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Message-ID: <6895a358-c993-74d5-d804-3b7130ee1559@os.amperecomputing.com>
Date: Tue, 3 Feb 2026 18:08:33 -0800 (PST)
From: Shubhang Kaushik <shubhang@...amperecomputing.com>
To: Christian Loehle <christian.loehle@....com>, linux-kernel@...r.kernel.org,
peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org
cc: juri.lelli@...hat.com, dietmar.eggemann@....com, kprateek.nayak@....com,
pierre.gondois@....com
Subject: Re: [PATCHv2] sched/fair: Skip SCHED_IDLE rq for SCHED_IDLE task
Hi Christian Loehle,
On Tue, 3 Feb 2026, Christian Loehle wrote:
> 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 <christian.loehle@....com>
> ---
> v2: Reword commit message, SCHED_IDLE aren't always preferred,
> but rather equivalent
> Factor out choose_sched_idle_rq() too (Both Vincent)
>
> kernel/sched/fair.c | 32 +++++++++++++++++++-------------
> 1 file changed, 19 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3eaeceda71b0..6510ab6eb44b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6832,9 +6832,15 @@ 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
> @@ -7400,7 +7406,7 @@ 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)) {
> @@ -7491,8 +7497,7 @@ 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;
> @@ -7565,7 +7570,8 @@ 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;
> }
> @@ -7600,7 +7606,7 @@ 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;
> }
>
> @@ -7722,7 +7728,7 @@ 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);
> @@ -7793,7 +7799,7 @@ 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;
>
> @@ -7801,7 +7807,7 @@ 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) ||
> @@ -7833,7 +7839,7 @@ 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)) {
>
> @@ -12261,7 +12267,7 @@ 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 */
> @@ -12299,7 +12305,7 @@ 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);
Usually sched_idle_rqs were treated as not-busy in several balancing
decisions to avoid yielding to background load. Does this change alter
that interpretation at the domain balancing level ?
> }
> sd->last_balance = jiffies;
> interval = get_sd_balance_interval(sd, busy);
> --
> 2.34.1
>
>
Regards,
Shubhang Kaushik
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