@@ -6293,6 +6293,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
/* Check a recently used CPU as a potential idle candidate: */
recent_used_cpu = p->recent_used_cpu;
+ p->recent_used_cpu = prev;
if (recent_used_cpu != prev &&
recent_used_cpu != target &&
cpus_share_cache(recent_used_cpu, target)) {
@@ -6789,9 +6790,6 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
} else if (wake_flags & WF_TTWU) { /* XXX always ? */
/* Fast path */
new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
-
- if (want_affine)
- current->recent_used_cpu = cpu;
}
rcu_read_unlock();
After select_idle_sibling, p->recent_used_cpu is set to the new target. However on the next wakeup, prev will be the same as recent_used_cpu unless the load balancer has moved the task since the last wakeup. It still works, but is less efficient than it can be after all the changes that went in since that reduce unnecessary migrations, load balancer changes etc. This patch preserves recent_used_cpu for longer. With tbench on a 2-socket CascadeLake machine, 80 logical CPUs, HT enabled 5.10.0-rc6 5.10.0-rc6 idlecandidate-v1r10 altrecent-v1r10 Hmean 1 505.67 ( 0.00%) 501.34 * -0.86%* Hmean 2 974.06 ( 0.00%) 981.39 * 0.75%* Hmean 4 1904.43 ( 0.00%) 1926.13 * 1.14%* Hmean 8 3721.02 ( 0.00%) 3799.86 * 2.12%* Hmean 16 6769.17 ( 0.00%) 6938.40 * 2.50%* Hmean 32 10312.58 ( 0.00%) 10632.11 * 3.10%* Hmean 64 13792.01 ( 0.00%) 13670.17 * -0.88%* Hmean 128 20963.44 ( 0.00%) 21456.33 * 2.35%* Hmean 256 20335.62 ( 0.00%) 21070.24 * 3.61%* Hmean 320 20147.25 ( 0.00%) 20624.92 * 2.37%* The benefit is marginal, the main impact is on how it affects p->recent_used_cpu and whether a domain search happens. From the schedstats patches and schedstat enabled Ops SIS Search 5653107942.00 5726545742.00 Ops SIS Domain Search 3365067916.00 3319768543.00 Ops SIS Scanned 112173512543.00 99194352541.00 Ops SIS Domain Scanned 109885472517.00 96787575342.00 Ops SIS Failures 2923185114.00 2950166441.00 Ops SIS Recent Used Hit 56547.00 118064916.00 Ops SIS Recent Used Miss 1590899250.00 354942791.00 Ops SIS Recent Attempts 1590955797.00 473007707.00 Ops SIS Search Efficiency 5.04 5.77 Ops SIS Domain Search Eff 3.06 3.43 Ops SIS Fast Success Rate 40.47 42.03 Ops SIS Success Rate 48.29 48.48 Ops SIS Recent Success Rate 0.00 24.96 (First interesting point is the ridiculous number of times runqueues are enabled -- almost 97 billion times over the course of 40 minutes) Note "Recent Used Hit" is over 2000 times more likely to succeed. The failure rate also increases by quite a lot but the cost is marginal even if the "Fast Success Rate" only increases by 2% overall. What cannot be observed from these stats is where the biggest impact as these stats cover low utilisation to over saturation. If graphed over time, the graphs show that the sched domain is only scanned at negligible rates until the machine is fully busy. With low utilisation, the "Fast Success Rate" is almost 100% until the machine is fully busy. For 320 clients, the success rate is close to 0% which is unsurprising. Signed-off-by: Mel Gorman <mgorman@techsingularity.net> --- kernel/sched/fair.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-)