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Date: Tue, 1 Mar 2022 18:17:27 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>
Cc: Juri Lelli <juri.lelli@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Mel Gorman <mgorman@...e.de>, Ben Segall <bsegall@...gle.com>,
Patrick Bellasi <patrick.bellasi@...bug.net>,
Valentin Schneider <valentin.schneider@....com>,
Vincent Donnefort <vincent.donnefort@....com>,
linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: Refactor cpu_util_without()
Except the 'task has no contribution or is new' condition at the
beginning of cpu_util_without(), a cpu_util_next(..., dst_cpu = -1)
call can replace the rest of this function.
The UTIL_EST specific check for a race between select_task_rq_fair()
and detach_task() in case of an enqueued or running WF_EXEC task has
to be moved to cpu_util_next().
This was initially introduced by commit c469933e7721
("sched/fair: Fix cpu_util_wake() for 'execl' type workloads").
UnixBench's `execl` throughput tests were run on the dual socket 40
CPUs Intel E5-2690 v2 machine to make sure the regression doesn't
occur again.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
There is still a lot of CPU utilization related code. cpu_util_without()
and cpu_util_next() are very similar. In fact the former can be
refactored to use a call to the latter to be able to remove some
redundancy.
kernel/sched/fair.c | 143 ++++++++++++++++++--------------------------
1 file changed, 59 insertions(+), 84 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 16874e112fe6..c084c2e29e40 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6511,6 +6511,64 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
return target;
}
+/*
+ * Predicts what cpu_util(@cpu) would return if @p was migrated (and enqueued)
+ * to @dst_cpu.
+ */
+static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
+{
+ struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
+ unsigned long util_est, util = READ_ONCE(cfs_rq->avg.util_avg);
+
+ /*
+ * If @p migrates from @cpu to another, remove its contribution. Or,
+ * if @p migrates from another CPU to @cpu, add its contribution. In
+ * the other cases, @cpu is not impacted by the migration, so the
+ * util_avg should already be correct.
+ */
+ if (task_cpu(p) == cpu && dst_cpu != cpu)
+ lsub_positive(&util, task_util(p));
+ else if (task_cpu(p) != cpu && dst_cpu == cpu)
+ util += task_util(p);
+
+ if (sched_feat(UTIL_EST)) {
+ util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued);
+
+ /*
+ * During wake-up, the task isn't enqueued yet and doesn't
+ * appear in the cfs_rq->avg.util_est.enqueued of any rq,
+ * so just add it (if needed) to "simulate" what will be
+ * cpu_util after the task has been enqueued.
+ */
+ if (dst_cpu == cpu)
+ util_est += _task_util_est(p);
+
+ /*
+ * Despite the following checks we still have a small window
+ * for a possible race, when an execl's select_task_rq_fair()
+ * races with LB's detach_task():
+ *
+ * detach_task()
+ * p->on_rq = TASK_ON_RQ_MIGRATING;
+ * ---------------------------------- A
+ * deactivate_task() \
+ * dequeue_task() + RaceTime
+ * util_est_dequeue() /
+ * ---------------------------------- B
+ *
+ * The additional check on "current == p" it's required to
+ * properly fix the execl regression and it helps in further
+ * reducing the chances for the above race.
+ */
+ if (unlikely(task_on_rq_queued(p) || current == p))
+ lsub_positive(&util_est, _task_util_est(p));
+
+ util = max(util, util_est);
+ }
+
+ return min(util, capacity_orig_of(cpu));
+}
+
/*
* cpu_util_without: compute cpu utilization without any contributions from *p
* @cpu: the CPU which utilization is requested
@@ -6526,19 +6584,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
*/
static unsigned long cpu_util_without(int cpu, struct task_struct *p)
{
- struct cfs_rq *cfs_rq;
- unsigned int util;
-
/* Task has no contribution or is new */
if (cpu != task_cpu(p) || !READ_ONCE(p->se.avg.last_update_time))
return cpu_util_cfs(cpu);
- cfs_rq = &cpu_rq(cpu)->cfs;
- util = READ_ONCE(cfs_rq->avg.util_avg);
-
- /* Discount task's util from CPU's util */
- lsub_positive(&util, task_util(p));
-
/*
* Covered cases:
*
@@ -6560,82 +6609,8 @@ static unsigned long cpu_util_without(int cpu, struct task_struct *p)
* estimation of the spare capacity on that CPU, by just
* considering the expected utilization of tasks already
* runnable on that CPU.
- *
- * Cases a) and b) are covered by the above code, while case c) is
- * covered by the following code when estimated utilization is
- * enabled.
*/
- if (sched_feat(UTIL_EST)) {
- unsigned int estimated =
- READ_ONCE(cfs_rq->avg.util_est.enqueued);
-
- /*
- * Despite the following checks we still have a small window
- * for a possible race, when an execl's select_task_rq_fair()
- * races with LB's detach_task():
- *
- * detach_task()
- * p->on_rq = TASK_ON_RQ_MIGRATING;
- * ---------------------------------- A
- * deactivate_task() \
- * dequeue_task() + RaceTime
- * util_est_dequeue() /
- * ---------------------------------- B
- *
- * The additional check on "current == p" it's required to
- * properly fix the execl regression and it helps in further
- * reducing the chances for the above race.
- */
- if (unlikely(task_on_rq_queued(p) || current == p))
- lsub_positive(&estimated, _task_util_est(p));
-
- util = max(util, estimated);
- }
-
- /*
- * Utilization (estimated) can exceed the CPU capacity, thus let's
- * clamp to the maximum CPU capacity to ensure consistency with
- * cpu_util.
- */
- return min_t(unsigned long, util, capacity_orig_of(cpu));
-}
-
-/*
- * Predicts what cpu_util(@cpu) would return if @p was migrated (and enqueued)
- * to @dst_cpu.
- */
-static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu)
-{
- struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
- unsigned long util_est, util = READ_ONCE(cfs_rq->avg.util_avg);
-
- /*
- * If @p migrates from @cpu to another, remove its contribution. Or,
- * if @p migrates from another CPU to @cpu, add its contribution. In
- * the other cases, @cpu is not impacted by the migration, so the
- * util_avg should already be correct.
- */
- if (task_cpu(p) == cpu && dst_cpu != cpu)
- lsub_positive(&util, task_util(p));
- else if (task_cpu(p) != cpu && dst_cpu == cpu)
- util += task_util(p);
-
- if (sched_feat(UTIL_EST)) {
- util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued);
-
- /*
- * During wake-up, the task isn't enqueued yet and doesn't
- * appear in the cfs_rq->avg.util_est.enqueued of any rq,
- * so just add it (if needed) to "simulate" what will be
- * cpu_util after the task has been enqueued.
- */
- if (dst_cpu == cpu)
- util_est += _task_util_est(p);
-
- util = max(util, util_est);
- }
-
- return min(util, capacity_orig_of(cpu));
+ return cpu_util_next(cpu, p, -1);
}
/*
--
2.25.1
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