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Subject[PATCH v2] perf: Store active software events in a hashlist
Date
Each time a software event triggers, we need to walk through
the entire list of events from the current cpu and task contexts
to retrieve a running perf event that matches.
We also need to check a matching perf event is actually counting.

This walk is wasteful and makes the event fast path scaling
down with a growing number of events running on the same
contexts.

To solve this, we store the running perf events in a hashlist to
get an immediate access to them against their type:event_id when
they trigger.

v2: - Fix SWEVENT_HLIST_SIZE definition (and re-learn some basic
maths along the way)
- Only allocate hlist for online cpus, but keep track of the
refcount on offline possible cpus too, so that we allocate it
if needed when it becomes online.
- Drop the kref use as it's not adapted to our tricks anymore.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
---
include/linux/perf_event.h | 12 ++
kernel/perf_event.c | 251 +++++++++++++++++++++++++++++++++-----------
2 files changed, 200 insertions(+), 63 deletions(-)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 6e96cc8..bf896d0 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -589,6 +589,14 @@ enum perf_group_flag {
PERF_GROUP_SOFTWARE = 0x1,
};

+#define SWEVENT_HLIST_BITS 8
+#define SWEVENT_HLIST_SIZE (1 << SWEVENT_HLIST_BITS)
+
+struct swevent_hlist {
+ struct hlist_head heads[SWEVENT_HLIST_SIZE];
+ struct rcu_head rcu_head;
+};
+
/**
* struct perf_event - performance event kernel representation:
*/
@@ -597,6 +605,7 @@ struct perf_event {
struct list_head group_entry;
struct list_head event_entry;
struct list_head sibling_list;
+ struct hlist_node hlist_entry;
int nr_siblings;
int group_flags;
struct perf_event *group_leader;
@@ -744,6 +753,9 @@ struct perf_cpu_context {
int active_oncpu;
int max_pertask;
int exclusive;
+ struct swevent_hlist *swevent_hlist;
+ struct mutex hlist_mutex;
+ int hlist_refcount;

/*
* Recursion avoidance:
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 4aa50ff..c953cfb 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -15,6 +15,7 @@
#include <linux/smp.h>
#include <linux/file.h>
#include <linux/poll.h>
+#include <linux/hash.h>
#include <linux/sysfs.h>
#include <linux/dcache.h>
#include <linux/percpu.h>
@@ -3965,36 +3966,6 @@ static void perf_swevent_add(struct perf_event *event, u64 nr,
perf_swevent_overflow(event, 0, nmi, data, regs);
}

-static int perf_swevent_is_counting(struct perf_event *event)
-{
- /*
- * The event is active, we're good!
- */
- if (event->state == PERF_EVENT_STATE_ACTIVE)
- return 1;
-
- /*
- * The event is off/error, not counting.
- */
- if (event->state != PERF_EVENT_STATE_INACTIVE)
- return 0;
-
- /*
- * The event is inactive, if the context is active
- * we're part of a group that didn't make it on the 'pmu',
- * not counting.
- */
- if (event->ctx->is_active)
- return 0;
-
- /*
- * We're inactive and the context is too, this means the
- * task is scheduled out, we're counting events that happen
- * to us, like migration events.
- */
- return 1;
-}
-
static int perf_tp_event_match(struct perf_event *event,
struct perf_sample_data *data);

@@ -4018,12 +3989,6 @@ static int perf_swevent_match(struct perf_event *event,
struct perf_sample_data *data,
struct pt_regs *regs)
{
- if (event->cpu != -1 && event->cpu != smp_processor_id())
- return 0;
-
- if (!perf_swevent_is_counting(event))
- return 0;
-
if (event->attr.type != type)
return 0;

@@ -4040,18 +4005,53 @@ static int perf_swevent_match(struct perf_event *event,
return 1;
}

-static void perf_swevent_ctx_event(struct perf_event_context *ctx,
- enum perf_type_id type,
- u32 event_id, u64 nr, int nmi,
- struct perf_sample_data *data,
- struct pt_regs *regs)
+static inline u64 swevent_hash(u64 type, u32 event_id)
{
+ u64 val = event_id | (type << 32);
+
+ return hash_64(val, SWEVENT_HLIST_BITS);
+}
+
+static struct hlist_head *
+find_swevent_head(struct perf_cpu_context *ctx, u64 type, u32 event_id)
+{
+ u64 hash;
+ struct swevent_hlist *hlist;
+
+ hash = swevent_hash(type, event_id);
+
+ hlist = rcu_dereference(ctx->swevent_hlist);
+ if (!hlist)
+ return NULL;
+
+ return &hlist->heads[hash];
+}
+
+static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
+ u64 nr, int nmi,
+ struct perf_sample_data *data,
+ struct pt_regs *regs)
+{
+ struct perf_cpu_context *cpuctx;
struct perf_event *event;
+ struct hlist_node *node;
+ struct hlist_head *head;

