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SubjectRe: [RFC][PATCH] perf: Rewrite core context handling
On Wed, Oct 10, 2018 at 12:45:59PM +0200, Peter Zijlstra wrote:
> Hi all,
>
> There have been various issues and limitations with the way perf uses
> (task) contexts to track events. Most notable is the single hardware PMU
> task context, which has resulted in a number of yucky things (both
> proposed and merged).
>
> Notably:
>
> - HW breakpoint PMU
> - ARM big.little PMU
> - Intel Branch Monitoring PMU
>
> Since we now track the events in RB trees, we can 'simply' add a pmu
> order to them and have them grouped that way, reducing to a single
> context. Of course, reality never quite works out that simple, and below
> ends up adding an intermediate data structure to bridge the context ->
> pmu mapping.
>
> Something a little like:
>
> ,------------------------[1:n]---------------------.
> V V
> perf_event_context <-[1:n]-> perf_event_pmu_context <--- perf_event
> ^ ^ | |
> `--------[1:n]---------' `-[n:1]-> pmu <-[1:n]-'
>
> This patch builds (provided you disable CGROUP_PERF), boots and survives
> perf-top without the machine catching fire.
>
> There's still a fair bit of loose ends (look for XXX), but I think this
> is the direction we should be going.

I think this is the right direction, as this is roughly what I suggested
before the RB-tree stuff. ;)

> Comments?

Vague things inline below.

> +/*
> + * ,------------------------[1:n]---------------------.
> + * V V
> + * perf_event_context <-[1:n]-> perf_event_pmu_context <--- perf_event
> + * ^ ^ | |
> + * `--------[1:n]---------' `-[n:1]-> pmu <-[1:n]-'
> + *
> + *
> + * XXX destroy epc when empty
> + * refcount, !rcu
> + *
> + * XXX epc locking
> + *
> + * event->pmu_ctx ctx->mutex && inactive
> + * ctx->pmu_ctx_list ctx->mutex && ctx->lock
> + *
> + */
> +struct perf_event_pmu_context {
> + struct pmu *pmu;
> + struct perf_event_context *ctx;
> +
> + struct list_head pmu_ctx_entry;
> +
> + struct list_head pinned_active;
> + struct list_head flexible_active;
> +
> + unsigned int embedded : 1;

Is this just for lifetime management (i.e. not attempting to free the
embedded epc)?

Do we need a flag? Can't we have the pmu hold a ref on its embedded epc,
and init that at pmu init time?

> +
> + unsigned int nr_events;
> + unsigned int nr_active;
> +
> + atomic_t refcount; /* event <-> epc */
> +
> + void *task_ctx_data; /* pmu specific data */
> +};
>
> struct perf_event_groups {
> struct rb_root tree;
> @@ -710,7 +749,6 @@ struct perf_event_groups {
> * Used as a container for task events and CPU events as well:
> */
> struct perf_event_context {
> - struct pmu *pmu;
> /*
> * Protect the states of the events in the list,
> * nr_active, and the list:
> @@ -723,20 +761,21 @@ struct perf_event_context {
> */
> struct mutex mutex;
>
> - struct list_head active_ctx_list;
> + struct list_head pmu_ctx_list;
> +
> struct perf_event_groups pinned_groups;
> struct perf_event_groups flexible_groups;
> struct list_head event_list;

I think that the groups lists and event list should be in the
perf_event_pmu_context.

That would make scheduling and rotating events a per-pmu thing, as we
want, without complicating the RB tree logic or requiring additional
hooks.

That may make the move_group case more complicated, though.

... and maybe I've missed some other headache with that?

>
> - struct list_head pinned_active;
> - struct list_head flexible_active;
> -
> int nr_events;
> int nr_active;
> int is_active;
> +
> + int nr_task_data;
> int nr_stat;
> int nr_freq;
> int rotate_disable;

Likewise these all seem to be PMU-specific (though I guess we care about
them in the ctx-switch fast paths?).

> +
> atomic_t refcount;
> struct task_struct *task;
>
> @@ -757,7 +796,6 @@ struct perf_event_context {
> #ifdef CONFIG_CGROUP_PERF
> int nr_cgroups; /* cgroup evts */
> #endif
> - void *task_ctx_data; /* pmu specific data */
> struct rcu_head rcu_head;
> };

[...]

> @@ -1528,6 +1498,11 @@ perf_event_groups_less(struct perf_event
> if (left->cpu > right->cpu)
> return false;
>
> + if (left->pmu_ctx->pmu < right->pmu_ctx->pmu)
> + return true;
> + if (left->pmu_ctx->pmu > right->pmu_ctx->pmu)
> + return false;
> +
> if (left->group_index < right->group_index)
> return true;
> if (left->group_index > right->group_index)
> @@ -1610,7 +1585,7 @@ del_event_from_groups(struct perf_event
> * Get the leftmost event in the @cpu subtree.
> */
> static struct perf_event *
> -perf_event_groups_first(struct perf_event_groups *groups, int cpu)
> +perf_event_groups_first(struct perf_event_groups *groups, int cpu, struct pmu *pmu)
> {
> struct perf_event *node_event = NULL, *match = NULL;
> struct rb_node *node = groups->tree.rb_node;
> @@ -1623,8 +1598,19 @@ perf_event_groups_first(struct perf_even
> } else if (cpu > node_event->cpu) {
> node = node->rb_right;
> } else {
> - match = node_event;
> - node = node->rb_left;
> + if (pmu) {
> + if (pmu < node_event->pmu_ctx->pmu) {
> + node = node->rb_left;
> + } else if (pmu > node_event->pmu_ctx->pmu) {
> + node = node->rb_right;
> + } else {
> + match = node_event;
> + node = node->rb_left;
> + }
> + } else {
> + match = node_event;
> + node = node->rb_left;
> + }
> }
> }
>
> @@ -1635,13 +1621,17 @@ perf_event_groups_first(struct perf_even
> * Like rb_entry_next_safe() for the @cpu subtree.
> */
> static struct perf_event *
> -perf_event_groups_next(struct perf_event *event)
> +perf_event_groups_next(struct perf_event *event, struct pmu *pmu)
> {
> struct perf_event *next;
>
> next = rb_entry_safe(rb_next(&event->group_node), typeof(*event), group_node);
> - if (next && next->cpu == event->cpu)
> + if (next && next->cpu == event->cpu) {
> + if (pmu && next->pmu_ctx->pmu != pmu)
> + return NULL;
> +
> return next;
> + }
>
> return NULL;
> }

This would be much nicer with a per-pmu event_list.

[...]

> + // XXX premature; what if this is allowed, but we get moved to a PMU
> + // that doesn't have this.
> if (is_sampling_event(event)) {
> if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
> err = -EOPNOTSUPP;

Ugh, could that happen for SW events moved into a HW context?

Thanks,
Mark.

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