From: Peter Zijlstra <peterz@infradead.org>
To: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Tejun Heo <tj@kernel.org>,
"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
Viresh Kumar <viresh.kumar@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 v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes
Date: Wed, 23 Jan 2019 10:16:34 +0100 [thread overview]
Message-ID: <20190123091634.GT27931@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190122153315.dhjl67sgpu74hmqv@e110439-lin>
On Tue, Jan 22, 2019 at 03:33:15PM +0000, Patrick Bellasi wrote:
> On 22-Jan 15:57, Peter Zijlstra wrote:
> > On Tue, Jan 22, 2019 at 02:01:15PM +0000, Patrick Bellasi wrote:
> > > Yes, I would say we have two options:
> > >
> > > 1) SCHED_FLAG_KEEP_POLICY enforces all the scheduling class specific
> > > attributes, but cross class attributes (e.g. uclamp)
> > >
> > > 2) add SCHED_KEEP_NICE, SCHED_KEEP_PRIO, and SCED_KEEP_PARAMS
> > > and use them in the if conditions in D)
> >
> > So the current KEEP_POLICY basically provides sched_setparam(), and
>
> But it's not exposed user-space.
Correct; not until your first patch indeed.
> > given we have that as a syscall, that is supposedly a useful
> > functionality.
>
> For uclamp is definitively useful to change clamps without the need to
> read beforehand the current policy params and use them in a following
> set syscall... which is racy pattern.
Right; but my argument was mostly that if sched_setparam() is a useful
interface, a 'pure' KEEP_POLICY would be too and your (1) looses that.
> > And I suppose the UTIL_CLAMP is !KEEP_UTIL; we could go either way
> > around with that flag.
>
> What about getting rid of the racy case above by exposing userspace
> only the new UTIL_CLAMP and, on:
>
> sched_setscheduler(flags: UTIL_CLAMP)
>
> we enforce the other two flags from the syscall:
>
> ---8<---
> SYSCALL_DEFINE3(sched_setattr)
> if (attr.sched_flags & SCHED_FLAG_KEEP_POLICY) {
> attr.sched_policy = SETPARAM_POLICY;
> attr.sched_flags |= (KEEP_POLICY|KEEP_PARAMS);
> }
> ---8<---
>
> This will not make possible to change class and set flags in one go,
> but honestly that's likely a very limited use-case, isn't it ?
So I must admit to not seeing much use for sched_setparam() (and its
equivalents) myself, but given it is an existing interface, I also think
it would be nice to cover that functionality in the sched_setattr()
call.
That is; I know of userspace priority-ceiling implementations using
sched_setparam(), but I don't see any reason why that wouldn't also work
with sched_setscheduler() (IOW always also set the policy).
> > > In both cases the goal should be to return from code block D).
> >
> > I don't think so; we really do want to 'goto change' for util changes
> > too I think. Why duplicate part of that logic?
>
> But that will force a dequeue/enqueue... isn't too much overhead just
> to change a clamp value?
These syscalls aren't what I consider fast paths anyway. However, there
are people that rely on the scheduler syscalls not to schedule
themselves, or rather be non-blocking (see for example that prio-ceiling
implementation).
And in that respect the newly introduced uclamp_mutex does appear to be
a problem.
Also; do you expect these clamp values to be changed often?
> Perhaps we can also end up with some wired
s/wired/weird/, right?
> side-effects like the task being preempted ?
Nothing worse than any other random reschedule would cause.
> Consider also that the uclamp_task_update_active() added by this patch
> not only has lower overhead but it will be use also by cgroups where
> we want to force update all the tasks on a cgroup's clamp change.
I haven't gotten that far; but I would prefer not to have two different
'change' paths in __sched_setscheduler().
