[PATCH 3/3] perf: Optimize sched_task() in a context switch

kan.liang
2020-11-06T21:31:19Z

From: Kan Liang <kan.liang@linux.intel.com>

Some calls to sched_task() in a context switch can be avoided. For
example, large PEBS only requires flushing the buffer in context switch
out. The current code still invokes the sched_task() for large PEBS in
context switch in.

The current code doesn't know and check the states of an event. It has
no idea which calls is unnecessary.

Add a new variable, sched_cb_state, to indicate the states of an event.
Add sched_cb_state to track the states per CPU context. Only invokes
the sched_task() when the relative states are set.

Split sched_cb_usage into sched_cb_task_usage and sched_cb_cpu_usage to
track per-task and per-CPU events respectively. Avoid going through the
sched_cb_list for pure per-task events. Avoid invoking sched_task() in
perf_event_context_sched_in/out for pure perf-CPU events.

Since the interface functions are changed, the specific codes are also
changed here.
- For power, the BHRB entries are reset in context switch in.
- For X86 large PEBS, the PEBS buffer are flushed in context switch out.
- For X86 normal LBR, the LBR registers are reset in context switch in.
- For X86 LBR call stack, the LBR registers are saved in context switch
out, and restored in context switch in.

Suggested-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
arch/powerpc/perf/core-book3s.c | 7 +++-
arch/x86/events/intel/ds.c | 8 +++--
arch/x86/events/intel/lbr.c | 15 ++++++--
include/linux/perf_event.h | 12 +++++--
kernel/events/core.c | 62 +++++++++++++++++++++++++++------
5 files changed, 86 insertions(+), 18 deletions(-)

diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
index 6586f7e71cfb..e5837fdf82e3 100644
--- a/arch/powerpc/perf/core-book3s.c
+++ b/arch/powerpc/perf/core-book3s.c
@@ -370,6 +370,7 @@ static void power_pmu_bhrb_reset(void)
static void power_pmu_bhrb_enable(struct perf_event *event)
{
struct cpu_hw_events *cpuhw = this_cpu_ptr(&cpu_hw_events);
+ int state = PERF_SCHED_CB_SW_IN;

if (!ppmu->bhrb_nr)
return;
@@ -380,7 +381,11 @@ static void power_pmu_bhrb_enable(struct perf_event *event)
cpuhw->bhrb_context = event->ctx;
}
cpuhw->bhrb_users++;
- perf_sched_cb_inc(event->ctx->pmu);
+
+ if (!(event->attach_state & PERF_ATTACH_TASK))
+ state |= PERF_SCHED_CB_CPU;
+
+ perf_sched_cb_inc(event->ctx->pmu, state);
}

static void power_pmu_bhrb_disable(struct perf_event *event)
diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index 444e5f061d04..f4f9d737ca85 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -1019,12 +1019,16 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc,
* that does not hurt:
*/
bool update = cpuc->n_pebs == 1;
+ int state = PERF_SCHED_CB_SW_OUT;

if (needed_cb != pebs_needs_sched_cb(cpuc)) {
+ if (!(event->attach_state & PERF_ATTACH_TASK))
+ state |= PERF_SCHED_CB_CPU;
+
if (!needed_cb)
- perf_sched_cb_inc(pmu);
+ perf_sched_cb_inc(pmu, state);
else
- perf_sched_cb_dec(pmu);
+ perf_sched_cb_dec(pmu, state);

update = true;
}
diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index 8961653c5dd2..e4c500580df5 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -660,6 +660,7 @@ void intel_pmu_lbr_add(struct perf_event *event)
{
struct kmem_cache *kmem_cache = event->pmu->task_ctx_cache;
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+ int state = PERF_SCHED_CB_SW_IN;

if (!x86_pmu.lbr_nr)
return;
@@ -693,7 +694,13 @@ void intel_pmu_lbr_add(struct perf_event *event)
*/
if (x86_pmu.intel_cap.pebs_baseline && event->attr.precise_ip > 0)
cpuc->lbr_pebs_users++;
- perf_sched_cb_inc(event->ctx->pmu);
+
+ if (!(event->attach_state & PERF_ATTACH_TASK))
+ state |= PERF_SCHED_CB_CPU;
+ if (event->attach_state & PERF_ATTACH_TASK_DATA)
+ state |= PERF_SCHED_CB_SW_OUT;
+
+ perf_sched_cb_inc(event->ctx->pmu, state);
if (!cpuc->lbr_users++ && !event->total_time_running)
intel_pmu_lbr_reset();

@@ -724,6 +731,7 @@ void release_lbr_buffers(void)
void intel_pmu_lbr_del(struct perf_event *event)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+ int state = 0;

if (!x86_pmu.lbr_nr)
return;
@@ -740,7 +748,10 @@ void intel_pmu_lbr_del(struct perf_event *event)
cpuc->lbr_users--;
WARN_ON_ONCE(cpuc->lbr_users < 0);
WARN_ON_ONCE(cpuc->lbr_pebs_users < 0);
- perf_sched_cb_dec(event->ctx->pmu);
+
+ if (!(event->attach_state & PERF_ATTACH_TASK))
+ state |= PERF_SCHED_CB_CPU;
+ perf_sched_cb_dec(event->ctx->pmu, state);
}

static inline bool vlbr_exclude_host(void)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index f7a84d1048b9..839dd65dd1f3 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -854,6 +854,10 @@ struct perf_event_context {
*/
#define PERF_NR_CONTEXTS 4

