[PATCH 1/5] perf: Optimize context reschedule for single PMU cases

Peter Zijlstra
2024-08-07T11:56:30Z

Currently re-scheduling a context will reschedule all active PMUs for
that context, even if it is known only a single event is added.

Namhyung reported that changing this to only reschedule the affected
PMU when possible provides significant performance gains under certain
conditions.

Therefore, allow partial context reschedules for a specific PMU, that
of the event modified.

While the patch looks somewhat noisy, it mostly just propagates a new
@pmu argument through the callchain and modifies the epc loop to only
pick the 'epc->pmu == @pmu' case.

Reported-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
kernel/events/core.c | 164 +++++++++++++++++++++++++++------------------------
1 file changed, 88 insertions(+), 76 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -685,30 +685,32 @@ do { \
___p; \
})

+#define for_each_epc(_epc, _ctx, _pmu, _cgroup) \
+ list_for_each_entry(_epc, &((_ctx)->pmu_ctx_list), pmu_ctx_entry) \
+ if (_cgroup && !_epc->nr_cgroups) \
+ continue; \
+ else if (_pmu && _epc->pmu != _pmu) \
+ continue; \
+ else
+
static void perf_ctx_disable(struct perf_event_context *ctx, bool cgroup)
{
struct perf_event_pmu_context *pmu_ctx;

- list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
- if (cgroup && !pmu_ctx->nr_cgroups)
- continue;
+ for_each_epc(pmu_ctx, ctx, NULL, cgroup)
perf_pmu_disable(pmu_ctx->pmu);
- }
}

static void perf_ctx_enable(struct perf_event_context *ctx, bool cgroup)
{
struct perf_event_pmu_context *pmu_ctx;

- list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
- if (cgroup && !pmu_ctx->nr_cgroups)
- continue;
+ for_each_epc(pmu_ctx, ctx, NULL, cgroup)
perf_pmu_enable(pmu_ctx->pmu);
- }
}

-static void ctx_sched_out(struct perf_event_context *ctx, enum event_type_t event_type);
-static void ctx_sched_in(struct perf_event_context *ctx, enum event_type_t event_type);
+static void ctx_sched_out(struct perf_event_context *ctx, struct pmu *pmu, enum event_type_t event_type);
+static void ctx_sched_in(struct perf_event_context *ctx, struct pmu *pmu, enum event_type_t event_type);

#ifdef CONFIG_CGROUP_PERF

@@ -865,7 +867,7 @@ static void perf_cgroup_switch(struct ta
perf_ctx_lock(cpuctx, cpuctx->task_ctx);
perf_ctx_disable(&cpuctx->ctx, true);

- ctx_sched_out(&cpuctx->ctx, EVENT_ALL|EVENT_CGROUP);
+ ctx_sched_out(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP);
/*
* must not be done before ctxswout due
* to update_cgrp_time_from_cpuctx() in
@@ -877,7 +879,7 @@ static void perf_cgroup_switch(struct ta
* perf_cgroup_set_timestamp() in ctx_sched_in()
* to not have to pass task around
*/
- ctx_sched_in(&cpuctx->ctx, EVENT_ALL|EVENT_CGROUP);
+ ctx_sched_in(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP);

perf_ctx_enable(&cpuctx->ctx, true);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
@@ -2656,7 +2658,8 @@ static void add_event_to_ctx(struct perf
}

static void task_ctx_sched_out(struct perf_event_context *ctx,
- enum event_type_t event_type)
+ struct pmu *pmu,
+ enum event_type_t event_type)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);

@@ -2666,18 +2669,19 @@ static void task_ctx_sched_out(struct pe
if (WARN_ON_ONCE(ctx != cpuctx->task_ctx))
return;

- ctx_sched_out(ctx, event_type);
+ ctx_sched_out(ctx, pmu, event_type);
}

static void perf_event_sched_in(struct perf_cpu_context *cpuctx,
- struct perf_event_context *ctx)
+ struct perf_event_context *ctx,
+ struct pmu *pmu)
{
- ctx_sched_in(&cpuctx->ctx, EVENT_PINNED);
+ ctx_sched_in(&cpuctx->ctx, pmu, EVENT_PINNED);
if (ctx)
- ctx_sched_in(ctx, EVENT_PINNED);
- ctx_sched_in(&cpuctx->ctx, EVENT_FLEXIBLE);
+ ctx_sched_in(ctx, pmu, EVENT_PINNED);
+ ctx_sched_in(&cpuctx->ctx, pmu, EVENT_FLEXIBLE);
if (ctx)
- ctx_sched_in(ctx, EVENT_FLEXIBLE);
+ ctx_sched_in(ctx, pmu, EVENT_FLEXIBLE);
}

