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Date: Fri, 24 Jul 2020 09:43:44 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: peterz@...radead.org
Cc: acme@...hat.com, mingo@...nel.org, linux-kernel@...r.kernel.org,
jolsa@...nel.org, eranian@...gle.com,
alexander.shishkin@...ux.intel.com, ak@...ux.intel.com,
like.xu@...ux.intel.com
Subject: Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP_COEXIST event
capability
On 7/24/2020 7:46 AM, peterz@...radead.org wrote:
> On Fri, Jul 24, 2020 at 12:55:43PM +0200, peterz@...radead.org wrote:
>>> + event_sched_out(event, cpuctx, ctx);
>>> + perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
>>> +}
>>
>> Ah, so the problem here is that ERROR is actually recoverable using
>> IOC_ENABLE. We don't want that either. Let me try and figure out of EXIT
>> would work.
>
> EXIT is difficult too.. Also, that COEXIST thing hurt my brian, can't we
> do a simpler SIBLING thing that simply requires the event to be a group
> sibling?
>
> The consequence is that SLOTS must be the leader, is that really a
> problem? You keep providing the {cycles, slots, metric-things} example,
> but afaict {slots, metric-thing, cycles} should work just fine too. > PERF_SAMPLE_READ with PERF_FORMAT_GROUP doesn't need to the the leader.
I'm not sure I get your point.
For the PERF_SAMPLE_READ with PERF_FORMAT_GROUP case, other events can
be the leader, e.g., {cycles, slots, metric-things}:S.
In this case, the SLOTS event is not a leader. I don't think we can
assume that the SLOTS event must be the leader.
I think the case should be a useful case. Users may want to read the
Topdown values for each sample.
Thanks,
Kan
>
> The thing is, with COEXIST, if you have:
>
> {cycles, slots, metric1, metric2}
>
> and you say that COEXIST events need to co-exist with at least one other
> COEXIST event you end up with a problem when: close(fd_slots) happens,
> because in the above case, there's two more COEXIST events, so the
> requirement holds, even through this is very much not what we want.
>
> You 'fixed' this by saying closing any COEXIST event will tear the group
> up, but that's 'weird' and unexpected if you were to do:
> close(fd_metric2).
> >
> So something like this.
>
> ---
> Subject: perf/core: Add a new PERF_EV_CAP_SIBLING event capability
>
> Current perf assumes that events in a group are independent. Close an
> event doesn't impact the value of the other events in the same group.
> If the closed event is a member, after the event closure, other events
> are still running like a group. If the closed event is a leader, other
> events are running as singleton events.
>
> Add PERF_EV_CAP_SIBLING to allow events to indicate they require being
> part of a group, and when the leader dies they cannot exist
> independently.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> include/linux/perf_event.h | 4 ++++
> kernel/events/core.c | 38 +++++++++++++++++++++++++++++++++-----
> 2 files changed, 37 insertions(+), 5 deletions(-)
>
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -576,9 +576,13 @@ typedef void (*perf_overflow_handler_t)(
> * PERF_EV_CAP_SOFTWARE: Is a software event.
> * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that can be read
> * from any CPU in the package where it is active.
> + * PERF_EV_CAP_SIBLING: An event with this flag must be a group sibling and
> + * cannot be a group leader. If an event with this flag is detached from the
> + * group it is scheduled out and moved into an unrecoverable ERROR state.
> */
> #define PERF_EV_CAP_SOFTWARE BIT(0)
> #define PERF_EV_CAP_READ_ACTIVE_PKG BIT(1)
> +#define PERF_EV_CAP_SIBLING BIT(2)
>
> #define SWEVENT_HLIST_BITS 8
> #define SWEVENT_HLIST_SIZE (1 << SWEVENT_HLIST_BITS)
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2133,8 +2133,24 @@ static inline struct list_head *get_even
> return event->attr.pinned ? &ctx->pinned_active : &ctx->flexible_active;
> }
>
> +/*
> + * Events that have PERF_EV_CAP_SIBLING require being part of a group and
> + * cannot exist on their own, schedule them out and move them into the ERROR
> + * state. Also see _perf_event_enable(), it will not be able to recover
> + * this ERROR state.
> + */
> +static inline void perf_remove_sibling_event(struct perf_event *event)
> +{
> + struct perf_event_context *ctx = event->ctx;
> + struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
> +
> + event_sched_out(event, cpuctx, ctx);
> + perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
> +}
> +
> static void perf_group_detach(struct perf_event *event)
> {
> + struct perf_event *leader = event->group_leader;
> struct perf_event *sibling, *tmp;
> struct perf_event_context *ctx = event->ctx;
>
> @@ -2153,7 +2169,7 @@ static void perf_group_detach(struct per
> /*
> * If this is a sibling, remove it from its group.
> */
> - if (event->group_leader != event) {
> + if (leader != event) {
> list_del_init(&event->sibling_list);
> event->group_leader->nr_siblings--;
> goto out;
> @@ -2166,6 +2182,9 @@ static void perf_group_detach(struct per
> */
> list_for_each_entry_safe(sibling, tmp, &event->sibling_list, sibling_list) {
>
> + if (sibling->event_caps & PERF_EV_CAP_SIBLING)
> + perf_remove_sibling_event(sibling);
> +
> sibling->group_leader = sibling;
> list_del_init(&sibling->sibling_list);
>
> @@ -2183,10 +2202,10 @@ static void perf_group_detach(struct per
> }
>
> out:
> - perf_event__header_size(event->group_leader);
> -
> - for_each_sibling_event(tmp, event->group_leader)
> + for_each_sibling_event(tmp, leader)
> perf_event__header_size(tmp);
> +
> + perf_event__header_size(leader);
> }
>
> static bool is_orphaned_event(struct perf_event *event)
> @@ -2979,6 +2998,7 @@ static void _perf_event_enable(struct pe
> raw_spin_lock_irq(&ctx->lock);
> if (event->state >= PERF_EVENT_STATE_INACTIVE ||
> event->state < PERF_EVENT_STATE_ERROR) {
> +out:
> raw_spin_unlock_irq(&ctx->lock);
> return;
> }
> @@ -2990,8 +3010,16 @@ static void _perf_event_enable(struct pe
> * has gone back into error state, as distinct from the task having
> * been scheduled away before the cross-call arrived.
> */
> - if (event->state == PERF_EVENT_STATE_ERROR)
> + if (event->state == PERF_EVENT_STATE_ERROR) {
> + /*
> + * Detached SIBLING events cannot leave ERROR state.
> + */
> + if (event->event_caps & PERF_EV_CAP_SIBLING &&
> + event->group_leader == event)
> + goto out;
> +
> event->state = PERF_EVENT_STATE_OFF;
> + }
> raw_spin_unlock_irq(&ctx->lock);
>
> event_function_call(event, __perf_event_enable, NULL);
>
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