Re: [PATCH RFC] remove jump_label optimization for perf sched events
From: Peter Zijlstra
Date: Thu Nov 17 2011 - 07:49:52 EST
On Thu, 2011-11-17 at 14:30 +0200, Gleb Natapov wrote:
> jump_lable patching is very expensive operation that involves pausing all
> cpus. The patching of perf_sched_events jump_label is easily controllable
> from userspace by unprivileged user. When user runs loop like this
> "while true; do perf stat -e cycles true; done" the performance of my
> test application that just increments a counter for one second drops by
> 4%. This is on a 16 cpu box with my test application using only one of
> them. An impact on a real server doing real work will be much worse.
> Performance of KVM PMU drops nearly 50% due to jump_lable for "perf
> record" since KVM PMU implementation creates and destroys perf event
> frequently.
Ideally we'd fix text_poke to not use stop_machine() we know how to, but
we haven't had the green light from Intel/AMD yet.
Rostedt was going to implement it anyway and see if anything breaks.
Also, virt might be able to pull something smart on text_poke() dunno.
That said, I'd much rather throttle this particular jump label than
remove it altogether, some people really don't like all this scheduler
hot path crap.
Something I've pondered for a while but never actually tried yet (and it
hasn't even seen a compiler) is something like the below, I don't think
there's any reason to have two scheduler hooks.
It wouldn't solve your problem, but having only one hooks does make it
easier to play around with throttling stuff.
---
include/linux/perf_event.h | 19 +++++--------------
kernel/events/core.c | 14 ++++++++++----
kernel/sched.c | 9 +--------
3 files changed, 16 insertions(+), 26 deletions(-)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 1e9ebe5..f1f621a 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -947,10 +947,8 @@ extern void perf_pmu_unregister(struct pmu *pmu);
extern int perf_num_counters(void);
extern const char *perf_pmu_name(void);
-extern void __perf_event_task_sched_in(struct task_struct *prev,
- struct task_struct *task);
-extern void __perf_event_task_sched_out(struct task_struct *prev,
- struct task_struct *next);
+extern void __perf_event_task_sched(struct task_struct *prev,
+ struct task_struct *next);
extern int perf_event_init_task(struct task_struct *child);
extern void perf_event_exit_task(struct task_struct *child);
extern void perf_event_free_task(struct task_struct *task);
@@ -1064,20 +1062,13 @@ perf_sw_event(u32 event_id, u64 nr, struct pt_regs *regs, u64 addr)
extern struct jump_label_key perf_sched_events;
-static inline void perf_event_task_sched_in(struct task_struct *prev,
- struct task_struct *task)
-{
- if (static_branch(&perf_sched_events))
- __perf_event_task_sched_in(prev, task);
-}
-
-static inline void perf_event_task_sched_out(struct task_struct *prev,
- struct task_struct *next)
+static inline void perf_event_task_sched(struct task_struct *prev,
+ struct task_struct *next)
{
perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, NULL, 0);
if (static_branch(&perf_sched_events))
- __perf_event_task_sched_out(prev, next);
+ __perf_event_task_sched(prev, next);
}
extern void perf_event_mmap(struct vm_area_struct *vma);
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2e41c8e..bf9bccb 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2019,8 +2019,8 @@ static void perf_event_context_sched_out(struct task_struct *task, int ctxn,
* accessing the event control register. If a NMI hits, then it will
* not restart the event.
*/
-void __perf_event_task_sched_out(struct task_struct *task,
- struct task_struct *next)
+static void
+__perf_event_task_sched_out(struct task_struct *task, struct task_struct *next)
{
int ctxn;
@@ -2199,8 +2199,8 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
* accessing the event control register. If a NMI hits, then it will
* keep the event running.
*/
-void __perf_event_task_sched_in(struct task_struct *prev,
- struct task_struct *task)
+static void
+__perf_event_task_sched_in(struct task_struct *prev, struct task_struct *task)
{
struct perf_event_context *ctx;
int ctxn;
@@ -2221,6 +2221,12 @@ void __perf_event_task_sched_in(struct task_struct *prev,
perf_cgroup_sched_in(prev, task);
}
+void __perf_event_task_sched(struct task_struct *prev, struct task_struct *next)
+{
+ __perf_event_task_sched_out(prev, next);
+ __perf_event_task_sched_in(prev, next);
+}
+
static u64 perf_calculate_period(struct perf_event *event, u64 nsec, u64 count)
{
u64 frequency = event->attr.sample_freq;
diff --git a/kernel/sched.c b/kernel/sched.c
index c9e3ab6..657bbc1 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3183,7 +3183,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
struct task_struct *next)
{
sched_info_switch(prev, next);
- perf_event_task_sched_out(prev, next);
+ perf_event_task_sched(prev, next);
fire_sched_out_preempt_notifiers(prev, next);
prepare_lock_switch(rq, next);
prepare_arch_switch(next);
@@ -3226,13 +3226,6 @@ static void finish_task_switch(struct rq *rq, struct task_struct *prev)
*/
prev_state = prev->state;
finish_arch_switch(prev);
-#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
- local_irq_disable();
-#endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
- perf_event_task_sched_in(prev, current);
-#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
- local_irq_enable();
-#endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
finish_lock_switch(rq, prev);
fire_sched_in_preempt_notifiers(current);
--
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