Date:   Wed, 12 Sep 2018 18:12:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....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>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups

On Wed, Sep 12, 2018 at 04:56:19PM +0100, Patrick Bellasi wrote:
> On 12-Sep 15:49, Peter Zijlstra wrote:
> > On Tue, Aug 28, 2018 at 02:53:10PM +0100, Patrick Bellasi wrote:

> > > +/**
> > > + * uclamp_map: reference counts a utilization "clamp value"
> > > + * @value:    the utilization "clamp value" required
> > > + * @se_count: the number of scheduling entities requiring the "clamp value"
> > > + * @se_lock:  serialize reference count updates by protecting se_count
> > 
> > Why do you have a spinlock to serialize a single value? Don't we have
> > atomics for that?
> 
> There are some code paths where it's used to protect clamp groups
> mapping and initialization, e.g.
> 
>       uclamp_group_get()
>           spin_lock()
>               // initialize clamp group (if required) and then...
>               se_count += 1
>           spin_unlock()
> 
> Almost all these paths are triggered from user-space and protected
> by a global uclamp_mutex, but fork/exit paths.
> 
> To serialize these paths I'm using the spinlock above, does it make
> sense ? Can we use the global uclamp_mutex on forks/exit too ?

OK, then your comment is misleading; it serializes both fields.

> One additional observations is that, if in the future we want to add a
> kernel space API, (e.g. driver asking for a new clamp value), maybe we
> will need to have a serialized non-sleeping uclamp_group_get() API ?

No idea; but if you want to go all fancy you can replace he whole
uclamp_map thing with something like:

struct uclamp_map {
	union {
		struct {
			unsigned long v : 10;
			unsigned long c : BITS_PER_LONG - 10;
		};
		atomic_long_t s;
	};
};

And use uclamp_map::c == 0 as unused (as per normal refcount
semantics) and atomic_long_cmpxchg() the whole thing using
uclamp_map::s.

> > > + *                          uclamp_maps is a matrix of
> > > + *          +------- UCLAMP_CNT by CONFIG_UCLAMP_GROUPS_COUNT+1 entries
> > > + *          |                                |
> > > + *          |                /---------------+---------------\
> > > + *          |               +------------+       +------------+
> > > + *          |  / UCLAMP_MIN | value      |       | value      |
> > > + *          |  |            | se_count   |...... | se_count   |
> > > + *          |  |            +------------+       +------------+
> > > + *          +--+            +------------+       +------------+
> > > + *             |            | value      |       | value      |
> > > + *             \ UCLAMP_MAX | se_count   |...... | se_count   |
> > > + *                          +-----^------+       +----^-------+
> > > + *                                |                   |
> > > + *                      uc_map =  +                   |
> > > + *                     &uclamp_maps[clamp_id][0]      +
> > > + *                                                clamp_value =
> > > + *                                       uc_map[group_id].value
> > > + */
> > > +static struct uclamp_map uclamp_maps[UCLAMP_CNT]
> > > +				    [CONFIG_UCLAMP_GROUPS_COUNT + 1]
> > > +				    ____cacheline_aligned_in_smp;
> > > +
> > 
> > I'm still completely confused by all this.
> > 
> > sizeof(uclamp_map) = 12
> > 
> > that array is 2*6=12 of those, so the whole thing is 144 bytes. which is
> > more than 2 (64 byte) cachelines.
> 
> This data structure is *not* used in the hot-path, that's why I did not
> care about fitting it exactly into few cache lines.
> 
> It's used to map a user-space "clamp value" into a kernel-space "clamp
> group" when user-space:
>  - changes a task specific clamp value
>  - changes a cgroup clamp value
>  - a task forks/exits
> 
> I assume we can consider all those as "slow" code paths, is that correct ?

Yep.

> > What's the purpose of that cacheline align statement?
> 
> In uclamp_maps, we still need to scan the array when a clamp value is
> changed from user-space, i.e. the cases reported above. Thus, that
> alignment is just to ensure that we minimize the number of cache lines
> used. Does that make sense ?
> 
> Maybe that alignment implicitly generated by the compiler ?

It is not, but if it really is a slow path, we shouldn't care about
alignment.

> > Note that without that apparently superfluous lock, it would be 8*12 =
> > 96 bytes, which is 1.5 lines and would indeed suggest you default to
> > GROUP_COUNT=7 by default to fill 2 lines.
> 
> Yes, will check better if we can count on just the uclamp_mutex

Well, if we don't care about performance (slow path) then keeping he
lock is fine, just the comment and alignment are misleading.

> > Why are the min and max things torn up like that?  I'm fairly sure I
> > asked some of that last time; but the above comments only try to explain
> > what, not why.
> 
> We use that organization to speedup scanning for clamp values of the
> same clamp_id. That's more important in the hot-path than above, where
> we need to scan struct uclamp_cpu when a new aggregated clamp value
> has to be computed. This is done in:
> 
>    [PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting
>    https://lore.kernel.org/lkml/20180828135324.21976-4-patrick.bellasi@arm.com/
> 
> Specifically:
> 
>    dequeue_task()
>      uclamp_cpu_put()
>        uclamp_cpu_put_id(clamp_id)
>          uclamp_cpu_update(clamp_id)
>             // Here we have an array scan by clamp_id
> 
> With the given data layout I reported above, when we update the
> min_clamp value (boost) we have all the data required in a single
> cache line.
> 
> If that makes sense, I can certainly improve the comment above to
> justify its layout.

OK, let me read on.

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