linux-kernel - Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll

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Date:   Sun, 25 Mar 2018 16:15:52 -0400
From:   Rik van Riel <riel@...riel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Ilsche <thomas.ilsche@...dresden.de>,
        Doug Smythies <dsmythies@...us.net>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Mike Galbraith <mgalbraith@...e.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

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.

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