Re: [PATCH] sched/stats: Fix run_delay over-count for proxy-migrated blocked donors

[albin_yang]

Hi Kayra,

Thanks for your review.

> ... sched_info_dequeue() ... Or Am I missing something?

No, you're right: the fake enqueue starts the counter at migration time, and
the bill is settled later in sched_info_dequeue() on the proxy_needs_return()
path (dequeue_block_task() with TASK_WAKING). The ~167 ms I measured is that
delta. I had written sched_info_arrive() charging it directly, which is wrong -
I'll fix the description.

> This should be a series instead of separate patches if this goes above.

Agreed - I'll resend as [PATCH v5 1/2] (DELAY_DEQUEUE fix) + [PATCH v5 2/2]
(this one).

> ... when B gets a kill signal B need to enqueue separately from A, so it
> starts to wait. But blocked_on is set, so B's counter never starts.

A case I hadn't considered, and my sanity check does not cover it: I only
verified that a donor waking normally and then contending for the CPU still
accumulates run_delay. A waiter enqueuing separately from its owner does
genuinely start waiting for a CPU.

So let me ask first: in that path, is blocked_on really still set when B is
enqueued separately? If yes, !task_is_blocked() would under-count B, and the
options as I see them are:

  a) a predicate that is also cleared on that path,
  b) clear blocked_on there (proxy-exec territory - for you/John/Peter),
  c) hold this patch until John's series lands and re-evaluate then,
  d) fix it at the source instead: give proxy_migrate_task() DEQUEUE_SAVE /
     ENQUEUE_RESTORE so the migration never touches sched_info at all. That
     sidesteps the blocked_on question entirely, but it also affects psi, rt,
     dl and sched_ext, so I'd want your/John's input first.

Do you know whether blocked_on is still set there? If it is, I'd still prefer
to take this now, since the under-count only shows up once John's series is in,
and revisit (or adjust) then - unless you'd rather wait.

Adding John to CC.

Thanks,
Wei Yang