On Wed, 2025-07-02 at 12:17 +0530, Madadi Vineeth Reddy wrote:
> Hi Tim,
>
> On 18/06/25 23:57, Tim Chen wrote:
> > Switching a process's preferred LLC generates lots of task
> > migrations across LLCs. To avoid frequent switches
> > of home LLC, implement the following policy:
> >
> > 1. Require a 2x occ change threshold to switch preferred LLC
> > 2. Don't discard preferred LLC for a task
> >
> > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > ---
> > kernel/sched/fair.c | 24 ++++++++++++++++--------
> > 1 file changed, 16 insertions(+), 8 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 6a2678f9d44a..7fb2322c5d9e 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1175,6 +1175,14 @@ static s64 update_curr_se(struct rq *rq, struct sched_entity *curr)
> > #define EPOCH_PERIOD (HZ/100) /* 10 ms */
> > #define EPOCH_OLD 5 /* 50 ms */
> >
> > +static int llc_id(int cpu)
> > +{
> > + if (cpu < 0)
> > + return -1;
> > +
> > + return per_cpu(sd_llc_id, cpu);
> > +}
> > +
> > void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
> > {
> > unsigned long epoch;
> > @@ -1299,6 +1307,7 @@ static void task_cache_work(struct callback_head *work)
> > struct task_struct *p = current;
> > struct mm_struct *mm = p->mm;
> > unsigned long m_a_occ = 0;
> > + unsigned long last_m_a_occ = 0;
> > int cpu, m_a_cpu = -1;
> > cpumask_var_t cpus;
> >
> > @@ -1337,11 +1346,13 @@ static void task_cache_work(struct callback_head *work)
> > per_cpu(sd_llc_id, i), occ, m_occ, m_cpu, nr);
> > }
> >
> > - a_occ /= nr;
> > + // a_occ /= nr;
>
> Is the above by mistake?
> I think we need to have average only and not the total value as that favors LLCs with
> larger size.
>
Actually Chen Yu and I have gone back and forth on this one. A
different perspective is dividing by nr will disfavor
LLCs of larger size. You will need way more tasks in larger
LLC to put the tasks there, which may cause over-stacking on the
smaller LLC. We find that not dividing by nr is more stable when
we bring CPU online/offline.
Tim
> Thanks,
> Madadi Vineeth Reddy
>
> > if (a_occ > m_a_occ) {
> > m_a_occ = a_occ;
> > m_a_cpu = m_cpu;
> > }
> > + if (llc_id(cpu) == llc_id(mm->mm_sched_cpu))
> > + last_m_a_occ = a_occ;
> >
> > trace_printk("(%d) a_occ: %ld m_a_occ: %ld\n",
> > per_cpu(sd_llc_id, cpu), a_occ, m_a_occ);
> > @@ -1355,13 +1366,10 @@ static void task_cache_work(struct callback_head *work)
> > }
> > }
> >
> > - /*
> > - * If the max average cache occupancy is 'small' we don't care.
> > - */
> > - if (m_a_occ < (NICE_0_LOAD >> EPOCH_OLD))
> > - m_a_cpu = -1;
> > -
> > - mm->mm_sched_cpu = m_a_cpu;
> > + if (m_a_occ > (2 * last_m_a_occ)) {
> > + /* avoid the bouncing of mm_sched_cpu */
> > + mm->mm_sched_cpu = m_a_cpu;
> > + }
> >
> > free_cpumask_var(cpus);
> > }
>