linux-kernel - Re: [RFC v3 1/5] sched/core: add capacity constraints to CPU controller

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Date:   Wed, 15 Mar 2017 11:20:20 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     "Joel Fernandes (Google)" <joel.opensrc@...il.com>
Cc:     Linux Kernel Mailing List <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>, Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC v3 1/5] sched/core: add capacity constraints to CPU
 controller

On 13-Mar 03:46, Joel Fernandes (Google) wrote:
> On Tue, Feb 28, 2017 at 6:38 AM, Patrick Bellasi
> <patrick.bellasi@....com> wrote:
> > The CPU CGroup controller allows to assign a specified (maximum)
> > bandwidth to tasks within a group, however it does not enforce any
> > constraint on how such bandwidth can be consumed.
> > With the integration of schedutil, the scheduler has now the proper
> > information about a task to select  the most suitable frequency to
> > satisfy tasks needs.
> [..]
> 
> > +static u64 cpu_capacity_min_read_u64(struct cgroup_subsys_state *css,
> > +                                    struct cftype *cft)
> > +{
> > +       struct task_group *tg;
> > +       u64 min_capacity;
> > +
> > +       rcu_read_lock();
> > +       tg = css_tg(css);
> > +       min_capacity = tg->cap_clamp[CAP_CLAMP_MIN];
> 
> Shouldn't the cap_clamp be accessed with READ_ONCE (and WRITE_ONCE in
> the write path) to avoid load-tearing?

tg->cap_clamp is an "unsigned int" and thus I would expect a single
memory access to write/read it, isn't it? I mean: I do not expect the
compiler "to mess" with these accesses.

However, if your concerns are more about overlapping read/write for the
same capacity from different threads, then perhaps we should better
use a mutex to serialize these two functions... not entirely convinced...

> Thanks,
> Joel

-- 
#include <best/regards.h>

Patrick Bellasi