Re: [PATCH] sched/numa: Fix scan period for remote private faults

[Zhan Xusheng]

From: Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>

On Tue, Aug 04, 2026 at 11:07:31AM +0800, Hongling Zeng wrote:
> This is wrong because for remote private memory, we should continue
> to the ratio calculation which can speed up scanning to migrate the
> memory to the local node.

I don't think the ratio calculation actually speeds scanning up in that
case, though. For the pure remote-private accesses you describe
(shared == 0, private > 0):

	ps_ratio = private * NUMA_PERIOD_SLOTS / (private + shared)
	         = private * 10 / (private + 0)
	         = 10

which is >= NUMA_PERIOD_THRESHOLD (7), so it takes the first branch:

	int slot = ps_ratio - NUMA_PERIOD_THRESHOLD;   /* 3 */
	diff = slot * period_slot;                     /* > 0 */

and numa_scan_period is *increased* (scan slower), not decreased. The
speed-up (else) branch is only reached when both ps_ratio < 7 and
lr_ratio < 7, which pure-private accesses (ps_ratio == 10) never satisfy.

So dropping the early return here doesn't speed scanning up; it just
grows the period by ~3 slots instead of doubling it. That might still be
a reasonable change, but the justification as written describes an effect
that doesn't seem to happen. Could you double-check, and share some
before/after numbers on a remote-private workload? A scan-rate change
like this really wants data behind it.

Two smaller things:

  - The comment rewrites (ps_ratio -> "private", lr_ratio -> "local") look
    like a reasonable cleanup on their own, but folding them into a
    behavioural change makes the patch harder to review -- perhaps split
    them out. (The lr_ratio branch also keeps the "shared ... moved by
    other tasks" sentence, which no longer fits a local-dominant branch.)

  - This is the same early return that other in-flight patches touch (the
    numa_faults_locality reset thread, where Peter suggested sharing the
    tail with the normal path). It may be worth coordinating so the
    changes don't collide.

Thanks,
Zhan Xusheng