static int __cpuidle poll_idle(struct cpuidle_device *dev,
struct cpuidle_driver *drv, int index)
{
u64 time_start = local_clock();

local_irq_enable();
if (!current_set_polling_and_test()) {
unsigned int loop_count = 0;

while (!need_resched()) {
cpu_relax();
cpu_relax();
cpu_relax();
cpu_relax();
cpu_relax();
cpu_relax();
cpu_relax();
cpu_relax();
if (loop_count++ < POLL_IDLE_COUNT)
continue;

loop_count = 0;
if (local_clock() - time_start > POLL_IDLE_TIME_LIMIT)
break;
}
}
current_clr_polling();

return index;
}

I will let you know how they perform.

--
All Rights Reversed.

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