From: Rik van Riel riel@surriel.com
[ Upstream commit c722f35b513f807629603bbf24640b1a48be21b5 ]
Mel Gorman did some nice work in 9fe1f127b913 ("sched/fair: Merge select_idle_core/cpu()"), resulting in the kernel being more efficient at finding an idle CPU, and in tasks spending less time waiting to be run, both according to the schedstats run_delay numbers, and according to measured application latencies. Yay.
The flip side of this is that we see more task migrations (about 30% more), higher cache misses, higher memory bandwidth utilization, and higher CPU use, for the same number of requests/second.
This is most pronounced on a memcache type workload, which saw a consistent 1-3% increase in total CPU use on the system, due to those increased task migrations leading to higher L2 cache miss numbers, and higher memory utilization. The exclusive L3 cache on Skylake does us no favors there.
On our web serving workload, that effect is usually negligible.
It appears that the increased number of CPU migrations is generally a good thing, since it leads to lower cpu_delay numbers, reflecting the fact that tasks get to run faster. However, the reduced locality and the corresponding increase in L2 cache misses hurts a little.
The patch below appears to fix the regression, while keeping the benefit of the lower cpu_delay numbers, by reintroducing select_idle_smt with a twist: when a socket has no idle cores, check to see if the sibling of "prev" is idle, before searching all the other CPUs.
This fixes both the occasional 9% regression on the web serving workload, and the continuous 2% CPU use regression on the memcache type workload.
With Mel's patches and this patch together, task migrations are still high, but L2 cache misses, memory bandwidth, and CPU time used are back down to what they were before. The p95 and p99 response times for the memcache type application improve by about 10% over what they were before Mel's patches got merged.
Signed-off-by: Rik van Riel riel@surriel.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Mel Gorman mgorman@techsingularity.net Acked-by: Vincent Guittot vincent.guittot@linaro.org Link: https://lkml.kernel.org/r/20210326151932.2c187840@imladris.surriel.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 47 ++++++++++++++++++++++++++++++++++++++------- 1 file changed, 40 insertions(+), 7 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 2a4041e3178f..54ca03bacddf 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6107,6 +6107,24 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu return -1; }
+/* + * Scan the local SMT mask for idle CPUs. + */ +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target) +{ + int cpu; + + for_each_cpu(cpu, cpu_smt_mask(target)) { + if (!cpumask_test_cpu(cpu, p->cpus_ptr) || + !cpumask_test_cpu(cpu, sched_domain_span(sd))) + continue; + if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) + return cpu; + } + + return -1; +} + #else /* CONFIG_SCHED_SMT */
static inline void set_idle_cores(int cpu, int val) @@ -6123,6 +6141,11 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma return __select_idle_cpu(core); }
+static inline int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target) +{ + return -1; +} + #endif /* CONFIG_SCHED_SMT */
/* @@ -6130,11 +6153,10 @@ static inline int select_idle_core(struct task_struct *p, int core, struct cpuma * 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, bool has_idle_core, int target) { struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask); int i, cpu, idle_cpu = -1, nr = INT_MAX; - bool smt = test_idle_cores(target, false); int this = smp_processor_id(); struct sched_domain *this_sd; u64 time; @@ -6145,7 +6167,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
- if (sched_feat(SIS_PROP) && !smt) { + if (sched_feat(SIS_PROP) && !has_idle_core) { u64 avg_cost, avg_idle, span_avg;
/* @@ -6165,7 +6187,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t }
for_each_cpu_wrap(cpu, cpus, target) { - if (smt) { + if (has_idle_core) { i = select_idle_core(p, cpu, cpus, &idle_cpu); if ((unsigned int)i < nr_cpumask_bits) return i; @@ -6179,10 +6201,10 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t } }
- if (smt) + if (has_idle_core) set_idle_cores(this, false);
- if (sched_feat(SIS_PROP) && !smt) { + if (sched_feat(SIS_PROP) && !has_idle_core) { time = cpu_clock(this) - time; update_avg(&this_sd->avg_scan_cost, time); } @@ -6237,6 +6259,7 @@ static inline bool asym_fits_capacity(int task_util, int cpu) */ static int select_idle_sibling(struct task_struct *p, int prev, int target) { + bool has_idle_core = false; struct sched_domain *sd; unsigned long task_util; int i, recent_used_cpu; @@ -6316,7 +6339,17 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) if (!sd) return target;
- i = select_idle_cpu(p, sd, target); + if (sched_smt_active()) { + has_idle_core = test_idle_cores(target, false); + + if (!has_idle_core && cpus_share_cache(prev, target)) { + i = select_idle_smt(p, sd, prev); + if ((unsigned int)i < nr_cpumask_bits) + return i; + } + } + + i = select_idle_cpu(p, sd, has_idle_core, target); if ((unsigned)i < nr_cpumask_bits) return i;