Re: [PATCH 2/4] sched/cache: Add PR_SCHED_CACHE prctl for per-mm control

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Yangyu,

On 7/22/2026 5:10 PM, Yangyu Chen wrote:
> Cache aware scheduling is currently controlled only through global
> debugfs knobs, but the right aggressiveness is workload and platform
> specific. A multi-threaded Verilator run is one example: its RSS is
> large while only a small part of it is hot, so an RSS-based footprint
> estimate should not decide whether it is aggregated; and packing its
> threads onto the SMT siblings of one LLC beats spreading them across
> LLCs on some platforms (e.g. AMD EPYC Turin) but not on others (e.g.
> EPYC Milan). Such choices cannot be made globally for the whole
> machine. Add a prctl interface to override the knobs per process
> (per mm_struct):
> 
>    prctl(PR_SCHED_CACHE, PR_SCHED_CACHE_SET, attr, value, 0);
>    prctl(PR_SCHED_CACHE, PR_SCHED_CACHE_GET, attr, &value, 0);
> 
> A single prctl command implements both directions, like
> PR_RSEQ_SLICE_EXTENSION. The attributes are a per-process enable
> (effective only while the feature is globally active), the two
> aggregation tolerances, the overaggr percentage (applied where a
> task's own migration is admitted; group level statistics span many
> processes and keep using the global value), and an inherit mask
> selecting which attributes an mm created by execve() keeps. fork()
> always inherits everything, and the mask itself lives on the
> task_struct so it survives both, which lets a numactl-like launcher
> configure a workload and exec it.
> 
> The overrides live in mm->sc_stat with -1 meaning "follow the global
> default"; GET stores the raw value through an int pointer so this
> sentinel round-trips without being mistaken for an errno.
> mm_init_sched() gains the creating task to tell fork (p != current)
> from exec (p == current) apart. A disabled mm has its preferred LLC
> invalidated at the existing invalidation points, so all group-level
> statistics self-neutralize.
> 
> Also sync the tools/perf/trace/beauty copy of prctl.h.
> 
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Yangyu Chen <cyy@cyyself.name>

[ ... ]

> +static int sched_cache_set_attr(unsigned long attr, unsigned long val)
> +{
> +	struct mm_struct *mm = current->mm;

As preparation work, should we first decouple sc_stat from
mm_struct and tie this stat to per-task task_struct? In this
way, we could have per-task cache preference control and extend
it to tasks/threads/process/cgroup if needed, which looks more
flexible IMO. We have a proposal here:
https://github.com/chen-yu-surf/linux/commit/bd43a0b6dd189d5091fb88630208cb7bf67b3165.patch

which introduces a pointer in task_struct:
struct sched_cache_group __rcu  *sched_cache_grp;

> +	bool def = sched_cache_val_default(val);
> +	int ival = def ? -1 : (int)val;
> +
> +	switch (attr) {
> +	case PR_SCHED_CACHE_ENABLE:
> +		if (!def && val > 1)
> +			return -EINVAL;
> +		WRITE_ONCE(mm->sc_stat.user_enabled, ival);
> +		/*
> +		 * Drop the preferred LLC hint on any change: a process
> +		 * that became disabled must stop being honored right
> +		 * away, and one that became enabled re-establishes the
> +		 * hint within an epoch anyway. This is best effort: an
> +		 * in-flight task_cache_work() scan re-checks the enable
> +		 * before publishing a new preference, and a lost race
> +		 * is corrected at the next tick.
> +		 */
> +		WRITE_ONCE(mm->sc_stat.cpu, -1);
> +		break;

After we switching from per mm_struct to per task control, we could provide
fine-gain control at task/process/process group granularity(similar to 
core-scheduling)

int prctl(PR_SCHED_CACHE, unsigned long subop, pid_t pid,
           unsigned long cookie, unsigned long type);
pid argument: the PID of the target task. 0 means "the calling task."
pid_type : PIDTYPE_PID targets the single thread,
            PIDTYPE_TGID the whole thread group and PIDTYPE_PGID the process
            group of the target task.

And the proposal is here:
https://github.com/chen-yu-surf/linux/commit/17718b7cef1d03948e9fd3bcd0b5a49aba7aae2d.patch

thanks,
Chenyu