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Subject[PATCH v9 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()
Date
Hello Steven,

I added several comments so I'm not sure if I could add your reviewed-by.

----->8-----
From f0710d99759ed28c1409a527166780899f00d236 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.park@lge.com>
Date: Tue, 22 Aug 2017 17:19:21 +0900
Subject: [PATCH v9 2/2] sched/rt: Add support for SD_PREFER_SIBLING on
find_lowest_rq()

It would be better to try to check other siblings first if
SD_PREFER_SIBLING is flaged when pushing tasks - migration.

Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
---
kernel/sched/rt.c | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 75 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 979b734..196ffc7 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1618,12 +1618,35 @@ static struct task_struct *pick_highest_pushable_task(struct rq *rq, int cpu)

static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);

+/*
+ * Find the first cpu in: mask & sd & ~prefer
+ */
+static int find_cpu(const struct cpumask *mask,
+ const struct sched_domain *sd,
+ const struct sched_domain *prefer)
+{
+ const struct cpumask *sds = sched_domain_span(sd);
+ const struct cpumask *ps = prefer ? sched_domain_span(prefer) : NULL;
+ int cpu;
+
+ for_each_cpu(cpu, mask) {
+ if (!cpumask_test_cpu(cpu, sds))
+ continue;
+ if (ps && cpumask_test_cpu(cpu, ps))
+ continue;
+ break;
+ }
+
+ return cpu;
+}
+
static int find_lowest_rq(struct task_struct *task)
{
- struct sched_domain *sd;
+ struct sched_domain *sd, *prefer = NULL;
struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
int this_cpu = smp_processor_id();
int cpu = task_cpu(task);
+ int fallback_cpu = -1;

/* Make sure the mask is initialized first */
if (unlikely(!lowest_mask))
@@ -1668,9 +1691,35 @@ static int find_lowest_rq(struct task_struct *task)
return this_cpu;
}

- best_cpu = cpumask_first_and(lowest_mask,
- sched_domain_span(sd));
+ /*
+ * If a cpu being in lowest_mask & current sd &
+ * ~prefer sd is valid, that becomes our choice.
+ * Of course, the lowest possible cpu is already
+ * under consideration through lowest_mask.
+ */
+ best_cpu = find_cpu(lowest_mask, sd, prefer);
+
if (best_cpu < nr_cpu_ids) {
+ /*
+ * If current domain is SD_PREFER_SIBLING
+ * flaged, we have to try to check other
+ * siblings first.
+ */
+ if (sd->flags & SD_PREFER_SIBLING) {
+ prefer = sd;
+
+ /*
+ * fallback_cpu should be one
+ * in the closest domain among
+ * SD_PREFER_SIBLING domains,
+ * in case that more than one
+ * SD_PREFER_SIBLING domains
+ * exist in the hierachy.
+ */
+ if (fallback_cpu == -1)
+ fallback_cpu = best_cpu;
+ continue;
+ }
rcu_read_unlock();
return best_cpu;
}
@@ -1679,6 +1728,29 @@ static int find_lowest_rq(struct task_struct *task)
rcu_read_unlock();

/*
+ * If fallback_cpu is valid, all our guesses failed *except* for
+ * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
+ *
+ * XXX: Consider the following example, 4 cores SMT2 system:
+ *
+ * LLC [0 - 7]
+ * SMT [0 1][2 3][4 5][6 7]
+ * o x o x x x x x
+ *
+ * where 'o': occupied and 'x': empty.
+ *
+ * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
+ * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
+ * in this case, we have to choose cpu4 for better work, instead
+ * of cpu3 that is fully loaded.
+ *
+ * We have to do the best if possible, but choose the second
+ * best here since that is too expensive to adopt.
+ */
+ if (fallback_cpu != -1)
+ return fallback_cpu;
+
+ /*
* And finally, if there were no matches within the domains
* just give the caller *something* to work with from the compatible
* locations.
--
1.9.1
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