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

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Message-ID: <20180405095858.GP3572@vireshk-i7>
Date:   Thu, 5 Apr 2018 15:28:58 +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 28-03-18, 10:07, Patrick Bellasi wrote:
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 2b124811947d..c840b0626735 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -201,43 +201,80 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
>  	return min(util, sg_cpu->max);
>  }

I like the general idea but there are few things which look incorrect
to me, even in the current code.

> -static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time, unsigned int flags)
> +/**
> + * sugov_set_iowait_boost updates the IO boost at each wakeup from IO.
> + * @sg_cpu: the sugov data for the CPU to boost
> + * @flags:  SCHED_CPUFREQ_IOWAIT if the task is waking up after an IO wait
> + *
> + * Each time a task wakes up after an IO operation, the CPU utilization can be
> + * boosted to a certain utilization which is doubled at each wakeup
> + * from IO, starting from the utilization of the minimum OPP to that of the
> + * maximum one.

You may also want to write here that the doubling of boost value is
restricted by rate_limit_us duration, its not that we double every
time this routine is called.

> + */
> +static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, unsigned int flags)
>  {

>  
> -		/* Clear iowait_boost if the CPU apprears to have been idle. */
> -		if (delta_ns > TICK_NSEC) {
> -			sg_cpu->iowait_boost = 0;
> -			sg_cpu->iowait_boost_pending = false;
> -		}

So this is the only difference in this routine, everything else is
re-arrangement IIUC.

There is a problem that I see in existing code as well as code after
this commit.

Consider this sequence of events on a platform where cpufreq policies
aren't shared, i.e. each CPU has his own policy.

sugov_set_iowait_boost() gets called multiple times for a CPU with
IOWAIT flag set that leads us to a big boost value, like fmax. The CPU
goes to idle then and the task wakes up after few ticks.  Because we
are first checking the IOWAIT flag in this routine, we will double the
iowait boost. Ideally, based on the TICK_NSEC logic we have, we should
have first set the iowait boost to 0 and then because the flag was
set, set the boost to fmin. So the order of this routine needs to get
fixed in the first patch.

The same problem can happen for cases where the policy is shared as
well, but chances are less.

> -static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, unsigned long *util,
> -			       unsigned long *max)
> +/**
> + * sugov_iowait_boost boosts a CPU after a wakeup from IO.
> + * @sg_cpu: the sugov data for the cpu to boost
> + * @time:   the update time from the caller
> + * @util:   the utilization to (eventually) boost
> + * @max:    the maximum value the utilization can be boosted to
> + *
> + * A CPU running a task which woken up after an IO operation can have its
> + * utilization boosted to speed up the completion of those IO operations.
> + * The IO boost value is increased each time a task wakes up from IO, in
> + * sugov_set_iowait_boost(), and it's instead decreased by this function,
> + * each time an increase has not been requested (!iowait_boost_pending).
> + *
> + * A CPU which also appears to have been idle for at least one tick has also
> + * its IO boost utilization reset.
> + *
> + * This mechanism is designed to boost high frequently IO waiting tasks, while
> + * being more conservative on tasks which does sporadic IO operations.
> + */
> +static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
> +			       unsigned long *util, unsigned long *max)
>  {
>  	unsigned int boost_util, boost_max;
>  
> -	if (!sg_cpu->iowait_boost)
> +	/* Clear 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 = 0;
> +			sg_cpu->iowait_boost_pending = false;
> +		}
>  		return;

This looks incorrect. I have read this 10 times and it looked
incorrect every single time :(

The "return" statement should be part of the if block itself ? Else we
will never boost.

> +	}
>  

Now we can reach here even on !sg_cpu->iowait_boost which wasn't the
case earlier. Though we will eventually return from the routine
without doing any damage, but we will waste some time running useless
if/else expressions.

Maybe still have something like

        if (!sg_cpu->iowait_boost)
                return;

??

-- 
viresh