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authorAdrian Hunter <adrian.hunter@intel.com>2015-07-21 09:44:02 (GMT)
committerArnaldo Carvalho de Melo <acme@redhat.com>2015-07-24 01:51:12 (GMT)
commit45ac1403f564f411c6a383a2448688ba8dd705a4 (patch)
tree50e1569b0dee4ba4e0730548a25322e3e0434ff6 /kernel/events
parent6e5259e9b5b711b325a8455feb3ed27cdd0af0da (diff)
downloadlinux-45ac1403f564f411c6a383a2448688ba8dd705a4.tar.xz
perf: Add PERF_RECORD_SWITCH to indicate context switches
There are already two events for context switches, namely the tracepoint sched:sched_switch and the software event context_switches. Unfortunately neither are suitable for use by non-privileged users for the purpose of synchronizing hardware trace data (e.g. Intel PT) to the context switch. Tracepoints are no good at all for non-privileged users because they need either CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= -1. On the other hand, kernel software events need either CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= 1. Now many distributions do default perf_event_paranoid to 1 making context_switches a contender, except it has another problem (which is also shared with sched:sched_switch) which is that it happens before perf schedules events out instead of after perf schedules events in. Whereas a privileged user can see all the events anyway, a non-privileged user only sees events for their own processes, in other words they see when their process was scheduled out not when it was scheduled in. That presents two problems to use the event: 1. the information comes too late, so tools have to look ahead in the event stream to find out what the current state is 2. if they are unlucky tracing might have stopped before the context-switches event is recorded. This new PERF_RECORD_SWITCH event does not have those problems and it also has a couple of other small advantages. It is easier to use because it is an auxiliary event (like mmap, comm and task events) which can be enabled by setting a single bit. It is smaller than sched:sched_switch and easier to parse. To make the event useful for privileged users also, if the context is cpu-wide then the event record will be PERF_RECORD_SWITCH_CPU_WIDE which is the same as PERF_RECORD_SWITCH except it also provides the next or previous pid/tid. Signed-off-by: Adrian Hunter <adrian.hunter@intel.com> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Jiri Olsa <jolsa@redhat.com> Cc: Andi Kleen <ak@linux.intel.com> Cc: Mathieu Poirier <mathieu.poirier@linaro.org> Cc: Pawel Moll <pawel.moll@arm.com> Cc: Stephane Eranian <eranian@google.com> Link: http://lkml.kernel.org/r/1437471846-26995-2-git-send-email-adrian.hunter@intel.com Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Diffstat (limited to 'kernel/events')
-rw-r--r--kernel/events/core.c103
1 files changed, 103 insertions, 0 deletions
diff --git a/kernel/events/core.c b/kernel/events/core.c
index d3dae34..ce21143 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -163,6 +163,7 @@ static atomic_t nr_mmap_events __read_mostly;
static atomic_t nr_comm_events __read_mostly;
static atomic_t nr_task_events __read_mostly;
static atomic_t nr_freq_events __read_mostly;
+static atomic_t nr_switch_events __read_mostly;
static LIST_HEAD(pmus);
static DEFINE_MUTEX(pmus_lock);
@@ -2619,6 +2620,9 @@ static void perf_pmu_sched_task(struct task_struct *prev,
local_irq_restore(flags);
}
+static void perf_event_switch(struct task_struct *task,
+ struct task_struct *next_prev, bool sched_in);
+
#define for_each_task_context_nr(ctxn) \
