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sched/cache: Skip cache-aware scheduling for single-threaded processes

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For a single thread, the current wakeup path tends to place it
on the same LLC where it was previously running with cache-hot
data. There is no need to enable cache-aware scheduling for
single-threaded processes for the following reasons:

1. Cache-aware scheduling primarily benefits multi-threaded
   processes where threads share data. Single-threaded processes
   typically have no inter-thread data sharing and thus gain little.

2. Enabling it incurs the additional overhead of tracking the
   thread's residency in the LLCs.

3. Bypassing single-threaded processes avoids excessive
   concentration of such tasks on a single LLC.

Nevertheless, this check can be omitted if users explicitly
provide hints for such single-threaded workloads where different
processes have shared memory, e.g., via prctl() or other interfaces
to be added in the future.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Co-developed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Tingyin Duan <tingyin.duan@gmail.com>
Link: https://patch.msgid.link/8a59a13aa58fdb48e410ecb2aabd97fe3ea5d256.1778703694.git.tim.c.chen@linux.intel.com
This commit is contained in:
Chen Yu 2026-05-13 13:39:14 -07:00 committed by Peter Zijlstra
parent deee5e27d5
commit 7b34bb1ca3
1 changed files with 16 additions and 4 deletions
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20
kernel/sched/fair.c
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@ -1384,8 +1384,12 @@ static int llc_id(int cpu)
return per_cpu(sd_llc_id, cpu);
}
static bool invalid_llc_nr(struct mm_struct *mm, int cpu)
static bool invalid_llc_nr(struct mm_struct *mm, struct task_struct *p,
int cpu)
{
if (get_nr_threads(p) <= 1)
return true;
return !fits_capacity((mm->sc_stat.nr_running_avg * cpu_smt_num_threads),
per_cpu(sd_llc_size, cpu));
}
@ -1581,7 +1585,7 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
* its preferred state.
*/
if (epoch - READ_ONCE(mm->sc_stat.epoch) > EPOCH_LLC_AFFINITY_TIMEOUT ||
invalid_llc_nr(mm, cpu_of(rq))) {
invalid_llc_nr(mm, p, cpu_of(rq))) {
if (mm->sc_stat.cpu != -1)
mm->sc_stat.cpu = -1;
}
@ -1687,9 +1691,9 @@ static inline void update_avg_scale(u64 *avg, u64 sample)
static void task_cache_work(struct callback_head *work)
{
int cpu, m_a_cpu = -1, nr_running = 0, curr_cpu;
unsigned long next_scan, now = jiffies;
struct task_struct *p = current, *cur;
int cpu, m_a_cpu = -1, nr_running = 0;
unsigned long curr_m_a_occ = 0;
struct mm_struct *mm = p->mm;
unsigned long m_a_occ = 0;
@ -1711,6 +1715,14 @@ static void task_cache_work(struct callback_head *work)
now + EPOCH_PERIOD))
return;
curr_cpu = task_cpu(p);
if (invalid_llc_nr(mm, p, curr_cpu)) {
if (mm->sc_stat.cpu != -1)
mm->sc_stat.cpu = -1;
return;
}
if (!zalloc_cpumask_var(&cpus, GFP_KERNEL))
return;
@ -10326,7 +10338,7 @@ static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
return mig_unrestricted;
/* skip cache aware load balance for too many threads */
if (invalid_llc_nr(mm, dst_cpu)) {
if (invalid_llc_nr(mm, p, dst_cpu)) {
if (mm->sc_stat.cpu != -1)
mm->sc_stat.cpu = -1;
return mig_unrestricted;