Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP_COEXIST event capability

["Liang, Kan" <kan.liang@linux.intel.com>]

On 7/24/2020 7:46 AM, peterz@infradead.org wrote:
> On Fri, Jul 24, 2020 at 12:55:43PM +0200, peterz@infradead.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@infradead.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);
>