- list_for_each_entry_rcu(event, &ctx->event_list, event_entry) {
+ cpuctx = &__get_cpu_var(perf_cpu_context);
+
+ rcu_read_lock();
+
+ head = find_swevent_head(cpuctx, type, event_id);
+
+ if (!head)
+ goto end;
+
+ hlist_for_each_entry_rcu(event, node, head, hlist_entry) {
if (perf_swevent_match(event, type, event_id, data, regs))
perf_swevent_add(event, nr, nmi, data, regs);
}
+end:
+ rcu_read_unlock();
}

int perf_swevent_get_recursion_context(void)
@@ -4089,27 +4089,6 @@ void perf_swevent_put_recursion_context(int rctx)
}
EXPORT_SYMBOL_GPL(perf_swevent_put_recursion_context);

-static void do_perf_sw_event(enum perf_type_id type, u32 event_id,
- u64 nr, int nmi,
- struct perf_sample_data *data,
- struct pt_regs *regs)
-{
- struct perf_cpu_context *cpuctx;
- struct perf_event_context *ctx;
-
- cpuctx = &__get_cpu_var(perf_cpu_context);
- rcu_read_lock();
- perf_swevent_ctx_event(&cpuctx->ctx, type, event_id,
- nr, nmi, data, regs);
- /*
- * doesn't really matter which of the child contexts the
- * events ends up in.
- */
- ctx = rcu_dereference(current->perf_event_ctxp);
- if (ctx)
- perf_swevent_ctx_event(ctx, type, event_id, nr, nmi, data, regs);
- rcu_read_unlock();
-}

void __perf_sw_event(u32 event_id, u64 nr, int nmi,
struct pt_regs *regs, u64 addr)
@@ -4135,16 +4114,28 @@ static void perf_swevent_read(struct perf_event *event)
static int perf_swevent_enable(struct perf_event *event)
{
struct hw_perf_event *hwc = &event->hw;
+ struct perf_cpu_context *cpuctx;
+ struct hlist_head *head;
+
+ cpuctx = &__get_cpu_var(perf_cpu_context);

if (hwc->sample_period) {
hwc->last_period = hwc->sample_period;
perf_swevent_set_period(event);
}
+
+ head = find_swevent_head(cpuctx, event->attr.type, event->attr.config);
+ if (WARN_ON_ONCE(!head))
+ return -EINVAL;
+
+ hlist_add_head_rcu(&event->hlist_entry, head);
+
return 0;
}

static void perf_swevent_disable(struct perf_event *event)
{
+ hlist_del_rcu(&event->hlist_entry);
}

static const struct pmu perf_ops_generic = {
@@ -4358,13 +4349,115 @@ static int perf_tp_event_match(struct perf_event *event,
return 0;
}

+static void swevent_hlist_release_rcu(struct rcu_head *rcu_head)
+{
+ struct swevent_hlist *hlist;
+
+ hlist = container_of(rcu_head, struct swevent_hlist, rcu_head);
+ kfree(hlist);
+}
+
+static void swevent_hlist_release(struct perf_cpu_context *cpuctx)
+{
+ struct swevent_hlist *hlist;
+
+ if (!cpuctx->swevent_hlist)
+ return;
+
+ hlist = cpuctx->swevent_hlist;
+ rcu_assign_pointer(cpuctx->swevent_hlist, NULL);
+ call_rcu(&hlist->rcu_head, swevent_hlist_release_rcu);
+}
+
+static void swevent_hlist_put_cpu(struct perf_event *event, int cpu)
+{
+ struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
+
+ mutex_lock(&cpuctx->hlist_mutex);
+
+ if (!--cpuctx->hlist_refcount)
+ swevent_hlist_release(cpuctx);
+
+ mutex_unlock(&cpuctx->hlist_mutex);
+}
+
+static void swevent_hlist_put(struct perf_event *event)
+{
+ int cpu;
+
+ if (event->cpu != -1) {
+ swevent_hlist_put_cpu(event, event->cpu);
+ return;
+ }
+
+ for_each_possible_cpu(cpu)
+ swevent_hlist_put_cpu(event, cpu);
+}
+
+static int swevent_hlist_get_cpu(struct perf_event *event, int cpu)
+{
+ struct perf_cpu_context *cpuctx = &per_cpu(perf_cpu_context, cpu);
+ int err = 0;
+
+ mutex_lock(&cpuctx->hlist_mutex);
+
+ if (!cpuctx->swevent_hlist && cpu_online(cpu)) {
+ struct swevent_hlist *hlist;
+
+ hlist = kzalloc(sizeof(*hlist), GFP_KERNEL);
+ if (!hlist) {
+ err = -ENOMEM;
+ goto exit;
+ }
+ rcu_assign_pointer(cpuctx->swevent_hlist, hlist);
+ }
+ cpuctx->hlist_refcount++;
+ exit:
+ mutex_unlock(&cpuctx->hlist_mutex);
+
+ return err;
+}
+
+static int swevent_hlist_get(struct perf_event *event)
+{
+ int err;
+ int cpu, failed_cpu;
+
+ if (event->cpu != -1)
+ return swevent_hlist_get_cpu(event, event->cpu);
+
+ get_online_cpus();
+ for_each_possible_cpu(cpu) {
+ err = swevent_hlist_get_cpu(event, cpu);
+ if (err) {
+ failed_cpu = cpu;
+ goto fail;
+ }
+ }
+ put_online_cpus();
+
+ return 0;
+ fail:
+ for_each_possible_cpu(cpu) {
+ if (cpu == failed_cpu)
+ break;
+ swevent_hlist_put_cpu(event, cpu);
+ }
+
+ put_online_cpus();
+ return err;
+}
+
static void tp_perf_event_destroy(struct perf_event *event)
{
perf_trace_disable(event->attr.config);
+ swevent_hlist_put(event);
}

