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

[Patrick Bellasi]

On 15-Mar 09:10, Paul E. McKenney wrote:
> On Wed, Mar 15, 2017 at 06:20:28AM -0700, Joel Fernandes wrote:
> > On Wed, Mar 15, 2017 at 4:20 AM, Patrick Bellasi
> > <patrick.bellasi@xxxxxxx> wrote:
> > > On 13-Mar 03:46, Joel Fernandes (Google) wrote:
> > >> On Tue, Feb 28, 2017 at 6:38 AM, Patrick Bellasi
> > >> <patrick.bellasi@xxxxxxx> 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.
> > 
> > This depends on compiler and arch. I'm not sure if its in practice
> > these days an issue, but see section on 'load tearing' in
> > Documentation/memory-barriers.txt . If compiler decided to break down
> > the access to multiple accesses due to some reason, then might be a
> > problem.
> 
> The compiler might also be able to inline cpu_capacity_min_read_u64()
> fuse the load from tg->cap_clamp[CAP_CLAMP_MIN] with other accesses.
> If min_capacity is used several times in the ensuing code, the compiler
> could reload multiple times from tg->cap_clamp[CAP_CLAMP_MIN], which at
> best might be a bit confusing.

That's actually an interesting case, however I don't think it applies
in this case since cpu_capacity_min_read_u64() is called only via
a function poninter and thus it will never be inlined. isn't it?

> > Adding Paul for his expert opinion on the matter ;)
> 
> My personal approach is to use READ_ONCE() and WRITE_ONCE() unless
> I can absolutely prove that the compiler cannot do any destructive
> optimizations.  And I not-infrequently find unsuspected opportunities
> for destructive optimization in my own code.  Your mileage may vary.  ;-)

I guess here the main concern from Joel is that such a pattern:

   u64 var = unsigned_int_value_from_memory;

can result is a couple of "load from memory" operations.

In that case a similar:

  unsigned_int_left_value = new_unsigned_int_value;

executed on a different thread can overlap with the previous memory
read operations and ending up in "var" containing a not consistent
value.

Question is: can this really happen, given the data types in use?


> 							Thanx, Paul

Thanks! ;-)

-- 
#include <best/regards.h>

Patrick Bellasi