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From: Patrick Bellasi <patrick.bellasi@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	Ingo Molnar <mingo@redhat.com>, Tejun Heo <tj@kernel.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Paul Turner <pjt@google.com>,
	Quentin Perret <quentin.perret@arm.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Juri Lelli <juri.lelli@redhat.com>, Todd Kjos <tkjos@google.com>,
	Joel Fernandes <joelaf@google.com>,
	Steve Muckle <smuckle@google.com>,
	Suren Baghdasaryan <surenb@google.com>
Subject: Re: [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups
Date: Wed, 12 Sep 2018 18:35:15 +0100
Message-ID: <20180912173515.GH1413@e110439-lin> (raw)
In-Reply-To: <20180912161218.GW24082@hirez.programming.kicks-ass.net>

On 12-Sep 18:12, Peter Zijlstra wrote:
> 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...
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
               This is actually a couple of function calls

> >               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.

Yes... that definitively needs an update.

> > 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;
> 	};
> };

That sounds really cool and scary at the same time :)

The v:10 requires that we never set SCHED_CAPACITY_SCALE>1024
or that we use it to track a percentage value (i.e. [0..100]).

One of the last patches introduces percentage values to userspace.
But, I was considering that in kernel space we should always track
full scale utilization values.

The c:(BITS_PER_LONG-10) restricts the range of concurrently active
SE refcounting the same clamp value. Which, for some 32bit systems is
only 4 milions among tasks and cgroups... maybe still reasonable...


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

Yes... that could work for the uclamp_map updates, but as I noted
above, I think I have other calls serialized by that lock... will look
better into what you suggest, thanks!


[...]

> > > 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.

Ok, will remove it.

> > > 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.

Ok

[...]

Cheers,
Patrick

-- 
#include <best/regards.h>

Patrick Bellasi

  reply index

Thread overview: 80+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-28 13:53 [PATCH v4 00/16] Add utilization clamping support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 01/16] sched/core: uclamp: extend sched_setattr to support utilization clamping Patrick Bellasi
2018-09-05 11:01   ` Juri Lelli
2018-08-28 13:53 ` [PATCH v4 02/16] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-05 10:45   ` Juri Lelli
2018-09-06 13:48     ` Patrick Bellasi
2018-09-06 14:13       ` Juri Lelli
2018-09-06  8:17   ` Juri Lelli
2018-09-06 14:00     ` Patrick Bellasi
2018-09-08 23:47   ` Suren Baghdasaryan
2018-09-12 10:32     ` Patrick Bellasi
2018-09-12 13:49   ` Peter Zijlstra
2018-09-12 15:56     ` Patrick Bellasi
2018-09-12 16:12       ` Peter Zijlstra
2018-09-12 17:35         ` Patrick Bellasi [this message]
2018-09-12 17:42           ` Peter Zijlstra
2018-09-12 17:52             ` Patrick Bellasi
2018-09-13 19:14               ` Peter Zijlstra
2018-09-14  8:51                 ` Patrick Bellasi
2018-09-12 16:24   ` Peter Zijlstra
2018-09-12 17:42     ` Patrick Bellasi
2018-09-13 19:20       ` Peter Zijlstra
2018-09-14  8:47         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 03/16] sched/core: uclamp: add CPU's clamp groups accounting Patrick Bellasi
2018-09-12 17:34   ` Peter Zijlstra
2018-09-12 17:44     ` Patrick Bellasi
2018-09-13 19:12   ` Peter Zijlstra
2018-09-14  9:07     ` Patrick Bellasi
2018-09-14 11:52       ` Peter Zijlstra
2018-09-14 13:41         ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 04/16] sched/core: uclamp: update CPU's refcount on clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 05/16] sched/core: uclamp: enforce last task UCLAMP_MAX Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 06/16] sched/cpufreq: uclamp: add utilization clamping for FAIR tasks Patrick Bellasi
2018-09-14  9:32   ` Peter Zijlstra
2018-09-14 13:19     ` Patrick Bellasi
2018-09-14 13:36       ` Peter Zijlstra
2018-09-14 13:57         ` Patrick Bellasi
2018-09-27 10:23           ` Quentin Perret
2018-08-28 13:53 ` [PATCH v4 07/16] sched/core: uclamp: extend cpu's cgroup controller Patrick Bellasi
2018-08-28 18:29   ` Randy Dunlap
2018-08-29  8:53     ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 08/16] sched/core: uclamp: propagate parent clamps Patrick Bellasi
2018-09-09  3:02   ` Suren Baghdasaryan
2018-09-12 12:51     ` Patrick Bellasi
2018-09-12 15:56       ` Suren Baghdasaryan
2018-09-11 15:18   ` Tejun Heo
2018-09-11 16:26     ` Patrick Bellasi
2018-09-11 16:28       ` Tejun Heo
2018-08-28 13:53 ` [PATCH v4 09/16] sched/core: uclamp: map TG's clamp values into CPU's clamp groups Patrick Bellasi
2018-09-09 18:52   ` Suren Baghdasaryan
2018-09-12 14:19     ` Patrick Bellasi
2018-09-12 15:53       ` Suren Baghdasaryan
2018-08-28 13:53 ` [PATCH v4 10/16] sched/core: uclamp: use TG's clamps to restrict Task's clamps Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 11/16] sched/core: uclamp: add system default clamps Patrick Bellasi
2018-09-10 16:20   ` Suren Baghdasaryan
2018-09-11 16:46     ` Patrick Bellasi
2018-09-11 19:25       ` Suren Baghdasaryan
2018-08-28 13:53 ` [PATCH v4 12/16] sched/core: uclamp: update CPU's refcount on TG's clamp changes Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 13/16] sched/core: uclamp: use percentage clamp values Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 14/16] sched/core: uclamp: request CAP_SYS_ADMIN by default Patrick Bellasi
2018-09-04 13:47   ` Juri Lelli
2018-09-06 14:40     ` Patrick Bellasi
2018-09-06 14:59       ` Juri Lelli
2018-09-06 17:21         ` Patrick Bellasi
2018-09-14 11:10       ` Peter Zijlstra
2018-09-14 14:07         ` Patrick Bellasi
2018-09-14 14:28           ` Peter Zijlstra
2018-09-17 12:27             ` Patrick Bellasi
2018-09-21  9:13               ` Peter Zijlstra
2018-09-24 15:14                 ` Patrick Bellasi
2018-09-24 15:56                   ` Peter Zijlstra
2018-09-24 17:23                     ` Patrick Bellasi
2018-09-24 16:26                   ` Peter Zijlstra
2018-09-24 17:19                     ` Patrick Bellasi
2018-09-25 15:49                   ` Peter Zijlstra
2018-09-26 10:43                     ` Patrick Bellasi
2018-09-27 10:00                     ` Quentin Perret
2018-09-26 17:51                 ` Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 15/16] sched/core: uclamp: add clamp group discretization support Patrick Bellasi
2018-08-28 13:53 ` [PATCH v4 16/16] sched/cpufreq: uclamp: add utilization clamping for RT tasks Patrick Bellasi

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