linux-kernel - Re: [PATCH v11 1/5] sched/core: uclamp: Extend CPU's cgroup controller

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Date:   Mon, 8 Jul 2019 12:08:41 +0100
From:   Quentin Perret <quentin.perret@....com>
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>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alessio Balsini <balsini@...roid.com>
Subject: Re: [PATCH v11 1/5] sched/core: uclamp: Extend CPU's cgroup
 controller

Hi Patrick,

On Monday 08 Jul 2019 at 09:43:53 (+0100), Patrick Bellasi wrote:
> +static inline int uclamp_scale_from_percent(char *buf, u64 *value)
> +{
> +	*value = SCHED_CAPACITY_SCALE;
> +
> +	buf = strim(buf);
> +	if (strncmp("max", buf, 4)) {
> +		s64 percent;
> +		int ret;
> +
> +		ret = cgroup_parse_float(buf, 2, &percent);
> +		if (ret)
> +			return ret;
> +
> +		percent <<= SCHED_CAPACITY_SHIFT;
> +		*value = DIV_ROUND_CLOSEST_ULL(percent, 10000);
> +	}
> +
> +	return 0;
> +}
> +
> +static inline u64 uclamp_percent_from_scale(u64 value)
> +{
> +	return DIV_ROUND_CLOSEST_ULL(value * 10000, SCHED_CAPACITY_SCALE);
> +}

FWIW, I tried the patches and realized these conversions result in a
'funny' behaviour from a user's perspective. Things like this happen:

   $ echo 20 > cpu.uclamp.min
   $ cat cpu.uclamp.min
   20.2
   $ echo 20.2 > cpu.uclamp.min
   $ cat cpu.uclamp.min
   20.21

Having looked at the code, I get why this is happening, but I'm not sure
if a random user will. It's not an issue per se, but it's just a bit
weird.

I guess one way to fix this would be to revert back to having a
1024-scale for the cgroup interface too ... Though I understand Tejun
wanted % for consistency with other things.

So, I'm not sure if this is still up for discussion, but in any case I
wanted to say I support your original idea of using a 1024-scale for the
cgroups interface, since that would solve the 'issue' above and keeps
things consistent with the per-task API too.

Thanks,
Quentin