linux-kernel - Re: [PATCH v2 2/3] perf/ibs: Fix interface via core pmu events

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Date:   Sun, 12 Mar 2023 15:54:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ravi Bangoria <ravi.bangoria@....com>
Cc:     namhyung@...nel.org, eranian@...gle.com, acme@...nel.org,
        mark.rutland@....com, jolsa@...nel.org, irogers@...gle.com,
        bp@...en8.de, x86@...nel.org, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org, sandipan.das@....com,
        ananth.narayan@....com, santosh.shukla@....com
Subject: Re: [PATCH v2 2/3] perf/ibs: Fix interface via core pmu events

On Thu, Mar 09, 2023 at 03:41:10PM +0530, Ravi Bangoria wrote:
> diff --git a/arch/x86/events/amd/core.c b/arch/x86/events/amd/core.c
> index 8c45b198b62f..81d67b899371 100644
> --- a/arch/x86/events/amd/core.c
> +++ b/arch/x86/events/amd/core.c
> @@ -371,10 +371,15 @@ static inline int amd_has_nb(struct cpu_hw_events *cpuc)
>  static int amd_pmu_hw_config(struct perf_event *event)
>  {
>  	int ret;
> +	u64 dummy;
>  
> -	/* pass precise event sampling to ibs: */
> -	if (event->attr.precise_ip && get_ibs_caps())
> -		return -ENOENT;
> +	if (event->attr.precise_ip) {
> +		/* pass precise event sampling to ibs by returning -ESRCH */
> +		if (get_ibs_caps() && !ibs_core_pmu_event(event, &dummy))
> +			return -ESRCH;
> +		else
> +			return -ENOENT;
> +	}
>  
>  	if (has_branch_stack(event) && !x86_pmu.lbr_nr)
>  		return -EOPNOTSUPP;
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f79fd8b87f75..e990c71ba34a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -11639,18 +11639,26 @@ static struct pmu *perf_init_event(struct perf_event *event)
>  			goto again;
>  		}
>  
> +		/*
> +		 * pmu->event_init() should return -ESRCH only when it
> +		 * wants to forward the event to other pmu.
> +		 */
> +		if (ret == -ESRCH)
> +			goto try_all;
> +
>  		if (ret)
>  			pmu = ERR_PTR(ret);
>  
>  		goto unlock;
>  	}
>  
> +try_all:
>  	list_for_each_entry_rcu(pmu, &pmus, entry, lockdep_is_held(&pmus_srcu)) {
>  		ret = perf_try_init_event(pmu, event);
>  		if (!ret)
>  			goto unlock;
>  
> -		if (ret != -ENOENT) {
> +		if (ret != -ENOENT && ret != -ESRCH) {
>  			pmu = ERR_PTR(ret);
>  			goto unlock;
>  		}

Urgh.. So amd_pmu_hw_config() knows what PMU it should be but has no
real way to communicate this, so you make it try all of them again.

Now, we already have a gruesome hack in there, and I'm thikning you
should use that instead of adding yet another one. Note:

		if (ret == -ENOENT && event->attr.type != type && !extended_type) {
			type = event->attr.type;
			goto again;

So if you have amd_pmu_hw_config() do:

	event->attr.type = ibs_pmu.type;
	return -ENOENT;

it should all just work no?

And now thinking about this, I'm thinking we can clean up the whole
swevent mess too, a little something like the below perhaps... Then it
might just be possible to remove that list_for_each_entry_rcu()
entirely.

Hmm?


---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f79fd8b87f75..26130d1ca40b 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9951,6 +9951,9 @@ static void sw_perf_event_destroy(struct perf_event *event)
 	swevent_hlist_put();
 }
 
+static struct pmu perf_cpu_clock; /* fwd declaration */
+static struct pmu perf_task_clock;
+
 static int perf_swevent_init(struct perf_event *event)
 {
 	u64 event_id = event->attr.config;
@@ -9966,7 +9969,11 @@ static int perf_swevent_init(struct perf_event *event)
 
 	switch (event_id) {
 	case PERF_COUNT_SW_CPU_CLOCK:
+		event->attr.type = perf_cpu_clock.type;
+		return -ENOENT;
+
 	case PERF_COUNT_SW_TASK_CLOCK:
+		event->attr.type = perf_task_clock.type;
 		return -ENOENT;
 
 	default: