linux-kernel - [PATCH 5/6] sched/deadline: Add helpers to get the correct root domain/span/dl

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Date:   Tue, 12 Jan 2021 16:53:44 +0100
From:   Daniel Bristot de Oliveira <bristot@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Marco Perronet <perronet@...-sws.org>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Li Zefan <lizefan@...wei.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Valentin Schneider <valentin.schneider@....com>,
        cgroups@...r.kernel.org
Subject: [PATCH 5/6] sched/deadline: Add helpers to get the correct root domain/span/dl_bw

task_cpu() is often used on SCHED_DEADLINE to access the root_domain
and associated information. However, the task_cpu() does not always
reflect the correct CPU. The reason being is shown in the code:

>From kernel/sched/core:
----- %< -----
 * p->on_rq <- { 0, 1 = TASK_ON_RQ_QUEUED, 2 = TASK_ON_RQ_MIGRATING }:
 *
 *   is set by activate_task() and cleared by deactivate_task(), under
 *   rq->lock. Non-zero indicates the task is runnable, the special
 *   ON_RQ_MIGRATING state is used for migration without holding both
 *   rq->locks. It indicates task_cpu() is not stable, see task_rq_lock().
[...]
 * task_cpu(p): is changed by set_task_cpu(), the rules are:
 *
 *  - Don't call set_task_cpu() on a blocked task:
 *
 *    We don't care what CPU we're not running on, this simplifies hotplug,
 *    the CPU assignment of blocked tasks isn't required to be valid.
----- >% -----

So, a sleeping task will not have its task_cpu() stable, and this is
causing problems on SCHED_DEADLINE.

In preparation for fixing this problems, we need to add helper functions
that return the dl_task_cpu, root_domain, and "root" dl_bw that
reflects the current placement/affinity of the task.

Note that these functions are only required on the code path that
can happen when the task is not queued.

Signed-off-by: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Mel Gorman <mgorman@...e.de>
Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
Cc: Li Zefan <lizefan@...wei.com>
Cc: Tejun Heo <tj@...nel.org>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: Valentin Schneider <valentin.schneider@....com>
Cc: linux-kernel@...r.kernel.org
Cc: cgroups@...r.kernel.org
---
 kernel/sched/sched.h | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 54881d99cebd..5f3f3cb5a6ff 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2393,6 +2393,37 @@ void __dl_update(struct dl_bw *dl_b, s64 bw)
 		rq->dl.extra_bw += bw;
 	}
 }
+
+static inline
+int dl_task_cpu(struct task_struct *p)
+{
+	/*
+	 * We can only rely on task_cpu() of runnable tasks.
+	 */
+	if (p->on_rq)
+		return task_cpu(p);
+
+	/*
+	 * The task_cpu() of non-runnable task migth be pointing a CPU
+	 * it cannot run anymore (see set_task_cpu()). Hence, let's
+	 * get a possible cpu from the current cpu_mask.
+	 */
+	return cpumask_any(p->cpus_ptr);
+
+}
+
+static inline
+struct root_domain *dl_task_rd(struct task_struct *p)
+{
+	int cpu = dl_task_cpu(p);
+	return cpu_rq(cpu)->rd;
+}
+
+static inline
+struct dl_bw *dl_task_root_bw(struct task_struct *p)
+{
+	return &dl_task_rd(p)->dl_bw;
+}
 #else
 static inline
 void __dl_update(struct dl_bw *dl_b, s64 bw)
@@ -2401,6 +2432,18 @@ void __dl_update(struct dl_bw *dl_b, s64 bw)
 
 	dl->extra_bw += bw;
 }
+
+static inline
+int dl_task_cpu(struct task_struct *p)
+{
+	return 0;
+}
+
+static inline
+struct dl_bw *dl_task_root_bw(struct task_struct *p)
+{
+	return &task_rq(p)->dl.dl_bw;
+}
 #endif
 
 
-- 
2.29.2