[PATCH 3/3] drm/i915/gt: Improve "race-to-idle" at low frequencies

[Belgaumkar, Vinay]

On 11/17/2021 2:49 PM, Vinay Belgaumkar wrote:
> From: Chris Wilson <chris at chris-wilson.co.uk>
> 
> While the power consumption is proportional to the frequency, there is
> also a static draw for active gates. The longer we are able to powergate
> (rc6), the lower the static draw. Thus there is a sweetspot in the
> frequency/power curve where we run at higher frequency in order to sleep
> longer, aka race-to-idle. This is more evident at lower frequencies, so
> let's look to bump the frequency if we think we will benefit by sleeping
> longer at the higher frequency and so conserving power.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Vinay Belgaumkar <vinay.belgaumkar at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>

Data collected does show some power savings.

Reviewed-by: Vinay Belgaumkar <vinay.belgaumkar at intel.com>
> ---
>   drivers/gpu/drm/i915/gt/intel_rps.c | 31 ++++++++++++++++++++++++-----
>   1 file changed, 26 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gt/intel_rps.c b/drivers/gpu/drm/i915/gt/intel_rps.c
> index 3675ac93ded0..6af3231982af 100644
> --- a/drivers/gpu/drm/i915/gt/intel_rps.c
> +++ b/drivers/gpu/drm/i915/gt/intel_rps.c
> @@ -63,6 +63,22 @@ static void set(struct intel_uncore *uncore, i915_reg_t reg, u32 val)
>   	intel_uncore_write_fw(uncore, reg, val);
>   }
>   
> +static bool race_to_idle(struct intel_rps *rps, u64 busy, u64 dt)
> +{
> +	unsigned int this = rps->cur_freq;
> +	unsigned int next = rps->cur_freq + 1;
> +	u64 next_dt = next * max(busy, dt);
> +
> +	/*
> +	 * Compare estimated time spent in rc6 at the next power bin. If
> +	 * we expect to sleep longer than the estimated increased power
> +	 * cost of running at a higher frequency, it will be reduced power
> +	 * consumption overall.
> +	 */
> +	return (((next_dt - this * busy) >> 10) * this * this >
> +		((next_dt - next * busy) >> 10) * next * next);
> +}
> +
>   static void rps_timer(struct timer_list *t)
>   {
>   	struct intel_rps *rps = from_timer(rps, t, timer);
> @@ -133,7 +149,7 @@ static void rps_timer(struct timer_list *t)
>   			if (!max_busy[i])
>   				break;
>   
> -			busy += div_u64(max_busy[i], 1 << i);
> +			busy += max_busy[i] >> i;
>   		}
>   		GT_TRACE(rps_to_gt(rps),
>   			 "busy:%lld [%d%%], max:[%lld, %lld, %lld], interval:%d\n",
> @@ -141,13 +157,18 @@ static void rps_timer(struct timer_list *t)
>   			 max_busy[0], max_busy[1], max_busy[2],
>   			 rps->pm_interval);
>   
> -		if (100 * busy > rps->power.up_threshold * dt &&
> -		    rps->cur_freq < rps->max_freq_softlimit) {
> +		if (rps->cur_freq < rps->max_freq_softlimit &&
> +		    race_to_idle(rps, max_busy[0], dt)) {
> +			rps->pm_iir |= GEN6_PM_RP_UP_THRESHOLD;
> +			rps->pm_interval = 1;
> +			schedule_work(&rps->work);
> +		} else if (rps->cur_freq < rps->max_freq_softlimit &&
> +			   100 * busy > rps->power.up_threshold * dt) {
>   			rps->pm_iir |= GEN6_PM_RP_UP_THRESHOLD;
>   			rps->pm_interval = 1;
>   			schedule_work(&rps->work);
> -		} else if (100 * busy < rps->power.down_threshold * dt &&
> -			   rps->cur_freq > rps->min_freq_softlimit) {
> +		} else if (rps->cur_freq > rps->min_freq_softlimit &&
> +			   100 * busy < rps->power.down_threshold * dt) {
>   			rps->pm_iir |= GEN6_PM_RP_DOWN_THRESHOLD;
>   			rps->pm_interval = 1;
>   			schedule_work(&rps->work);
>