From: Christian Loehle Date: Tue, 3 Feb 2026 18:49:39 +0000 (+0000) Subject: sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=fd54d81c2c0e6cffd5470c2c27fbb04d0ebe7da0;p=thirdparty%2Flinux.git sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task CPUs whose rq only have SCHED_IDLE tasks running are considered to be equivalent to truly idle CPUs during wakeup path. For fork and exec SCHED_IDLE is even preferred. This is based on the assumption that the SCHED_IDLE CPU is not in an idle state and might be in a higher P-state, allowing the task/wakee to run immediately without sharing the rq. However this assumption doesn't hold if the wakee has SCHED_IDLE policy itself, as it will share the rq with existing SCHED_IDLE tasks. In this case, we are better off continuing to look for a truly idle CPU. On a Intel Xeon 2-socket with 64 logical cores in total this yields for kernel compilation using SCHED_IDLE: +---------+----------------------+----------------------+--------+ | workers | mainline (seconds) | patch (seconds) | delta% | +=========+======================+======================+========+ | 1 | 4384.728 ± 21.085 | 3843.250 ± 16.235 | -12.35 | | 2 | 2242.513 ± 2.099 | 1971.696 ± 2.842 | -12.08 | | 4 | 1199.324 ± 1.823 | 1033.744 ± 1.803 | -13.81 | | 8 | 649.083 ± 1.959 | 559.123 ± 4.301 | -13.86 | | 16 | 370.425 ± 0.915 | 325.906 ± 4.623 | -12.02 | | 32 | 234.651 ± 2.255 | 217.266 ± 0.253 | -7.41 | | 64 | 202.286 ± 1.452 | 197.977 ± 2.275 | -2.13 | | 128 | 217.092 ± 1.687 | 212.164 ± 1.138 | -2.27 | +---------+----------------------+----------------------+--------+ Signed-off-by: Christian Loehle Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Vincent Guittot Tested-by: K Prateek Nayak Link: https://patch.msgid.link/20260203184939.2138022-1-christian.loehle@arm.com --- diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 966e252822154..d57c02e82f3a1 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7064,9 +7064,15 @@ static int sched_idle_rq(struct rq *rq) rq->nr_running); } -static int sched_idle_cpu(int cpu) +static int choose_sched_idle_rq(struct rq *rq, struct task_struct *p) { - return sched_idle_rq(cpu_rq(cpu)); + return sched_idle_rq(rq) && !task_has_idle_policy(p); +} + +static int choose_idle_cpu(int cpu, struct task_struct *p) +{ + return available_idle_cpu(cpu) || + choose_sched_idle_rq(cpu_rq(cpu), p); } static void @@ -7631,7 +7637,7 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct * if (!sched_core_cookie_match(rq, p)) continue; - if (sched_idle_cpu(i)) + if (choose_sched_idle_rq(rq, p)) return i; if (available_idle_cpu(i)) { @@ -7722,8 +7728,7 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas static inline int __select_idle_cpu(int cpu, struct task_struct *p) { - if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) && - sched_cpu_cookie_match(cpu_rq(cpu), p)) + if (choose_idle_cpu(cpu, p) && sched_cpu_cookie_match(cpu_rq(cpu), p)) return cpu; return -1; @@ -7796,7 +7801,8 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu if (!available_idle_cpu(cpu)) { idle = false; if (*idle_cpu == -1) { - if (sched_idle_cpu(cpu) && cpumask_test_cpu(cpu, cpus)) { + if (choose_sched_idle_rq(cpu_rq(cpu), p) && + cpumask_test_cpu(cpu, cpus)) { *idle_cpu = cpu; break; } @@ -7831,7 +7837,7 @@ static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int t */ if (!cpumask_test_cpu(cpu, sched_domain_span(sd))) continue; - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) + if (choose_idle_cpu(cpu, p)) return cpu; } @@ -7953,7 +7959,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target) for_each_cpu_wrap(cpu, cpus, target) { unsigned long cpu_cap = capacity_of(cpu); - if (!available_idle_cpu(cpu) && !sched_idle_cpu(cpu)) + if (!choose_idle_cpu(cpu, p)) continue; fits = util_fits_cpu(task_util, util_min, util_max, cpu); @@ -8024,7 +8030,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) */ lockdep_assert_irqs_disabled(); - if ((available_idle_cpu(target) || sched_idle_cpu(target)) && + if (choose_idle_cpu(target, p) && asym_fits_cpu(task_util, util_min, util_max, target)) return target; @@ -8032,7 +8038,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) * If the previous CPU is cache affine and idle, don't be stupid: */ if (prev != target && cpus_share_cache(prev, target) && - (available_idle_cpu(prev) || sched_idle_cpu(prev)) && + choose_idle_cpu(prev, p) && asym_fits_cpu(task_util, util_min, util_max, prev)) { if (!static_branch_unlikely(&sched_cluster_active) || @@ -8064,7 +8070,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target) if (recent_used_cpu != prev && recent_used_cpu != target && cpus_share_cache(recent_used_cpu, target) && - (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)) && + choose_idle_cpu(recent_used_cpu, p) && cpumask_test_cpu(recent_used_cpu, p->cpus_ptr) && asym_fits_cpu(task_util, util_min, util_max, recent_used_cpu)) { @@ -12531,7 +12537,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle) { int continue_balancing = 1; int cpu = rq->cpu; - int busy = idle != CPU_IDLE && !sched_idle_cpu(cpu); + int busy = idle != CPU_IDLE && !sched_idle_rq(rq); unsigned long interval; struct sched_domain *sd; /* Earliest time when we have to do rebalance again */ @@ -12569,7 +12575,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle) * state even if we migrated tasks. Update it. */ idle = idle_cpu(cpu); - busy = !idle && !sched_idle_cpu(cpu); + busy = !idle && !sched_idle_rq(rq); } sd->last_balance = jiffies; interval = get_sd_balance_interval(sd, busy);