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SubjectRe: [RFC 2/2] perf: Sharing PMU counters across compatible events
On Fri, May 04, 2018 at 04:11:02PM -0700, Song Liu wrote:
> +/*
> + * If an dup event is already active, add this event as follower, and
> + * return 0; otherwise, return -EAGAIN
> + *
> + * RFC NOTE: this an o(1) operation
> + */
> +static int event_dup_try_add_follower(struct perf_event *event,
> + struct perf_cpu_context *cpuctx)
> +{
> + struct perf_event_dup *pdup;
> +
> + if (event->dup_id >= cpuctx->dup_event_count)
> + return -EAGAIN;
> +
> + pdup = &cpuctx->dup_event_list[event->dup_id];
> + if (list_empty(&pdup->active_dup))
> + return -EAGAIN;
> +
> + list_add_tail(&event->dup_sibling_entry, &pdup->active_dup);
> + pdup->master->pmu->read(pdup->master);
> + event->dup_base_count = pdup_read_count(pdup);
> + event->dup_base_child_count = pdup_read_child_count(pdup);
> + return 0;
> +}

> +/*
> + * remove event from the dup list; if it is the master, and there are
> + * other active events, promote another event as the new master.
> + *
> + * return 0, if it is there are more active events in this dup;
> + * return -EAGAIN, if it is the last active event
> + *
> + * RFC NOTE: this an o(1) operation
> + */
> +static int event_dup_sched_out(struct perf_event *event,
> + struct perf_cpu_context *cpuctx)
> +{
> + struct perf_event_dup *pdup;
> +
> + if (event->dup_id >= cpuctx->dup_event_count)
> + return -EAGAIN;
> +
> + pdup = &cpuctx->dup_event_list[event->dup_id];
> + list_del_init(&event->dup_sibling_entry);
> + if (event == pdup->master ) {
> + if (list_empty(&pdup->active_dup)) {
> + pdup->master = NULL;
> + return -EAGAIN;

This one is really odd.. -EAGAIN doesn't make sense for the last event.
I see how you got here, but yuck.

> + } else {
> + struct perf_event *new_master;
> +
> + new_master = list_first_entry(
> + &cpuctx->dup_event_list[event->dup_id].active_dup,
> + struct perf_event, dup_sibling_entry);
> + event_dup_sync(new_master, cpuctx);
> + pdup_switch_master(pdup, event, new_master);
> + pdup->master = new_master;
> + }
> + }
> + return 0;
> +}

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