/*
@@ -2695,16 +2699,12 @@ static void perf_event_sched_in(struct p
* event_type is a bit mask of the types of events involved. For CPU events,
* event_type is only either EVENT_PINNED or EVENT_FLEXIBLE.
*/
-/*
- * XXX: ctx_resched() reschedule entire perf_event_context while adding new
- * event to the context or enabling existing event in the context. We can
- * probably optimize it by rescheduling only affected pmu_ctx.
- */
static void ctx_resched(struct perf_cpu_context *cpuctx,
struct perf_event_context *task_ctx,
- enum event_type_t event_type)
+ struct pmu *pmu, enum event_type_t event_type)
{
bool cpu_event = !!(event_type & EVENT_CPU);
+ struct perf_event_pmu_context *epc;

/*
* If pinned groups are involved, flexible groups also need to be
@@ -2715,10 +2715,14 @@ static void ctx_resched(struct perf_cpu_

event_type &= EVENT_ALL;

- perf_ctx_disable(&cpuctx->ctx, false);
+ for_each_epc(epc, &cpuctx->ctx, pmu, false)
+ perf_pmu_disable(epc->pmu);
+
if (task_ctx) {
- perf_ctx_disable(task_ctx, false);
- task_ctx_sched_out(task_ctx, event_type);
+ for_each_epc(epc, task_ctx, pmu, false)
+ perf_pmu_disable(epc->pmu);
+
+ task_ctx_sched_out(task_ctx, pmu, event_type);
}

/*
@@ -2729,15 +2733,19 @@ static void ctx_resched(struct perf_cpu_
* - otherwise, do nothing more.
*/
if (cpu_event)
- ctx_sched_out(&cpuctx->ctx, event_type);
+ ctx_sched_out(&cpuctx->ctx, pmu, event_type);
else if (event_type & EVENT_PINNED)
- ctx_sched_out(&cpuctx->ctx, EVENT_FLEXIBLE);
+ ctx_sched_out(&cpuctx->ctx, pmu, EVENT_FLEXIBLE);

- perf_event_sched_in(cpuctx, task_ctx);
+ perf_event_sched_in(cpuctx, task_ctx, pmu);

- perf_ctx_enable(&cpuctx->ctx, false);
- if (task_ctx)
- perf_ctx_enable(task_ctx, false);
+ for_each_epc(epc, &cpuctx->ctx, pmu, false)
+ perf_pmu_enable(epc->pmu);
+
+ if (task_ctx) {
+ for_each_epc(epc, task_ctx, pmu, false)
+ perf_pmu_enable(epc->pmu);
+ }
}

void perf_pmu_resched(struct pmu *pmu)
@@ -2746,7 +2754,7 @@ void perf_pmu_resched(struct pmu *pmu)
struct perf_event_context *task_ctx = cpuctx->task_ctx;

perf_ctx_lock(cpuctx, task_ctx);
- ctx_resched(cpuctx, task_ctx, EVENT_ALL|EVENT_CPU);
+ ctx_resched(cpuctx, task_ctx, pmu, EVENT_ALL|EVENT_CPU);
perf_ctx_unlock(cpuctx, task_ctx);
}

@@ -2802,9 +2810,10 @@ static int __perf_install_in_context(vo
#endif

if (reprogram) {
- ctx_sched_out(ctx, EVENT_TIME);
+ ctx_sched_out(ctx, NULL, EVENT_TIME);
add_event_to_ctx(event, ctx);
- ctx_resched(cpuctx, task_ctx, get_event_type(event));
+ ctx_resched(cpuctx, task_ctx, event->pmu_ctx->pmu,
+ get_event_type(event));
} else {
add_event_to_ctx(event, ctx);
}
@@ -2948,7 +2957,7 @@ static void __perf_event_enable(struct p
return;

if (ctx->is_active)
- ctx_sched_out(ctx, EVENT_TIME);
+ ctx_sched_out(ctx, NULL, EVENT_TIME);

perf_event_set_state(event, PERF_EVENT_STATE_INACTIVE);
perf_cgroup_event_enable(event, ctx);
@@ -2957,7 +2966,7 @@ static void __perf_event_enable(struct p
return;

if (!event_filter_match(event)) {
- ctx_sched_in(ctx, EVENT_TIME);
+ ctx_sched_in(ctx, NULL, EVENT_TIME);
return;
}

