On Thu, Jan 20, 2022 at 11:31:56AM +0800, Huangzhaoyang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
>
> When a new threshold breaching stall happens after a psi event was
> generated and within the window duration, the new event is not
> generated because the events are rate-limited to one per window. If
> after that no new stall is recorded then the event will not be
> generated even after rate-limiting duration has passed. This is
> happening because with no new stall, window_update will not be called
> even though threshold was previously breached. To fix this, record
> threshold breaching occurrence and generate the event once window
> duration is passed.
>
> Suggested-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
Good catch. The change makes sense to me.
However, I had to re-read the discussion to understand *why*
triggering once per window can be a practical problem. Could you
please include the lkmd scenario you mentioned?
Suren, even though it's your suggested code, can you please also add
ack/review tags? Thanks!
Some minor inline comments:
> diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> index 0a23300..87b694a 100644
> --- a/include/linux/psi_types.h
> +++ b/include/linux/psi_types.h
> @@ -132,6 +132,8 @@ struct psi_trigger {
>
> /* Refcounting to prevent premature destruction */
> struct kref refcount;
> +
> + bool threshold_breach;
Something like bool pending_event would be more descriptive, IMO.
Also please remember to add a short comment like we have for the other
struct members. For example:
/* Deferred event(s) from previous ratelimit window */
> @@ -524,24 +524,29 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> */
> list_for_each_entry(t, &group->triggers, node) {
> u64 growth;
> + bool trigger_stalled =
> + group->polling_total[t->state] != total[t->state];
Triggers don't stall, they trigger on stalls. How about this:
bool new_stall;
u64 growth;
new_stall = group->polling_total[t->state] != total[t->state];
(order local declarations by length, avoid line wraps where possible)
> - /* Check for stall activity */
> - if (group->polling_total[t->state] == total[t->state])
> - continue;
> -
> - /*
> - * Multiple triggers might be looking at the same state,
> - * remember to update group->polling_total[] once we've
> - * been through all of them. Also remember to extend the
> - * polling time if we see new stall activity.
> - */
> - new_stall = true;
> -
> - /* Calculate growth since last update */
> - growth = window_update(&t->win, now, total[t->state]);
> - if (growth < t->threshold)
> + /* Check for stall activity or a previous threshold breach */
> + if (!trigger_stalled && !t->threshold_breach)
> continue;
This could use a bit more explanation imo:
/*
* Check for new stall activity, as well as deferred
* events that occurred in the last window after the
* trigger had already fired (we want to ratelimit
* events without dropping any).
*/
if (!new_stall && !t->pending_event)
continue;
> + if (trigger_stalled) {
> + /*
> + * Multiple triggers might be looking at the same state,
> + * remember to update group->polling_total[] once we've
> + * been through all of them. Also remember to extend the
> + * polling time if we see new stall activity.
> + */
> + new_stall = true;
and then rename this flag `update_total'.
> + /* Calculate growth since last update */
> + growth = window_update(&t->win, now, total[t->state]);
> + if (growth < t->threshold)
> + continue;
> +
> + t->threshold_breach = true;
> + }
> /* Limit event signaling to once per window */
> if (now < t->last_event_time + t->win.size)
> continue;
> @@ -550,6 +555,8 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> if (cmpxchg(&t->event, 0, 1) == 0)
> wake_up_interruptible(&t->event_wait);
> t->last_event_time = now;
> + /* Reset threshold breach flag once event got generated */
> + t->threshold_breach = false;
> }
>
> if (new_stall)
> @@ -1152,6 +1159,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
> t->last_event_time = 0;
> init_waitqueue_head(&t->event_wait);
> kref_init(&t->refcount);
> + t->threshold_breach = false;
>
> mutex_lock(&group->trigger_lock);
Thanks!
