Re: [PATCH v6] cpufreq: schedutil: Make iowait boost more energy efficient

[Rafael J. Wysocki]

On Saturday, July 22, 2017 12:47:53 AM Joel Fernandes wrote:
> Currently the iowait_boost feature in schedutil makes the frequency go to max
> on iowait wakeups.  This feature was added to handle a case that Peter
> described where the throughput of operations involving continuous I/O requests
> [1] is reduced due to running at a lower frequency, however the lower
> throughput itself causes utilization to be low and hence causing frequency to
> be low hence its "stuck".
> 
> Instead of going to max, its also possible to achieve the same effect by
> ramping up to max if there are repeated in_iowait wakeups happening. This patch
> is an attempt to do that. We start from a lower frequency (policy->min)
> and double the boost for every consecutive iowait update until we reach the
> maximum iowait boost frequency (iowait_boost_max).
> 
> I ran a synthetic test (continuous O_DIRECT writes in a loop) on an x86 machine
> with intel_pstate in passive mode using schedutil. In this test the iowait_boost
> value ramped from 800MHz to 4GHz in 60ms. The patch achieves the desired improved
> throughput as the existing behavior.
> 
> Also while at it, make iowait_boost and iowait_boost_max as unsigned int since
> its unit is kHz and this is consistent with struct cpufreq_policy.
> 
> [1] https://patchwork.kernel.org/patch/9735885/
> 
> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> Cc: Len Brown <lenb@xxxxxxxxxx>
> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Suggested-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
> ---
> Viresh, made slight modifications to the last approach we agreed on using, but
> nothing we didn't already discuss. I also dropped the RFC tag since I think
> this is increasingly now becoming final (or has become final if no one else has
> any other objection).
> 
>  kernel/sched/cpufreq_schedutil.c | 37 +++++++++++++++++++++++++++++++------
>  1 file changed, 31 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 622eed1b7658..0c0b6c8c15fc 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -53,6 +53,7 @@ struct sugov_cpu {
>  	struct update_util_data update_util;
>  	struct sugov_policy *sg_policy;
>  
> +	bool iowait_boost_pending;
>  	unsigned long iowait_boost;
>  	unsigned long iowait_boost_max;
>  	u64 last_update;
> @@ -172,30 +173,53 @@ static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>  				   unsigned int flags)
>  {
>  	if (flags & SCHED_CPUFREQ_IOWAIT) {
> -		sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> +		if (sg_cpu->iowait_boost_pending)
> +			return;
> +
> +		sg_cpu->iowait_boost_pending = true;
> +
> +		if (sg_cpu->iowait_boost) {
> +			sg_cpu->iowait_boost = min(sg_cpu->iowait_boost << 1,
> +						   sg_cpu->iowait_boost_max);

I would do

			sg_cpu->iowait_boost <<= 1;
			if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
				sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;

as that's easeir to read.

The rest of the patch is fine by me.

Thanks,
Rafael