@@ -2966,7 +2975,7 @@ static void __perf_event_enable(struct p
* then don't put it on unless the group is on.
*/
if (leader != event && leader->state != PERF_EVENT_STATE_ACTIVE) {
- ctx_sched_in(ctx, EVENT_TIME);
+ ctx_sched_in(ctx, NULL, EVENT_TIME);
return;
}

@@ -2974,7 +2983,7 @@ static void __perf_event_enable(struct p
if (ctx->task)
WARN_ON_ONCE(task_ctx != ctx);

- ctx_resched(cpuctx, task_ctx, get_event_type(event));
+ ctx_resched(cpuctx, task_ctx, event->pmu_ctx->pmu, get_event_type(event));
}

/*
@@ -3276,8 +3285,17 @@ static void __pmu_ctx_sched_out(struct p
perf_pmu_enable(pmu);
}

+/*
+ * Be very careful with the @pmu argument since this will change ctx state.
+ * The @pmu argument works for ctx_resched(), because that is symmetric in
+ * ctx_sched_out() / ctx_sched_in() usage and the ctx state ends up invariant.
+ *
+ * However, if you were to be asymmetrical, you could end up with messed up
+ * state, eg. ctx->is_active cleared even though most EPCs would still actually
+ * be active.
+ */
static void
-ctx_sched_out(struct perf_event_context *ctx, enum event_type_t event_type)
+ctx_sched_out(struct perf_event_context *ctx, struct pmu *pmu, enum event_type_t event_type)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
struct perf_event_pmu_context *pmu_ctx;
@@ -3331,11 +3349,8 @@ ctx_sched_out(struct perf_event_context

is_active ^= ctx->is_active; /* changed bits */

- list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
- if (cgroup && !pmu_ctx->nr_cgroups)
- continue;
+ for_each_epc(pmu_ctx, ctx, pmu, cgroup)
__pmu_ctx_sched_out(pmu_ctx, is_active);
- }
}

/*
@@ -3579,7 +3594,7 @@ perf_event_context_sched_out(struct task

inside_switch:
perf_ctx_sched_task_cb(ctx, false);
- task_ctx_sched_out(ctx, EVENT_ALL);
+ task_ctx_sched_out(ctx, NULL, EVENT_ALL);

perf_ctx_enable(ctx, false);
raw_spin_unlock(&ctx->lock);
@@ -3877,29 +3892,22 @@ static void pmu_groups_sched_in(struct p
merge_sched_in, &can_add_hw);
}

-static void ctx_groups_sched_in(struct perf_event_context *ctx,
- struct perf_event_groups *groups,
- bool cgroup)
+static void __pmu_ctx_sched_in(struct perf_event_pmu_context *pmu_ctx,
+ enum event_type_t event_type)
{
- struct perf_event_pmu_context *pmu_ctx;
-
- list_for_each_entry(pmu_ctx, &ctx->pmu_ctx_list, pmu_ctx_entry) {
- if (cgroup && !pmu_ctx->nr_cgroups)
- continue;
- pmu_groups_sched_in(ctx, groups, pmu_ctx->pmu);
- }
-}
+ struct perf_event_context *ctx = pmu_ctx->ctx;

-static void __pmu_ctx_sched_in(struct perf_event_context *ctx,
- struct pmu *pmu)
-{
- pmu_groups_sched_in(ctx, &ctx->flexible_groups, pmu);
+ if (event_type & EVENT_PINNED)
+ pmu_groups_sched_in(ctx, &ctx->pinned_groups, pmu_ctx->pmu);
+ if (event_type & EVENT_FLEXIBLE)
+ pmu_groups_sched_in(ctx, &ctx->flexible_groups, pmu_ctx->pmu);
}

static void
-ctx_sched_in(struct perf_event_context *ctx, enum event_type_t event_type)
+ctx_sched_in(struct perf_event_context *ctx, struct pmu *pmu, enum event_type_t event_type)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
+ struct perf_event_pmu_context *pmu_ctx;
int is_active = ctx->is_active;
bool cgroup = event_type & EVENT_CGROUP;