next prev parent reply other threads:[~2019-01-23 9:16 UTC|newest]
Thread overview: 89+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-15 10:14 [PATCH v6 00/16] Add utilization clamping support Patrick Bellasi
2019-01-15 10:14 ` [PATCH v6 01/16] sched/core: Allow sched_setattr() to use the current policy Patrick Bellasi
2019-01-25 13:56 ` Alessio Balsini
2019-01-15 10:14 ` [PATCH v6 02/16] sched/core: uclamp: Extend sched_setattr() to support utilization clamping Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 03/16] sched/core: uclamp: Map TASK's clamp values into CPU's clamp buckets Patrick Bellasi
2019-01-21 10:15 ` Peter Zijlstra
2019-01-21 12:27 ` Patrick Bellasi
2019-01-21 12:51 ` Peter Zijlstra
2019-01-21 15:05 ` Peter Zijlstra
2019-01-21 15:34 ` Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 04/16] sched/core: uclamp: Add CPU's clamp buckets refcounting Patrick Bellasi
2019-01-21 14:59 ` Peter Zijlstra
2019-01-21 15:23 ` Patrick Bellasi
2019-01-21 16:12 ` Peter Zijlstra
2019-01-21 16:33 ` Patrick Bellasi
2019-01-22 9:45 ` Peter Zijlstra
2019-01-22 10:31 ` Patrick Bellasi
2019-01-21 15:17 ` Peter Zijlstra
2019-01-21 15:54 ` Patrick Bellasi
2019-01-22 10:03 ` Peter Zijlstra
2019-01-22 10:53 ` Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 05/16] sched/core: uclamp: Update CPU's refcount on clamp changes Patrick Bellasi
2019-01-21 15:33 ` Peter Zijlstra
2019-01-21 15:44 ` Patrick Bellasi
2019-01-22 9:37 ` Peter Zijlstra
2019-01-22 10:43 ` Patrick Bellasi
2019-01-22 13:28 ` Peter Zijlstra
2019-01-22 14:01 ` Patrick Bellasi
2019-01-22 14:57 ` Peter Zijlstra
2019-01-22 15:33 ` Patrick Bellasi
2019-01-23 9:16 ` Peter Zijlstra [this message]
2019-01-23 14:14 ` Patrick Bellasi
2019-01-23 18:59 ` Peter Zijlstra
2019-01-24 11:21 ` Patrick Bellasi
2019-01-24 12:38 ` Peter Zijlstra
2019-01-15 10:15 ` [PATCH v6 06/16] sched/core: uclamp: Enforce last task UCLAMP_MAX Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 07/16] sched/core: uclamp: Add system default clamps Patrick Bellasi
2019-01-22 13:56 ` Peter Zijlstra
2019-01-22 14:43 ` Patrick Bellasi
2019-01-22 15:13 ` Peter Zijlstra
2019-01-22 15:41 ` Patrick Bellasi
2019-01-23 9:22 ` Peter Zijlstra
2019-01-23 14:19 ` Patrick Bellasi
2019-01-23 19:10 ` Peter Zijlstra
2019-01-15 10:15 ` [PATCH v6 08/16] sched/cpufreq: uclamp: Add utilization clamping for FAIR tasks Patrick Bellasi
2019-01-22 10:37 ` Rafael J. Wysocki
2019-01-22 11:02 ` Patrick Bellasi
2019-01-22 11:04 ` Rafael J. Wysocki
2019-01-22 11:27 ` Patrick Bellasi
2019-01-22 15:21 ` Peter Zijlstra
2019-01-22 15:45 ` Patrick Bellasi
2019-01-22 17:13 ` Peter Zijlstra
2019-01-22 18:18 ` Patrick Bellasi
2019-01-23 9:52 ` Peter Zijlstra
2019-01-23 14:24 ` Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 09/16] sched/cpufreq: uclamp: Add utilization clamping for RT tasks Patrick Bellasi
2019-01-22 12:30 ` Quentin Perret
2019-01-22 12:37 ` Patrick Bellasi
2019-01-23 10:28 ` Peter Zijlstra
2019-01-23 14:33 ` Patrick Bellasi
2019-01-23 10:49 ` Peter Zijlstra
2019-01-23 14:40 ` Patrick Bellasi
2019-01-23 20:11 ` Peter Zijlstra
2019-01-24 12:30 ` Patrick Bellasi
2019-01-24 12:38 ` Patrick Bellasi
2019-01-24 15:12 ` Peter Zijlstra
2019-01-24 16:00 ` Patrick Bellasi
2019-01-24 15:31 ` Peter Zijlstra
2019-01-24 16:14 ` Patrick Bellasi
2019-01-24 15:33 ` Peter Zijlstra
2019-01-24 15:15 ` Peter Zijlstra
2019-01-24 16:05 ` Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 10/16] sched/core: Add uclamp_util_with() Patrick Bellasi
2019-01-23 13:33 ` Peter Zijlstra
2019-01-23 14:51 ` Patrick Bellasi
2019-01-23 19:22 ` Peter Zijlstra
2019-01-15 10:15 ` [PATCH v6 11/16] sched/fair: Add uclamp support to energy_compute() Patrick Bellasi
2019-01-22 12:13 ` Quentin Perret
2019-01-22 12:45 ` Patrick Bellasi
2019-01-22 13:29 ` Quentin Perret
2019-01-22 14:26 ` Patrick Bellasi
2019-01-22 14:39 ` Quentin Perret
2019-01-22 15:01 ` Patrick Bellasi
2019-01-22 15:14 ` Quentin Perret
2019-01-15 10:15 ` [PATCH v6 12/16] sched/core: uclamp: Extend CPU's cgroup controller Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 13/16] sched/core: uclamp: Propagate parent clamps Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 14/16] sched/core: uclamp: Map TG's clamp values into CPU's clamp buckets Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 15/16] sched/core: uclamp: Use TG's clamps to restrict TASK's clamps Patrick Bellasi
2019-01-15 10:15 ` [PATCH v6 16/16] sched/core: uclamp: Update CPU's refcount on TG's clamp changes Patrick Bellasi
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