[RFC PATCH] perf/x86: improve the event scheduling to avoid unnecessary pmu_stop/start

Wen Yang
2022-02-17T02:45:58Z

This issue has been there for a long time, we could reproduce it as follows:

1, run a script that periodically collects perf data, eg:
while true
do
perf stat -e cache-misses,cache-misses,cache-misses -C 1 sleep 2
perf stat -e cache-misses -C 1 sleep 2
sleep 1
done

2, run another one to capture the IPC, eg:
perf stat -e cycles:D,instructions:D -C 1 -I 1000

Then we could observe that the counter used by cycles:D changes frequently:
crash> struct cpu_hw_events.n_events,assign,event_list,events ffff88bf7f44f420
n_events = 3
assign = {33, 1, 32, 0, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
event_list = {0xffff88bf77b85000, 0xffff88b72db82000, 0xffff88b72db85800, 0xffff88ff6cfcb000, 0xffff88ff609f1800, 0xffff88ff609f1800, 0xffff88ff5f46a800, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
events = {0x0, 0xffff88b72db82000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xffff88b72db85800, 0xffff88bf77b85000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}

crash> struct cpu_hw_events.n_events,assign,event_list,events ffff88bf7f44f420
n_events = 6
assign = {33, 3, 32, 0, 1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
event_list = {0xffff88bf77b85000, 0xffff88b72db82000, 0xffff88b72db85800, 0xffff88bf46c34000, 0xffff88bf46c35000, 0xffff88bf46c30000, 0xffff88ff5f46a800, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
events = {0xffff88bf46c34000, 0xffff88bf46c35000, 0xffff88bf46c30000, 0xffff88b72db82000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xffff88b72db85800, 0xffff88bf77b85000, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}

The reason is that the NMI watchdog permanently consumes one FP
(*cycles*). Therefore, when the above shell script obtains *cycles*
again, only one GP can be used, and its weight is 5.
But other events (like *cache-misses*) have a weight of 4,
so the counter used by *cycles* will often be taken away, as in
the raw data above:
[1]
n_events = 3
assign = {33, 1, 32, ...}
-->
n_events = 6
assign = {33, 3, 32, 0, 1, 2, ...}

So it will cause unnecessary pmu_stop/start.

The current event scheduling algorithm is more than 10 years old:
https://lwn.net/Articles/357631/

We wish it could be optimized a bit.
In this patch, the fields cnt_mask and evt_mask are added to indicate
currently used counters and events so that the used ones can be skipped
in __perf_sched_find_counter and perf_sched_next_event functions to avoid
unnecessary pmu_stop/start.

[1]:
32: INTEL_PMC_IDX_FIXED_INSTRUCTIONS
33: INTEL_PMC_IDX_FIXED_CPU_CYCLES
34: INTEL_PMC_IDX_FIXED_REF_CYCLES
0,1,2,3: GP

Signed-off-by: Wen Yang <simon.wy@alibaba-inc.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: Wen Yang <wenyang@linux.alibaba.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-perf-users@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
arch/x86/events/core.c | 40 +++++++++++++++++++++++++++++++---------
arch/x86/events/intel/uncore.c | 2 +-
arch/x86/events/perf_event.h | 3 ++-
3 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
index e686c5e..1a47e31 100644
--- a/arch/x86/events/core.c
+++ b/arch/x86/events/core.c
@@ -796,6 +796,8 @@ struct perf_sched {
int max_events;
int max_gp;
int saved_states;
+ u64 cnt_mask;
+ u64 evt_mask;
struct event_constraint **constraints;
struct sched_state state;
struct sched_state saved[SCHED_STATES_MAX];
@@ -805,7 +807,7 @@ struct perf_sched {
* Initialize iterator that runs through all events and counters.
*/
static void perf_sched_init(struct perf_sched *sched, struct event_constraint **constraints,
- int num, int wmin, int wmax, int gpmax)
+ int num, int wmin, int wmax, int gpmax, u64 cnt_mask, u64 evt_mask)
{
int idx;