@@ -3935,12 +3943,16 @@ ctx_sched_in(struct perf_event_context *
* First go through the list and put on any pinned groups
* in order to give them the best chance of going on.
*/
- if (is_active & EVENT_PINNED)
- ctx_groups_sched_in(ctx, &ctx->pinned_groups, cgroup);
+ if (is_active & EVENT_PINNED) {
+ for_each_epc(pmu_ctx, ctx, pmu, cgroup)
+ __pmu_ctx_sched_in(pmu_ctx, EVENT_PINNED);
+ }

/* Then walk through the lower prio flexible groups */
- if (is_active & EVENT_FLEXIBLE)
- ctx_groups_sched_in(ctx, &ctx->flexible_groups, cgroup);
+ if (is_active & EVENT_FLEXIBLE) {
+ for_each_epc(pmu_ctx, ctx, pmu, cgroup)
+ __pmu_ctx_sched_in(pmu_ctx, EVENT_FLEXIBLE);
+ }
}

static void perf_event_context_sched_in(struct task_struct *task)
@@ -3983,10 +3995,10 @@ static void perf_event_context_sched_in(
*/
if (!RB_EMPTY_ROOT(&ctx->pinned_groups.tree)) {
perf_ctx_disable(&cpuctx->ctx, false);
- ctx_sched_out(&cpuctx->ctx, EVENT_FLEXIBLE);
+ ctx_sched_out(&cpuctx->ctx, NULL, EVENT_FLEXIBLE);
}

- perf_event_sched_in(cpuctx, ctx);
+ perf_event_sched_in(cpuctx, ctx, NULL);

perf_ctx_sched_task_cb(cpuctx->task_ctx, true);

@@ -4327,14 +4339,14 @@ static bool perf_rotate_context(struct p
update_context_time(&cpuctx->ctx);
__pmu_ctx_sched_out(cpu_epc, EVENT_FLEXIBLE);
rotate_ctx(&cpuctx->ctx, cpu_event);
- __pmu_ctx_sched_in(&cpuctx->ctx, pmu);
+ __pmu_ctx_sched_in(cpu_epc, EVENT_FLEXIBLE);
}

if (task_event)
rotate_ctx(task_epc->ctx, task_event);

if (task_event || (task_epc && cpu_event))
- __pmu_ctx_sched_in(task_epc->ctx, pmu);
+ __pmu_ctx_sched_in(task_epc, EVENT_FLEXIBLE);

perf_pmu_enable(pmu);
perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
@@ -4400,7 +4412,7 @@ static void perf_event_enable_on_exec(st

cpuctx = this_cpu_ptr(&perf_cpu_context);
perf_ctx_lock(cpuctx, ctx);
- ctx_sched_out(ctx, EVENT_TIME);
+ ctx_sched_out(ctx, NULL, EVENT_TIME);

list_for_each_entry(event, &ctx->event_list, event_entry) {
enabled |= event_enable_on_exec(event, ctx);
@@ -4412,9 +4424,9 @@ static void perf_event_enable_on_exec(st
*/
if (enabled) {
clone_ctx = unclone_ctx(ctx);
- ctx_resched(cpuctx, ctx, event_type);
+ ctx_resched(cpuctx, ctx, NULL, event_type);
} else {
- ctx_sched_in(ctx, EVENT_TIME);
+ ctx_sched_in(ctx, NULL, EVENT_TIME);
}
perf_ctx_unlock(cpuctx, ctx);

@@ -13202,7 +13214,7 @@ static void perf_event_exit_task_context
* in.
*/
raw_spin_lock_irq(&child_ctx->lock);
- task_ctx_sched_out(child_ctx, EVENT_ALL);
+ task_ctx_sched_out(child_ctx, NULL, EVENT_ALL);

/*
* Now that the context is inactive, destroy the task <-> ctx relation
@@ -13751,7 +13763,7 @@ static void __perf_event_exit_context(vo
struct perf_event *event;

raw_spin_lock(&ctx->lock);
- ctx_sched_out(ctx, EVENT_TIME);
+ ctx_sched_out(ctx, NULL, EVENT_TIME);
list_for_each_entry(event, &ctx->event_list, event_entry)
__perf_remove_from_context(event, cpuctx, ctx, (void *)DETACH_GROUP);
raw_spin_unlock(&ctx->lock);