linux-kernel - Re: [PATCH] cpufreq/schedutil: Cleanup and document iowait boost

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Date:   Tue, 10 Apr 2018 16:26:06 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...roid.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>
Subject: Re: [PATCH] cpufreq/schedutil: Cleanup and document iowait boost

On 10-04-18, 11:43, Patrick Bellasi wrote:
> On 05-Apr 15:28, Viresh Kumar wrote:
> What about this new version for the two functions,
> just compile tested:
> 
> ---8<---
> 
> static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
> 				   unsigned int flags)
> {
> 	bool iowait = flags & SCHED_CPUFREQ_IOWAIT;
> 
> 	/* Reset boost if the CPU appears to have been idle enough */
> 	if (sg_cpu->iowait_boost) {
> 		s64 delta_ns = time - sg_cpu->last_update;
> 
> 		if (delta_ns > TICK_NSEC) {
> 			sg_cpu->iowait_boost = iowait
> 				? sg_cpu->sg_policy->policy->min : 0;

Yeah, I see you are trying to optimize it a bit here but this makes
things more confusing I would say :)

I would just set iowait_boost to 0 and drop the return; from below and
let the code fall through and reach the end of this routine.

> 			sg_cpu->iowait_boost_pending = iowait;
> 			return;
> 		}
> 	}
> 
> 	/* Boost only tasks waking up after IO */
> 	if (!iowait)
> 		return;
> 
> 	/* Ensure IO boost doubles only one time at each frequency increase */
> 	if (sg_cpu->iowait_boost_pending)
> 		return;
> 	sg_cpu->iowait_boost_pending = true;
> 
> 	/* Double the IO boost at each frequency increase */
> 	if (sg_cpu->iowait_boost) {
> 		sg_cpu->iowait_boost <<= 1;
> 		if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
> 			sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
> 		return;
> 	}
> 
> 	/* At first wakeup after IO, start with minimum boost */
> 	sg_cpu->iowait_boost = sg_cpu->sg_policy->policy->min;
> }
> 
> static void sugov_iowait_boost(struct sugov_cpu *sg_cpu,
> 			       unsigned long *util, unsigned long *max)
> {
> 	unsigned int boost_util, boost_max;
> 
>         /* No IOWait boost active */
>         if (!sg_cpu->iowait_boost)
>                 return;
> 
> 	/* An IO waiting task has just woken up, use the boost value */
> 	if (sg_cpu->iowait_boost_pending) {
> 		sg_cpu->iowait_boost_pending = false;
> 	} else {
> 		/* Reduce the boost value otherwise */
> 		sg_cpu->iowait_boost >>= 1;
> 		if (sg_cpu->iowait_boost < sg_cpu->sg_policy->policy->min) {
> 			sg_cpu->iowait_boost = 0;
> 			return;
> 		}
> 	}
> 
> 	boost_util = sg_cpu->iowait_boost;
> 	boost_max = sg_cpu->iowait_boost_max;
> 
> 	/*
> 	 * A CPU is boosted only if its current utilization is smaller then
> 	 * the current IO boost level.
> 	 */
> 	if (*util * boost_max < *max * boost_util) {
> 		*util = boost_util;
> 		*max = boost_max;
> 	}
> }

So this is quite different than what you proposed, it is only fixing
the existing problem which I pointed out to. Looks fine, not much
changed really from the current state of code.

-- 
viresh