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Message-ID: <5a46f38f-2398-41a9-8811-55a4dacb61a8@arm.com>
Date: Wed, 4 Feb 2026 09:53:24 +0000
From: Christian Loehle <christian.loehle@....com>
To: Shubhang Kaushik <shubhang@...amperecomputing.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
On 2/4/26 02:08, Shubhang Kaushik wrote:
> 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 ?
>
Like Prateek already mentioned, the load-balancing didn't change.
(I did s/sched_idle_cpu(cpu)/sched_idle_rq(rq)/ though because
sched_idle_cpu didn't have many callers left and removed it
entirely).
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