Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

From: Qais Yousef <qais.yousef@arm.com>
To: Quentin Perret <qperret@google.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jonathan Corbet <corbet@lwn.net>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Patrick Bellasi <patrick.bellasi@matbug.net>,
	Pavan Kondeti <pkondeti@codeaurora.org>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value
Date: Mon, 20 Apr 2020 16:08:40 +0100	[thread overview]
Message-ID: <20200420150839.ysh3ijfjftyzg7jw@e107158-lin.cambridge.arm.com> (raw)
In-Reply-To: <20200415101122.GA14447@google.com>

Hi Quentin

On 04/15/20 11:11, Quentin Perret wrote:
> Hi Qais,
> 
> On Friday 03 Apr 2020 at 13:30:19 (+0100), Qais Yousef wrote:
> <snip>
> > +	/*
> > +	 * The new value will be applied to all RT tasks the next time they
> > +	 * wakeup, assuming the task is using the system default and not a user
> > +	 * specified value. In the latter we shall leave the value as the user
> > +	 * requested.
> > +	 */
> >  	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max ||
> >  	    sysctl_sched_uclamp_util_max > SCHED_CAPACITY_SCALE) {
> >  		result = -EINVAL;
> >  		goto undo;
> >  	}
> >  
> > +	if (sysctl_sched_rt_default_uclamp_util_min > SCHED_CAPACITY_SCALE) {
> > +		result = -EINVAL;
> > +		goto undo;
> > +	}
> 
> Hmm, checking:
> 
> 	if (sysctl_sched_rt_default_uclamp_util_min > sysctl_sched_uclamp_util_min)
> 
> would probably make sense too, but then that would make writing in
> sysctl_sched_uclamp_util_min cumbersome for sysadmins as they'd need to
> lower the rt default first. Is that the reason for checking against
> SCHED_CAPACITY_SCALE? That might deserve a comment or something.

There's no need for that extra diff. That constraint will be applied
automatically when calculating the effective value.

The check for SCHED_CAPACITY_SCALE is a a range check. The possible value is
[0:SCHED_CAPACITY_SCALE].

Does this answer your question? I could add a comment that all the uclamp
sysctls need to be within this range.

> 
> <snip>
> > @@ -1241,9 +1293,13 @@ static void uclamp_fork(struct task_struct *p)
> >  	for_each_clamp_id(clamp_id) {
> >  		unsigned int clamp_value = uclamp_none(clamp_id);
> >  
> > -		/* By default, RT tasks always get 100% boost */
> > +		/*
> > +		 * By default, RT tasks always get 100% boost, which the admins
> > +		 * are allowed to change via
> > +		 * sysctl_sched_rt_default_uclamp_util_min knob.
> > +		 */
> >  		if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> > -			clamp_value = uclamp_none(UCLAMP_MAX);
> > +			clamp_value = sysctl_sched_rt_default_uclamp_util_min;
> >  
> >  		uclamp_se_set(&p->uclamp_req[clamp_id], clamp_value, false);
> >  	}
> 
> And that, as per 20200414161320.251897-1-qperret@google.com, should not
> be there :)

Yep saw it. Thanks for fixing it!

> 
> Otherwise the patch pretty looks good to me!

Cheers

--
Qais Yousef