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sched/topology: Allow multiple domains to claim sched_domain_shared

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Recent optimizations of sd->shared assignment moved to allocating a
single instance of per-CPU sched_domain_shared objects per s_data.

Recent optimizations to select_idle_capacity() moved the sd->shared
assignments to "sd_asym" domain when ASYM_CPUCAPACITY is detected but
cache-aware scheduling mandates the presence of "sd_llc_shared" to
compute and cache per-LLC statistics.

Use an "alloc_flags" union in sched_domain_shared to claim a
sched_domain_shared object per sched_domain. Allocation starts searching
for an available / matching sched_domain_shared instance from the first
CPU of sched_domain_span(sd) (sd can be sd_llc, or sd_asym). If the
shared object is claimed by another domain, the instance corresponding
to next CPU in the domain span is explored until a matching / available
instance is found.

In case of a single CPU in sched_domain_span(), the domain will be
degenerated and a temporary overlap of ->shared objects across different
domains is acceptable.

"alloc_flags" forms a union with "nr_idle_scan" and the stale flags are
left as is when the sd->shared is published. The expectation is for the
first load balancing instance to correct the value just like the current
behavior, except the initial value is no longer 0.

Originally-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
K Prateek Nayak
2026-05-19 05:14:23 +00:00
committed by Peter Zijlstra
parent a26d9208c1
commit 9e005ed211
2 changed files with 69 additions and 10 deletions
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include/linux/sched/topology.h
+15 -1
View File
@@ -67,7 +67,21 @@ struct sched_domain_shared {
atomic_t ref;
atomic_t nr_busy_cpus;
int has_idle_cores;
int nr_idle_scan;
union {
int nr_idle_scan;
/*
* Used during allocation to claim the sched_domain_shared
* object at multiple levels.
*
* Note: between build and the first periodic LB tick, which
* rewrites the union via update_idle_cpu_scan(), readers of
* nr_idle_scan may observe the transient SD_* flag value as
* the scan bound. The flag bits are small positive integers,
* so the effect is just a slightly relaxed scan bound for one
* window and self-heals on the first tick.
*/
int alloc_flags;
};
#ifdef CONFIG_SCHED_CACHE
unsigned long util_avg;
unsigned long capacity;
kernel/sched/topology.c
+54 -9
View File
@@ -623,6 +623,12 @@ static void free_sched_groups(struct sched_group *sg, int free_sgc)
} while (sg != first);
}
static void free_sched_domain_shared(struct sched_domain_shared *sds)
{
if (sds && atomic_dec_and_test(&sds->ref))
kfree(sds);
}
static void destroy_sched_domain(struct sched_domain *sd)
{
/*
@@ -631,9 +637,7 @@ static void destroy_sched_domain(struct sched_domain *sd)
* dropping group/capacity references, freeing where none remain.
*/
free_sched_groups(sd->groups, 1);
if (sd->shared && atomic_dec_and_test(&sd->shared->ref))
kfree(sd->shared);
free_sched_domain_shared(sd->shared);
#ifdef CONFIG_SCHED_CACHE
/* only the bottom sd has llc_counts array */
@@ -755,7 +759,14 @@ cpu_attach_domain(struct sched_domain *sd, struct root_domain *rd, int cpu)
/* Pick reference to parent->shared. */
if (parent->shared) {
WARN_ON_ONCE(tmp->shared);
/*
* It is safe to free a sd->shared that
* has not been published yet. If a
* sd->shared was published, the refcount
* will end up being non-zero and it will
* not be freed here.
*/
free_sched_domain_shared(tmp->shared);
tmp->shared = parent->shared;
parent->shared = NULL;
}
@@ -2916,11 +2927,45 @@ static void adjust_numa_imbalance(struct sched_domain *sd_llc)
}
}
static void init_sched_domain_shared(struct s_data *d, struct sched_domain *sd)
static void
init_sched_domain_shared(struct s_data *d, struct sched_domain *sd, int flags)
{
int sd_id = cpumask_first(sched_domain_span(sd));
struct sched_domain_shared *sds = NULL;
int cpu;
/*
* Multiple domains can try to claim a shared object like
* SD_ASYM_CPUCAPACITY and SD_SHARE_LLC which can alias to
* same cpumask_first(sched_domain_span(sd)) CPU and can
* cause "nr_idle_scan" to be populated incorrectly during
* load balancing.
*
* Find the first CPU in sched_domain_span(sd) with an
* unclaimed domain (!alloc_flags) or where the alloc_flag
* matches the requested flag (SD_* flag)
*
* If the domain only has single CPU, allow temporary overlap
* in allocation since the domains will be degenerated later.
*/
for_each_cpu(cpu, sched_domain_span(sd)) {
sds = *per_cpu_ptr(d->sds, cpu);
if (!sds->alloc_flags ||
sd->span_weight == 1 ||
sds->alloc_flags == flags) {
sds->alloc_flags = flags;
sd->shared = sds;
break;
}
}
/*
* Use the sd_shared corresponding to the last
* CPU in the span if none are avaialable.
*/
if (WARN_ON_ONCE(!sd->shared))
sd->shared = sds;
sd->shared = *per_cpu_ptr(d->sds, sd_id);
/*
* nr_busy_cpus is consumed only by the NOHZ kick path via
* sd_balance_shared; on the asym-capacity path it is initialized but
@@ -2960,7 +3005,7 @@ static bool claim_asym_sched_domain_shared(struct s_data *d, int cpu)
if (!sd_asym || (sd_asym->flags & SD_NUMA))
return false;
init_sched_domain_shared(d, sd_asym);
init_sched_domain_shared(d, sd_asym, SD_ASYM_CPUCAPACITY);
return true;
}
@@ -3115,7 +3160,7 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
sd = sd->parent;
if (sd->flags & SD_SHARE_LLC) {
init_sched_domain_shared(&d, sd);
init_sched_domain_shared(&d, sd, SD_SHARE_LLC);
/*
* In presence of higher domains, adjust the