Re: [PATCH v2 17/23] sched/cache: Record the number of active threads per process for cache-aware scheduling

[Chen, Yu C]

On 12/11/2025 12:51 AM, Peter Zijlstra wrote:
> On Wed, Dec 03, 2025 at 03:07:36PM -0800, Tim Chen wrote:
> > From: Chen Yu <yu.c.chen@xxxxxxxxx>
> >
> > A performance regression was observed by Prateek when running hackbench
> > with many threads per process (high fd count). To avoid this, processes
> > with a large number of active threads are excluded from cache-aware
> > scheduling.
> >
> > With sched_cache enabled, record the number of active threads in each
> > process during the periodic task_cache_work(). While iterating over
> > CPUs, if the currently running task belongs to the same process as the
> > task that launched task_cache_work(), increment the active thread count.
> >
> > This number will be used by subsequent patch to inhibit cache aware
> > load balance.
> >
> > Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> > Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
> > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> > ---
> >
> > Notes:
> >      v1->v2: No change.
> >
> >   include/linux/mm_types.h |  1 +
> >   kernel/sched/fair.c      | 11 +++++++++--
> >   2 files changed, 10 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 1ea16ef90566..04743983de4d 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1043,6 +1043,7 @@ struct mm_struct {
> >   		raw_spinlock_t mm_sched_lock;
> >   		unsigned long mm_sched_epoch;
> >   		int mm_sched_cpu;
> > +		u64 nr_running_avg ____cacheline_aligned_in_smp;
>
> This is unlikely to do what you hope it does, it will place this
> variable on a new cacheline, but will not ensure this variable is the
> only one in that line. Notably ogtables_bytes (the next field in this
> structure) will share the line.
>
> It might all be less dodgy if you stick these here fields in their own
> structure, a little like mm_mm_cid or so.

Got it, will do.

> >   #endif
> >   
> >   #ifdef CONFIG_MMU
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 580a967efdac..2f38ad82688f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -1421,11 +1421,11 @@ static void task_tick_cache(struct rq *rq, struct task_struct *p)
> >   
> >   static void __no_profile task_cache_work(struct callback_head *work)
> >   {
> > -	struct task_struct *p = current;
> > +	struct task_struct *p = current, *cur;
> >   	struct mm_struct *mm = p->mm;
> >   	unsigned long m_a_occ = 0;
> >   	unsigned long curr_m_a_occ = 0;
> > -	int cpu, m_a_cpu = -1;
> > +	int cpu, m_a_cpu = -1, nr_running = 0;
> >   	cpumask_var_t cpus;
> >   
> >   	WARN_ON_ONCE(work != &p->cache_work);
> > @@ -1458,6 +1458,12 @@ static void __no_profile task_cache_work(struct callback_head *work)
> >   					m_occ = occ;
> >   					m_cpu = i;
> >   				}
>
> 	guard(rcu)();

OK.

> > +				rcu_read_lock();
> > +				cur = rcu_dereference(cpu_rq(i)->curr);
> > +				if (cur && !(cur->flags & (PF_EXITING | PF_KTHREAD)) &&
> > +				    cur->mm == mm)
> > +					nr_running++;
> > +				rcu_read_unlock();
> >   			}
> >   
> >   			/*
> > @@ -1501,6 +1507,7 @@ static void __no_profile task_cache_work(struct callback_head *work)
> >   		mm->mm_sched_cpu = m_a_cpu;
> >   	}
> >   
> > +	update_avg(&mm->nr_running_avg, nr_running);
> >   	free_cpumask_var(cpus);
> >   }
>
> Its a wee bit weird to introduce nr_running_avg without its user. Makes
> it hard to see what's what.

OK, will put the user together.

thanks,
Chenyu