linux-kernel - Re: The tick is active on idle adaptive-tick CPUs when /dev/cpu_dma

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Date:   Sat, 6 Jul 2019 15:02:11 +0200
From:   Thomas Lindroth <thomas.lindroth@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: The tick is active on idle adaptive-tick CPUs when
 /dev/cpu_dma_latency is used

On 7/6/19 1:06 PM, Rafael J. Wysocki wrote:
> The patch is below, but note that it adds the tick stopping overhead to the idle loop
> for CPUs that are not adaptive-tick and when PM QoS latency constraints are used
> which is not desirable in general.
> 
> Please test it, but as I said above, the real solution appears to be to treat adaptive-tick
> CPUs in a special way in the idle loop.
> 
> ---
>   drivers/cpuidle/governors/menu.c |   16 +++++-----------
>   1 file changed, 5 insertions(+), 11 deletions(-)
> 
> Index: linux-pm/drivers/cpuidle/governors/menu.c
> ===================================================================
> --- linux-pm.orig/drivers/cpuidle/governors/menu.c
> +++ linux-pm/drivers/cpuidle/governors/menu.c
> @@ -302,9 +302,10 @@ static int menu_select(struct cpuidle_dr
>   	     !drv->states[0].disabled && !dev->states_usage[0].disable)) {
>   		/*
>   		 * In this case state[0] will be used no matter what, so return
> -		 * it right away and keep the tick running.
> +		 * it right away and keep the tick running if state[0] is a
> +		 * polling one.
>   		 */
> -		*stop_tick = false;
> +		*stop_tick = !!(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
>   		return 0;
>   	}
>   
> @@ -395,16 +396,9 @@ static int menu_select(struct cpuidle_dr
>   
>   			return idx;
>   		}
> -		if (s->exit_latency > latency_req) {
> -			/*
> -			 * If we break out of the loop for latency reasons, use
> -			 * the target residency of the selected state as the
> -			 * expected idle duration so that the tick is retained
> -			 * as long as that target residency is low enough.
> -			 */
> -			predicted_us = drv->states[idx].target_residency;
> +		if (s->exit_latency > latency_req)
>   			break;
> -		}
> +
>   		idx = i;
>   	}

I tested the patch and it appears to work. Idle CPUs now have ticks disabled even
when /dev/cpu_dma_latency is used.

I also want to thank you for your work on the idle loop redesign. Overall it works
much better than before. I used to have a problem where idle CPUs would end up
doing C0 polling for a long time resulting in a big performance drop on the HT
sibling. When benchmarking I always had to offline siblings to get consistent
results. That problem was fixed in the redesign.