static const struct pmu *tp_perf_event_init(struct perf_event *event)
{
+ int err;
+
/*
* Raw tracepoint data is a severe data leak, only allow root to
* have these.
@@ -4378,6 +4471,11 @@ static const struct pmu *tp_perf_event_init(struct perf_event *event)
return NULL;

event->destroy = tp_perf_event_destroy;
+ err = swevent_hlist_get(event);
+ if (err) {
+ perf_trace_disable(event->attr.config);
+ return ERR_PTR(err);
+ }

return &perf_ops_generic;
}
@@ -4478,6 +4576,7 @@ static void sw_perf_event_destroy(struct perf_event *event)
WARN_ON(event->parent);

atomic_dec(&perf_swevent_enabled[event_id]);
+ swevent_hlist_put(event);
}

static const struct pmu *sw_perf_event_init(struct perf_event *event)
@@ -4516,6 +4615,12 @@ static const struct pmu *sw_perf_event_init(struct perf_event *event)
case PERF_COUNT_SW_ALIGNMENT_FAULTS:
case PERF_COUNT_SW_EMULATION_FAULTS:
if (!event->parent) {
+ int err;
+
+ err = swevent_hlist_get(event);
+ if (err)
+ return ERR_PTR(err);
+
atomic_inc(&perf_swevent_enabled[event_id]);
event->destroy = sw_perf_event_destroy;
}
@@ -5388,6 +5493,7 @@ static void __init perf_event_init_all_cpus(void)

for_each_possible_cpu(cpu) {
cpuctx = &per_cpu(perf_cpu_context, cpu);
+ mutex_init(&cpuctx->hlist_mutex);
__perf_event_init_context(&cpuctx->ctx, NULL);
}
}
@@ -5401,6 +5507,16 @@ static void __cpuinit perf_event_init_cpu(int cpu)
spin_lock(&perf_resource_lock);
cpuctx->max_pertask = perf_max_events - perf_reserved_percpu;
spin_unlock(&perf_resource_lock);
+
+ mutex_lock(&cpuctx->hlist_mutex);
+ if (&cpuctx->hlist_refcount > 0) {
+ struct swevent_hlist *hlist;
+
+ hlist = kzalloc(sizeof(*hlist), GFP_KERNEL);
+ WARN_ON_ONCE(!hlist);
+ rcu_assign_pointer(cpuctx->swevent_hlist, hlist);
+ }
+ mutex_unlock(&cpuctx->hlist_mutex);
}

#ifdef CONFIG_HOTPLUG_CPU
@@ -5414,6 +5530,15 @@ static void __perf_event_exit_cpu(void *info)
__perf_event_remove_from_context(event);
list_for_each_entry_safe(event, tmp, &ctx->flexible_groups, group_entry)
__perf_event_remove_from_context(event);
+
+ mutex_lock(&cpuctx->hlist_mutex);
+ /*
+ * We may have interrupted a triggering software event. So release
+ * the hlist in an rcu safe fashion
+ */
+ swevent_hlist_release(cpuctx);
+ mutex_unlock(&cpuctx->hlist_mutex);
+
}
static void perf_event_exit_cpu(int cpu)
{
--
1.6.2.3


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