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Date: Wed, 6 Dec 2017 12:42:04 +0100
From: Jiri Olsa <jolsa@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Namhyung Kim <namhyung@...nel.org>,
linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC] Sharing PMU counters across compatible events
On Fri, Dec 01, 2017 at 06:19:50AM -0800, Tejun Heo wrote:
> Hello,
>
> There are cases where a single PMU event, let's say instructions, is
> interesting for different subsets of the system. For example, it
> could be interesting to monitor instructions system-wide, at cgroup
> level and per each thread.
>
> This could easily be me not knowing better but I can't think of a way
> to do the above without consuming multiple PMU counters to monitor
> instructions, which quickly gets out of hand with the product of
> multiple domains and counters. Getting broken down numbers and adding
> up doesn't work at cgroup (the numbers aren't mutually exclusive) or
> thread level.
>
> If there's a way to achieve this using the existing features, I'd be
> glad to learn about it. If not, the patch at the bottom is a crude
> half-working proof-of-concept to share PMU counters across compatible
> events.
>
> In a nutshell, while adding events, it looks whether there already is
> a compatible event. If so, instead of enabling the new event, it gets
> linked to the already enabled one (the master event) and count of the
> dup event is updated by adding the delta of the master event.
>
> That patch is just a proof of concept. It's missing counter
> propagation somewhere and the dup counts end up somewhat lower than
> they should be. The master selection and switching are half-broken.
> Event compatibility testing is barely implemented, so on and so forth.
> However, it does allow gathering events for different targets without
> consuming multiple PMU counters.
>
> What do you think? Would this be something worth pursuing?
I see this rather on the hw level, since it concerns HW counters
I think we could detect same (alias) events at the time counters
are added/removed on/from the cpu and share their HW part like
counter idx, regs and such (struct hw_perf_event_cpu in my changes)
this way it'd be completely transparent for generic code
I made some untested (just compile) changes and attaching the
top patch, that has the main logic, please check all changes
in here:
git://git.kernel.org/pub/scm/linux/kernel/git/jolsa/perf.git
perf/alias_2
there's big change due to changing the perf_event::hw stuff
but the rest is quite simple (attached).. not completely sure
yet if I'm missing some functionality which would break due
to this change
thoughts?
thanks,
jirka
---
arch/x86/events/core.c | 58 +++++++++++++++++++++++++++++++++++++++++++++-
include/linux/perf_event.h | 4 ++++
kernel/events/core.c | 1 +
3 files changed, 62 insertions(+), 1 deletion(-)
diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index 2bb5d1b2a622..737857f07881 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -1189,6 +1189,54 @@ void x86_pmu_enable_event(struct perf_event *event)
ARCH_PERFMON_EVENTSEL_ENABLE);
}
+static bool is_alias(struct perf_event *alias, struct perf_event *event)
+{
+ if (is_sampling_event(alias))
+ return false;
+
+ return memcmp(&alias->attr, &event->attr, sizeof(alias->attr));
+}
+
+static int alias_add(struct cpu_hw_events *cpuc, struct perf_event *event)
+{
+ struct perf_event *master;
+ int n;
+
+ for (n = 0; n < cpuc->n_events; n++) {
+ if (is_alias(event, cpuc->event_list[n]))
+ break;
+ }
+
+ if (n == cpuc->n_events)
+ return 0;
+
+ master = cpuc->event_list[n];
+ event->hw.cpu = master->hw.cpu;
+ list_add_tail(&event->hw.alias, &master->hw.master);
+ return 1;
+}
+
+static int alias_del(struct cpu_hw_events *cpuc, struct perf_event *event)
+{
+ if (event->hw.cpu != &event->hw.cpu_local)
+ return 0;
+
+ event->hw.cpu = &event->hw.cpu_local;
+ list_del(&event->hw.alias);
+ return 1;
+}
+
+static void alias_update(struct perf_event *event)
+{
+ struct perf_event *alias;
+
+ list_for_each_entry(alias, &event->hw.master, hw.alias) {
+ alias->count = event->count;
+ alias->hw.prev_count = event->hw.prev_count;
+ alias->hw.period_left = event->hw.period_left;
+ }
+}
+
/*
* Add a single event to the PMU.
*
@@ -1200,7 +1248,10 @@ static int x86_pmu_add(struct perf_event *event, int flags)
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
struct hw_perf_event *hwc;
int assign[X86_PMC_IDX_MAX];
- int n, n0, ret;
+ int n, n0, ret = 0;
+
+ if (alias_add(cpuc, event))
+ goto out;
hwc = &event->hw;
@@ -1383,11 +1434,16 @@ static void x86_pmu_del(struct perf_event *event, int flags)
if (cpuc->txn_flags & PERF_PMU_TXN_ADD)
goto do_del;
+ if (alias_del(cpuc, event))
+ goto do_del;
+
/*
* Not a TXN, therefore cleanup properly.
*/
x86_pmu_stop(event, PERF_EF_UPDATE);
+ alias_update(event);
+
for (i = 0; i < cpuc->n_events; i++) {
if (event == cpuc->event_list[i])
break;
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index b8bc5ddfbffd..6a09914a3555 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -136,6 +136,10 @@ struct hw_perf_event {
struct { /* hardware */
struct hw_perf_event_cpu *cpu;
struct hw_perf_event_cpu cpu_local;
+ union {
+ struct list_head master;
+ struct list_head alias;
+ };
};
struct { /* software */
struct hrtimer hrtimer;
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5879c67d90e4..9cf36b2b533f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9513,6 +9513,7 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
INIT_LIST_HEAD(&event->active_event_entry);
INIT_LIST_HEAD(&event->dup_list);
INIT_LIST_HEAD(&event->addr_filters.list);
+ INIT_LIST_HEAD(&event->hw.master);
INIT_HLIST_NODE(&event->hlist_entry);
--
2.13.6
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