@@ -814,6 +816,8 @@ static void perf_sched_init(struct perf_sched *sched, struct event_constraint **
sched->max_weight = wmax;
sched->max_gp = gpmax;
sched->constraints = constraints;
+ sched->cnt_mask = cnt_mask;
+ sched->evt_mask = evt_mask;

for (idx = 0; idx < num; idx++) {
if (constraints[idx]->weight == wmin)
@@ -822,7 +826,10 @@ static void perf_sched_init(struct perf_sched *sched, struct event_constraint **

sched->state.event = idx; /* start with min weight */
sched->state.weight = wmin;
- sched->state.unassigned = num;
+ sched->state.unassigned = num - hweight_long(evt_mask);
+
+ while (sched->evt_mask & BIT_ULL(sched->state.event))
+ sched->state.event++;
}

static void perf_sched_save_state(struct perf_sched *sched)
@@ -874,6 +881,9 @@ static bool __perf_sched_find_counter(struct perf_sched *sched)
for_each_set_bit_from(idx, c->idxmsk, X86_PMC_IDX_MAX) {
u64 mask = BIT_ULL(idx);

+ if (sched->cnt_mask & mask)
+ continue;
+
if (sched->state.used & mask)
continue;

@@ -890,6 +900,9 @@ static bool __perf_sched_find_counter(struct perf_sched *sched)
if (c->flags & PERF_X86_EVENT_PAIR)
mask |= mask << 1;

+ if (sched->cnt_mask & mask)
+ continue;
+
if (sched->state.used & mask)
continue;

@@ -934,7 +947,10 @@ static bool perf_sched_next_event(struct perf_sched *sched)

do {
/* next event */
- sched->state.event++;
+ do {
+ sched->state.event++;
+ } while (sched->evt_mask & BIT_ULL(sched->state.event));
+
if (sched->state.event >= sched->max_events) {
/* next weight */
sched->state.event = 0;
@@ -954,11 +970,11 @@ static bool perf_sched_next_event(struct perf_sched *sched)
* Assign a counter for each event.
*/
int perf_assign_events(struct event_constraint **constraints, int n,
- int wmin, int wmax, int gpmax, int *assign)
+ int wmin, int wmax, int gpmax, u64 cnt_mask, u64 evt_mask, int *assign)
{
struct perf_sched sched;

- perf_sched_init(&sched, constraints, n, wmin, wmax, gpmax);
+ perf_sched_init(&sched, constraints, n, wmin, wmax, gpmax, cnt_mask, evt_mask);

do {
if (!perf_sched_find_counter(&sched))
@@ -978,7 +994,8 @@ int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign)
struct perf_event *e;
int n0, i, wmin, wmax, unsched = 0;
struct hw_perf_event *hwc;
- u64 used_mask = 0;
+ u64 cnt_mask = 0;
+ u64 evt_mask = 0;

/*
* Compute the number of events already present; see x86_pmu_add(),
@@ -1038,10 +1055,11 @@ int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign)
mask |= mask << 1;

/* not already used */
- if (used_mask & mask)
+ if (cnt_mask & mask)
break;

- used_mask |= mask;
+ cnt_mask |= mask;
+ evt_mask |= BIT_ULL(i);

if (assign)
assign[i] = hwc->idx;
@@ -1075,7 +1093,11 @@ int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign)
}

unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
- wmax, gpmax, assign);
+ wmax, gpmax, cnt_mask, evt_mask, assign);
+ if (unsched) {
+ unsched = perf_assign_events(cpuc->event_constraint, n, wmin,
+ wmax, gpmax, 0, 0, assign);
+ }
}

/*
diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
index e497da9..8afff7a 100644
--- a/arch/x86/events/intel/uncore.c
+++ b/arch/x86/events/intel/uncore.c
@@ -480,7 +480,7 @@ static int uncore_assign_events(struct intel_uncore_box *box, int assign[], int
/* slow path */
if (i != n)
ret = perf_assign_events(box->event_constraint, n,
- wmin, wmax, n, assign);
+ wmin, wmax, n, 0, 0, assign);

if (!assign || ret) {
for (i = 0; i < n; i++)
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 150261d..2ae1e98 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -1131,7 +1131,8 @@ static inline void __x86_pmu_enable_event(struct hw_perf_event *hwc,
void x86_pmu_enable_all(int added);

int perf_assign_events(struct event_constraint **constraints, int n,
- int wmin, int wmax, int gpmax, int *assign);
+ int wmin, int wmax, int gpmax, u64 cnt_mask, u64 evt_mask,
+ int *assign);
int x86_schedule_events(struct cpu_hw_events *cpuc, int n, int *assign);

void x86_pmu_stop(struct perf_event *event, int flags);
--
1.8.3.1