[PATCH 4/4] sched/fair: Avoid revisiting CPUs multiple times during select_idle_sibling
From: Mel Gorman
Date: Mon Dec 07 2020 - 04:17:04 EST
select_idle_core() potentially searches a number of CPUs for idle candidates
before select_idle_cpu() clears the mask and revisits the same CPUs. This
patch moves the initialisation of select_idle_mask to the top-level and
reuses the same mask across both select_idle_core and select_idle_cpu.
select_idle_smt() is left alone as the cost of checking one SMT sibling
is marginal relative to calling __clear_cpumask_cpu() for every CPU
visited by select_idle_core().
With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled
5.10.0-rc6 5.10.0-rc6
altrecent-v2 singlepass-v2
Hmean 1 502.05 ( 0.00%) 498.53 * -0.70%*
Hmean 2 983.65 ( 0.00%) 982.33 * -0.13%*
Hmean 4 1920.24 ( 0.00%) 1929.34 * 0.47%*
Hmean 8 3663.96 ( 0.00%) 3536.94 * -3.47%*
Hmean 16 6545.58 ( 0.00%) 6560.21 * 0.22%*
Hmean 32 10253.73 ( 0.00%) 10374.30 * 1.18%*
Hmean 64 12506.31 ( 0.00%) 11692.26 * -6.51%*
Hmean 128 21967.13 ( 0.00%) 21705.80 * -1.19%*
Hmean 256 21534.52 ( 0.00%) 21223.50 * -1.44%*
Hmean 320 21070.14 ( 0.00%) 21023.31 * -0.22%*
As before, results are somewhat workload and machine-specific with a mix
of good and bad. For example, netperf UDP_STREAM on an 80-cpu Broadwell
machine shows;
5.10.0-rc6 5.10.0-rc6
altrecent-v2 singlepass-v2
Hmean send-64 232.78 ( 0.00%) 258.02 * 10.85%*
Hmean send-128 464.69 ( 0.00%) 511.53 * 10.08%*
Hmean send-256 904.72 ( 0.00%) 999.40 * 10.46%*
Hmean send-1024 3512.08 ( 0.00%) 3868.86 * 10.16%*
Hmean send-2048 6777.61 ( 0.00%) 7435.05 * 9.70%*
Hmean send-3312 10352.45 ( 0.00%) 11321.43 * 9.36%*
Hmean send-4096 12660.58 ( 0.00%) 13577.08 * 7.24%*
Hmean send-8192 19240.36 ( 0.00%) 21321.34 * 10.82%*
Hmean send-16384 29691.27 ( 0.00%) 31296.47 * 5.41%*
The real question is -- is it better to scan CPU runqueues just once
where possible or scan multiple times? This patch scans once but it must
do additional work to track that scanning so for shallow scans it's a
loss and or deep scans, it's a win.
The main downside to this patch is that cpumask manipulations are
more complex because two cpumasks are involved. The alternative is that
select_idle_core() would scan the full domain and always return a CPU which
was previously considered. Alternatively, the depth of select_idle_core()
could be limited. Similarly, select_idle_core() often scans when no idle
core is available as test_idle_cores is very race-prone and maybe there
is a better approach to determining if select_idle_core() is likely to
succeed or not.
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/fair.c | 40 +++++++++++++++++++++++++++++-----------
1 file changed, 29 insertions(+), 11 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 00c3b526a5bd..3b1736dc5bde 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6064,10 +6064,11 @@ void __update_idle_core(struct rq *rq)
* there are no idle cores left in the system; tracked through
* sd_llc->shared->has_idle_cores and enabled through update_idle_core() above.
*/
-static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
+static int select_idle_core(struct task_struct *p, struct sched_domain *sd,
+ int target, struct cpumask *cpus_scan)
{
int idle_candidate = -1;
- struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
+ struct cpumask *cpus;
int core, cpu;
if (!static_branch_likely(&sched_smt_present))
@@ -6076,12 +6077,21 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
if (!test_idle_cores(target, false))
return -1;
- cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
+ /*
+ * Repurpose load_balance_mask to avoid rescanning cores while
+ * cpus_scan tracks the cpu if select_idle_cpu() is necessary.
+ * In this context, it should be impossible to enter LB and
+ * clobber load_balance_mask.
+ */
+ cpus = this_cpu_cpumask_var_ptr(load_balance_mask);
+ cpumask_copy(cpus, cpus_scan);
for_each_cpu_wrap(core, cpus, target) {
bool idle = true;
for_each_cpu(cpu, cpu_smt_mask(core)) {
+ __cpumask_clear_cpu(cpu, cpus_scan);
+
if (!available_idle_cpu(cpu)) {
idle = false;
break;
@@ -6130,7 +6140,8 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t
#else /* CONFIG_SCHED_SMT */
-static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd, int target)
+static inline int select_idle_core(struct task_struct *p, struct sched_domain *sd,
+ int target, struct cpumask *cpus)
{
return -1;
}
@@ -6147,9 +6158,9 @@ static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd
* comparing the average scan cost (tracked in sd->avg_scan_cost) against the
* average idle time for this rq (as found in rq->avg_idle).
*/
-static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
+static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd,
+ int target, struct cpumask *cpus)
{
- struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
struct sched_domain *this_sd;
u64 avg_cost, avg_idle;
u64 time;
@@ -6176,9 +6187,6 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
}
time = cpu_clock(this);
-
- cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
-
for_each_cpu_wrap(cpu, cpus, target) {
if (!--nr)
return -1;
@@ -6240,6 +6248,7 @@ static inline bool asym_fits_capacity(int task_util, int cpu)
static int select_idle_sibling(struct task_struct *p, int prev, int target)
{
struct sched_domain *sd;
+ struct cpumask *cpus_scan;
unsigned long task_util;
int i, recent_used_cpu;
@@ -6319,11 +6328,20 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
if (!sd)
return target;
- i = select_idle_core(p, sd, target);
+ /*
+ * Init the select_idle_mask. select_idle_core() will mask
+ * out the CPUs that have already been limited to limit the
+ * search in select_idle_cpu(). Further clearing is not
+ * done as select_idle_smt checks only one CPU.
+ */
+ cpus_scan = this_cpu_cpumask_var_ptr(select_idle_mask);
+ cpumask_and(cpus_scan, sched_domain_span(sd), p->cpus_ptr);
+
+ i = select_idle_core(p, sd, target, cpus_scan);
if ((unsigned)i < nr_cpumask_bits)
return i;
- i = select_idle_cpu(p, sd, target);
+ i = select_idle_cpu(p, sd, target, cpus_scan);
if ((unsigned)i < nr_cpumask_bits)
return i;
--
2.26.2