+#define PERF_SCHED_CB_CPU 0x1
+#define PERF_SCHED_CB_SW_IN 0x2
+#define PERF_SCHED_CB_SW_OUT 0x4
+
/**
* struct perf_event_cpu_context - per cpu event context structure
*/
@@ -874,7 +878,9 @@ struct perf_cpu_context {
#endif

struct list_head sched_cb_entry;
- int sched_cb_usage;
+ int sched_cb_task_usage;
+ int sched_cb_cpu_usage;
+ int sched_cb_state;

int online;
/*
@@ -967,8 +973,8 @@ extern const struct perf_event_attr *perf_event_attrs(struct perf_event *event);
extern void perf_event_print_debug(void);
extern void perf_pmu_disable(struct pmu *pmu);
extern void perf_pmu_enable(struct pmu *pmu);
-extern void perf_sched_cb_dec(struct pmu *pmu);
-extern void perf_sched_cb_inc(struct pmu *pmu);
+extern void perf_sched_cb_dec(struct pmu *pmu, int state);
+extern void perf_sched_cb_inc(struct pmu *pmu, int state);
extern int perf_event_task_disable(void);
extern int perf_event_task_enable(void);

diff --git a/kernel/events/core.c b/kernel/events/core.c
index df0df5514097..6a19b8fa0dd9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3384,6 +3384,32 @@ static void perf_event_sync_stat(struct perf_event_context *ctx,
}
}

+static inline bool perf_pmu_sched_in_ctx(struct perf_cpu_context *cpuctx)
+{
+ return !!(cpuctx->sched_cb_state & PERF_SCHED_CB_SW_IN);
+}
+
+static inline bool perf_pmu_sched_out_ctx(struct perf_cpu_context *cpuctx)
+{
+ return !!(cpuctx->sched_cb_state & PERF_SCHED_CB_SW_OUT);
+}
+
+static inline bool perf_pmu_has_sched_task(struct perf_cpu_context *cpuctx, bool sched_in)
+{
+ struct pmu *pmu = cpuctx->ctx.pmu;
+
+ if (!pmu->sched_task)
+ return false;
+
+ if (sched_in && perf_pmu_sched_in_ctx(cpuctx))
+ return true;
+
+ if (!sched_in && perf_pmu_sched_out_ctx(cpuctx))
+ return true;
+
+ return false;
+}
+
static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
struct task_struct *next)
{
@@ -3433,7 +3459,7 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,

perf_pmu_disable(pmu);

- if (cpuctx->sched_cb_usage && pmu->sched_task)
+ if (cpuctx->sched_cb_task_usage && perf_pmu_has_sched_task(cpuctx, false))
pmu->sched_task(ctx, false);

/*
@@ -3473,7 +3499,7 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
raw_spin_lock(&ctx->lock);
perf_pmu_disable(pmu);

- if (cpuctx->sched_cb_usage && pmu->sched_task)
+ if (cpuctx->sched_cb_task_usage && perf_pmu_has_sched_task(cpuctx, false))
pmu->sched_task(ctx, false);
task_ctx_sched_out(cpuctx, ctx, EVENT_ALL);

@@ -3484,22 +3510,37 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,

static DEFINE_PER_CPU(struct list_head, sched_cb_list);

-void perf_sched_cb_dec(struct pmu *pmu)
+void perf_sched_cb_dec(struct pmu *pmu, int state)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);

+ if (!(state & PERF_SCHED_CB_CPU)) {
+ --cpuctx->sched_cb_task_usage;
+ goto end;
+ }
+
this_cpu_dec(perf_sched_cb_usages);

- if (!--cpuctx->sched_cb_usage)
+ if (!--cpuctx->sched_cb_cpu_usage)
list_del(&cpuctx->sched_cb_entry);
+end:
+ if (!cpuctx->sched_cb_cpu_usage && !cpuctx->sched_cb_task_usage)
+ cpuctx->sched_cb_state = 0;
}

-void perf_sched_cb_inc(struct pmu *pmu)
+void perf_sched_cb_inc(struct pmu *pmu, int state)
{
struct perf_cpu_context *cpuctx = this_cpu_ptr(pmu->pmu_cpu_context);

- if (!cpuctx->sched_cb_usage++)
+ cpuctx->sched_cb_state |= state;
+
+ if (!(state & PERF_SCHED_CB_CPU)) {
+ cpuctx->sched_cb_task_usage++;
+ return;
+ }
+
+ if (!cpuctx->sched_cb_cpu_usage++)
list_add(&cpuctx->sched_cb_entry, this_cpu_ptr(&sched_cb_list));

this_cpu_inc(perf_sched_cb_usages);
@@ -3544,8 +3585,8 @@ static void perf_pmu_sched_task(struct task_struct *prev,
/* will be handled in perf_event_context_sched_in/out */
if (cpuctx->task_ctx)
continue;
-
- __perf_pmu_sched_task(cpuctx, sched_in);
+ if (perf_pmu_has_sched_task(cpuctx, sched_in))
+ __perf_pmu_sched_task(cpuctx, sched_in);
}
}

@@ -3809,7 +3850,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,

cpuctx = __get_cpu_context(ctx);
if (cpuctx->task_ctx == ctx) {
- if (cpuctx->sched_cb_usage)
+ if (cpuctx->sched_cb_task_usage &&
+ (cpuctx->sched_cb_state & PERF_SCHED_CB_SW_IN))
__perf_pmu_sched_task(cpuctx, true);
return;
}
@@ -3835,7 +3877,7 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
perf_event_sched_in(cpuctx, ctx, task);

- if (cpuctx->sched_cb_usage && pmu->sched_task)
+ if (cpuctx->sched_cb_task_usage && perf_pmu_has_sched_task(cpuctx, true))
pmu->sched_task(cpuctx->task_ctx, true);

perf_pmu_enable(pmu);
--
2.17.1