for ((ctxn) = 0; (ctxn) < perf_nr_task_contexts; (ctxn)++)
@@ -2641,6 +2645,9 @@ void __perf_event_task_sched_out(struct task_struct *task,
if (__this_cpu_read(perf_sched_cb_usages))
perf_pmu_sched_task(task, next, false);
+ if (atomic_read(&nr_switch_events))
+ perf_event_switch(task, next, false);
+
for_each_task_context_nr(ctxn)
perf_event_context_sched_out(task, ctxn, next);
@@ -2831,6 +2838,9 @@ void __perf_event_task_sched_in(struct task_struct *prev,
if (atomic_read(this_cpu_ptr(&perf_cgroup_events)))
perf_cgroup_sched_in(prev, task);
+ if (atomic_read(&nr_switch_events))
+ perf_event_switch(task, prev, true);
+
if (__this_cpu_read(perf_sched_cb_usages))
perf_pmu_sched_task(prev, task, true);
}
@@ -3454,6 +3464,10 @@ static void unaccount_event(struct perf_event *event)
atomic_dec(&nr_task_events);
if (event->attr.freq)
atomic_dec(&nr_freq_events);
+ if (event->attr.context_switch) {
+ static_key_slow_dec_deferred(&perf_sched_events);
+ atomic_dec(&nr_switch_events);
+ }
if (is_cgroup_event(event))
static_key_slow_dec_deferred(&perf_sched_events);
if (has_branch_stack(event))
@@ -5982,6 +5996,91 @@ void perf_log_lost_samples(struct perf_event *event, u64 lost)
}
/*
+ * context_switch tracking
+ */
+
+struct perf_switch_event {
+ struct task_struct *task;
+ struct task_struct *next_prev;
+
+ struct {
+ struct perf_event_header header;
+ u32 next_prev_pid;
+ u32 next_prev_tid;
+ } event_id;
+};
+
+static int perf_event_switch_match(struct perf_event *event)
+{
+ return event->attr.context_switch;
+}
+
+static void perf_event_switch_output(struct perf_event *event, void *data)
+{
+ struct perf_switch_event *se = data;
+ struct perf_output_handle handle;
+ struct perf_sample_data sample;
+ int ret;
+
+ if (!perf_event_switch_match(event))
+ return;
+
+ /* Only CPU-wide events are allowed to see next/prev pid/tid */
+ if (event->ctx->task) {
+ se->event_id.header.type = PERF_RECORD_SWITCH;
+ se->event_id.header.size = sizeof(se->event_id.header);
+ } else {
+ se->event_id.header.type = PERF_RECORD_SWITCH_CPU_WIDE;
+ se->event_id.header.size = sizeof(se->event_id);
+ se->event_id.next_prev_pid =
+ perf_event_pid(event, se->next_prev);
+ se->event_id.next_prev_tid =
+ perf_event_tid(event, se->next_prev);
+ }
+
+ perf_event_header__init_id(&se->event_id.header, &sample, event);
+
+ ret = perf_output_begin(&handle, event, se->event_id.header.size);
+ if (ret)
+ return;
+
+ if (event->ctx->task)
+ perf_output_put(&handle, se->event_id.header);
+ else
+ perf_output_put(&handle, se->event_id);
+
+ perf_event__output_id_sample(event, &handle, &sample);
+
+ perf_output_end(&handle);
+}
+
+static void perf_event_switch(struct task_struct *task,
+ struct task_struct *next_prev, bool sched_in)
+{
+ struct perf_switch_event switch_event;
+
+ /* N.B. caller checks nr_switch_events != 0 */
+
+ switch_event = (struct perf_switch_event){
+ .task = task,
+ .next_prev = next_prev,
+ .event_id = {
+ .header = {
+ /* .type */
+ .misc = sched_in ? 0 : PERF_RECORD_MISC_SWITCH_OUT,
+ /* .size */
+ },
+ /* .next_prev_pid */
+ /* .next_prev_tid */
+ },
+ };
+
+ perf_event_aux(perf_event_switch_output,
+ &switch_event,
+ NULL);
+}
+
+/*
* IRQ throttle logging
*/
@@ -7479,6 +7578,10 @@ static void account_event(struct perf_event *event)
if (atomic_inc_return(&nr_freq_events) == 1)
tick_nohz_full_kick_all();
}
+ if (event->attr.context_switch) {
+ atomic_inc(&nr_switch_events);
+ static_key_slow_inc(&perf_sched_events.key);
+ }
if (has_branch_stack(event))
static_key_slow_inc(&perf_sched_events.key);
if (is_cgroup_event(event))