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Date: Tue, 31 Mar 2009 16:20:32 +0530
From: Gautham R Shenoy <ego@...ibm.com>
To: "Ingo Molnar" <mingo@...e.hu>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org,
Suresh Siddha <suresh.b.siddha@...el.com>,
"Balbir Singh" <balbir@...ibm.com>,
Gautham R Shenoy <ego@...ibm.com>
Subject: [PATCH v4 4/5] sched: Arbitrate the nomination of preferred_wakeup_cpu
Currently for sched_mc/smt_power_savings = 2, we consolidate tasks
by having a preferred_wakeup_cpu which will be used for all the
further wake ups.
This preferred_wakeup_cpu is currently nominated by find_busiest_group()
when we perform load-balancing at sched_domains which has
SD_POWERSAVINGS_BALANCE flag set.
However, on systems which are multi-threaded and multi-core, we can
have multiple sched_domains in the same hierarchy with
SD_POWERSAVINGS_BALANCE flag set.
Currently we don't have any arbitration mechanism as to while performing
load balancing at which sched_domain in the hierarchy should
find_busiest_group(sd) nominate the preferred_wakeup_cpu.
Hence can overwrite valid nominations made previously thereby
causing the preferred_wakup_cpu to ping-pong,
thereby preventing us from effectively consolidating tasks.
Fix this by means of an arbitration algorithm, where in we nominate the
preferred_wakeup_cpu while performing load balancing at a particular
sched_domain if that sched_domain:
- is the topmost power aware sched_domain.
OR
- contains the previously nominated preferred wake up cpu in it's span.
This will help to further fine tune the wake-up biasing logic by
identifying a partially busy core within a CPU package instead of
potentially waking up a completely idle core.
Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
---
kernel/sched.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++-------
1 files changed, 49 insertions(+), 7 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 36d116b..193bb67 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -549,6 +549,14 @@ struct root_domain {
* This is triggered at POWERSAVINGS_BALANCE_WAKEUP(2).
*/
unsigned int preferred_wakeup_cpu;
+
+ /*
+ * top_powersavings_sd_lvl records the level of the highest
+ * sched_domain that has the SD_POWERSAVINGS_BALANCE flag set.
+ *
+ * Used to arbitrate nomination of the preferred_wakeup_cpu.
+ */
+ enum sched_domain_level top_powersavings_sd_lvl;
#endif
};
@@ -3439,9 +3447,11 @@ static inline void update_sd_power_savings_stats(struct sched_group *group,
* Returns 1 if there is potential to perform power-savings balance.
* Else returns 0.
*/
-static inline int check_power_save_busiest_group(struct sd_lb_stats *sds,
- int this_cpu, unsigned long *imbalance)
+static inline int check_power_save_busiest_group(struct sched_domain *sd,
+ struct sd_lb_stats *sds, int this_cpu, unsigned long *imbalance)
{
+ struct root_domain *my_rd = cpu_rq(this_cpu)->rd;
+
if (!sds->power_savings_balance)
return 0;
@@ -3452,8 +3462,25 @@ static inline int check_power_save_busiest_group(struct sd_lb_stats *sds,
*imbalance = sds->min_load_per_task;
sds->busiest = sds->group_min;
- if (active_power_savings_level >= POWERSAVINGS_BALANCE_WAKEUP) {
- cpu_rq(this_cpu)->rd->preferred_wakeup_cpu =
+ /*
+ * To avoid overwriting of preferred_wakeup_cpu nominations
+ * while performing load-balancing at various sched_domain
+ * levels, we define an arbitration mechanism wherein
+ * we nominates a preferred_wakeup_cpu while load balancing
+ * at a particular sched_domain sd if:
+ *
+ * - sd is the highest sched_domain in the hierarchy having the
+ * SD_POWERSAVINGS_BALANCE flag set.
+ *
+ * OR
+ *
+ * - sd contains the previously nominated preferred_wakeup_cpu
+ * in it's span.
+ */
+ if (sd->level == my_rd->top_powersavings_sd_lvl ||
+ cpumask_test_cpu(my_rd->preferred_wakeup_cpu,
+ sched_domain_span(sd))) {
+ my_rd->preferred_wakeup_cpu =
group_first_cpu(sds->group_leader);
}
@@ -3473,8 +3500,8 @@ static inline void update_sd_power_savings_stats(struct sched_group *group,
return;
}
-static inline int check_power_save_busiest_group(struct sd_lb_stats *sds,
- int this_cpu, unsigned long *imbalance)
+static inline int check_power_save_busiest_group(struct sched_domain *sd,
+ struct sd_lb_stats *sds, int this_cpu, unsigned long *imbalance)
{
return 0;
}
@@ -3838,7 +3865,7 @@ out_balanced:
* There is no obvious imbalance. But check if we can do some balancing
* to save power.
*/
- if (check_power_save_busiest_group(&sds, this_cpu, imbalance))
+ if (check_power_save_busiest_group(sd, &sds, this_cpu, imbalance))
return sds.busiest;
ret:
*imbalance = 0;
@@ -8059,6 +8086,8 @@ static int __build_sched_domains(const struct cpumask *cpu_map,
struct root_domain *rd;
cpumask_var_t nodemask, this_sibling_map, this_core_map, send_covered,
tmpmask;
+ struct sched_domain *sd;
+
#ifdef CONFIG_NUMA
cpumask_var_t domainspan, covered, notcovered;
struct sched_group **sched_group_nodes = NULL;
@@ -8334,6 +8363,19 @@ static int __build_sched_domains(const struct cpumask *cpu_map,
err = 0;
+ rd->preferred_wakeup_cpu = UINT_MAX;
+ rd->top_powersavings_sd_lvl = SD_LV_NONE;
+
+ if (active_power_savings_level < POWERSAVINGS_BALANCE_WAKEUP)
+ goto free_tmpmask;
+
+ /* Record the level of the highest power-aware sched_domain */
+ for_each_domain(first_cpu(*cpu_map), sd) {
+ if (!(sd->flags & SD_POWERSAVINGS_BALANCE))
+ continue;
+ rd->top_powersavings_sd_lvl = sd->level;
+ }
+
free_tmpmask:
free_cpumask_var(tmpmask);
free_send_covered:
--
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