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

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Date:   Tue, 10 Apr 2018 12:57:56 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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-Apr 16:26, Viresh Kumar wrote:
> 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.

Yes, that's possible... althought it's in the same scope of the
optimization you suggested for the next function, bail out ASAP to
avoid other branches.

> > 			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.

Yes, maybe... I think we still get the benefit to consolidate all the
iowait boost code into just these two function, while instead
currently the reset is in sugov_next_freq_shared().

Moreoverer, IMO it's easy to read and with a documentation more
aligned... but, lemme post a v2 and then we will see if it still makes
sense or I should just drop it.

-- 
#include <best/regards.h>

Patrick Bellasi