| Message ID | 20201207091516.24683-3-mgorman@techsingularity.net (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Reduce worst-case scanning of runqueues in select_idle_sibling | expand |
On 07/12/2020 10:15, Mel Gorman wrote: > 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 I'm confused here. Isn't current->recent_used_cpu set to 'cpu = smp_processor_id()' after sis()? Looking at v5.10-rc6. [...] > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 23934dbac635..01b38fc17bca 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6274,6 +6274,7 @@ 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) && p->recent_used_cpu is already set to prev in this if condition. asym_fits_capacity(task_util, recent_used_cpu)) { /* * Replace recent_used_cpu with prev as it is a potential * candidate for the next wake: */ p->recent_used_cpu = prev; return recent_used_cpu; > @@ -6765,9 +6766,6 @@ 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();
On Tue, Dec 08, 2020 at 10:57:29AM +0100, Dietmar Eggemann wrote: > On 07/12/2020 10:15, Mel Gorman wrote: > > 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 > > I'm confused here. Isn't current->recent_used_cpu set to 'cpu = > smp_processor_id()' after sis()? Looking at v5.10-rc6. If you are referring to this; if (want_affine) current->recent_used_cpu = cpu; then it gets removed by the path. That replaces recent_used_cpu with the wakers CPU which still works but the hit rate is lower. > > [...] > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 23934dbac635..01b38fc17bca 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6274,6 +6274,7 @@ 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) && > > p->recent_used_cpu is already set to prev in this if condition. > That can be removed as redundant, I'll fix it.
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 23934dbac635..01b38fc17bca 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6274,6 +6274,7 @@ 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) && @@ -6765,9 +6766,6 @@ 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 baseline-v2 altrecent-v2 Hmean 1 508.39 ( 0.00%) 502.05 * -1.25%* Hmean 2 986.70 ( 0.00%) 983.65 * -0.31%* Hmean 4 1914.55 ( 0.00%) 1920.24 * 0.30%* Hmean 8 3702.37 ( 0.00%) 3663.96 * -1.04%* Hmean 16 6573.11 ( 0.00%) 6545.58 * -0.42%* Hmean 32 10142.57 ( 0.00%) 10253.73 * 1.10%* Hmean 64 14348.40 ( 0.00%) 12506.31 * -12.84%* Hmean 128 21842.59 ( 0.00%) 21967.13 * 0.57%* Hmean 256 20813.75 ( 0.00%) 21534.52 * 3.46%* Hmean 320 20684.33 ( 0.00%) 21070.14 * 1.87%* The different was marginal except for 64 threads which showed in the baseline that the result was very unstable where as the patch was much more stable. This is somewhat machine specific as on a separate 80-cpu Broadwell machine the same test reported. 5.10.0-rc6 5.10.0-rc6 baseline-v2 altrecent-v2 Hmean 1 310.36 ( 0.00%) 291.81 * -5.98%* Hmean 2 340.86 ( 0.00%) 547.22 * 60.54%* Hmean 4 912.29 ( 0.00%) 1063.21 * 16.54%* Hmean 8 2116.40 ( 0.00%) 2103.60 * -0.60%* Hmean 16 4232.90 ( 0.00%) 4362.92 * 3.07%* Hmean 32 8442.03 ( 0.00%) 8642.10 * 2.37%* Hmean 64 11733.91 ( 0.00%) 11473.66 * -2.22%* Hmean 128 17727.24 ( 0.00%) 16784.23 * -5.32%* Hmean 256 16089.23 ( 0.00%) 16110.79 * 0.13%* Hmean 320 15992.60 ( 0.00%) 16071.64 * 0.49%* schedstats were not used in this series but from an earlier debugging effort, the schedstats after the test run were as follows; 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 With the patch, "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(-)