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perf/core: Add a new read format to get a number of lost samples
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Sometimes we want to know an accurate number of samples even if it's
lost.  Currenlty PERF_RECORD_LOST is generated for a ring-buffer which
might be shared with other events.  So it's hard to know per-event
lost count.

Add event->lost_samples field and PERF_FORMAT_LOST to retrieve it from
userspace.

Original-patch-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220616180623.1358843-1-namhyung@kernel.org
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Namhyung Kim authored and Peter Zijlstra committed Jun 28, 2022
1 parent f0fe9f3 commit 119a784
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Showing 4 changed files with 28 additions and 5 deletions.
2 changes: 2 additions & 0 deletions include/linux/perf_event.h
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Original file line number Diff line number Diff line change
Expand Up @@ -759,6 +759,8 @@ struct perf_event {
struct pid_namespace *ns;
u64 id;

atomic64_t lost_samples;

u64 (*clock)(void);
perf_overflow_handler_t overflow_handler;
void *overflow_handler_context;
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5 changes: 4 additions & 1 deletion include/uapi/linux/perf_event.h
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Original file line number Diff line number Diff line change
Expand Up @@ -301,13 +301,15 @@ enum {
* { u64 time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
* { u64 time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
* { u64 id; } && PERF_FORMAT_ID
* { u64 lost; } && PERF_FORMAT_LOST
* } && !PERF_FORMAT_GROUP
*
* { u64 nr;
* { u64 time_enabled; } && PERF_FORMAT_TOTAL_TIME_ENABLED
* { u64 time_running; } && PERF_FORMAT_TOTAL_TIME_RUNNING
* { u64 value;
* { u64 id; } && PERF_FORMAT_ID
* { u64 lost; } && PERF_FORMAT_LOST
* } cntr[nr];
* } && PERF_FORMAT_GROUP
* };
Expand All @@ -317,8 +319,9 @@ enum perf_event_read_format {
PERF_FORMAT_TOTAL_TIME_RUNNING = 1U << 1,
PERF_FORMAT_ID = 1U << 2,
PERF_FORMAT_GROUP = 1U << 3,
PERF_FORMAT_LOST = 1U << 4,

PERF_FORMAT_MAX = 1U << 4, /* non-ABI */
PERF_FORMAT_MAX = 1U << 5, /* non-ABI */
};

#define PERF_ATTR_SIZE_VER0 64 /* sizeof first published struct */
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21 changes: 18 additions & 3 deletions kernel/events/core.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1819,6 +1819,9 @@ static void __perf_event_read_size(struct perf_event *event, int nr_siblings)
if (event->attr.read_format & PERF_FORMAT_ID)
entry += sizeof(u64);

if (event->attr.read_format & PERF_FORMAT_LOST)
entry += sizeof(u64);

if (event->attr.read_format & PERF_FORMAT_GROUP) {
nr += nr_siblings;
size += sizeof(u64);
Expand Down Expand Up @@ -5260,11 +5263,15 @@ static int __perf_read_group_add(struct perf_event *leader,
values[n++] += perf_event_count(leader);
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(leader);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&leader->lost_samples);

for_each_sibling_event(sub, leader) {
values[n++] += perf_event_count(sub);
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(sub);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&sub->lost_samples);
}

raw_spin_unlock_irqrestore(&ctx->lock, flags);
Expand Down Expand Up @@ -5321,7 +5328,7 @@ static int perf_read_one(struct perf_event *event,
u64 read_format, char __user *buf)
{
u64 enabled, running;
u64 values[4];
u64 values[5];
int n = 0;

values[n++] = __perf_event_read_value(event, &enabled, &running);
Expand All @@ -5331,6 +5338,8 @@ static int perf_read_one(struct perf_event *event,
values[n++] = running;
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(event);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&event->lost_samples);

if (copy_to_user(buf, values, n * sizeof(u64)))
return -EFAULT;
Expand Down Expand Up @@ -6858,7 +6867,7 @@ static void perf_output_read_one(struct perf_output_handle *handle,
u64 enabled, u64 running)
{
u64 read_format = event->attr.read_format;
u64 values[4];
u64 values[5];
int n = 0;

values[n++] = perf_event_count(event);
Expand All @@ -6872,6 +6881,8 @@ static void perf_output_read_one(struct perf_output_handle *handle,
}
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(event);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&event->lost_samples);

__output_copy(handle, values, n * sizeof(u64));
}
Expand All @@ -6882,7 +6893,7 @@ static void perf_output_read_group(struct perf_output_handle *handle,
{
struct perf_event *leader = event->group_leader, *sub;
u64 read_format = event->attr.read_format;
u64 values[5];
u64 values[6];
int n = 0;

values[n++] = 1 + leader->nr_siblings;
Expand All @@ -6900,6 +6911,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
values[n++] = perf_event_count(leader);
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(leader);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&leader->lost_samples);

__output_copy(handle, values, n * sizeof(u64));

Expand All @@ -6913,6 +6926,8 @@ static void perf_output_read_group(struct perf_output_handle *handle,
values[n++] = perf_event_count(sub);
if (read_format & PERF_FORMAT_ID)
values[n++] = primary_event_id(sub);
if (read_format & PERF_FORMAT_LOST)
values[n++] = atomic64_read(&sub->lost_samples);

__output_copy(handle, values, n * sizeof(u64));
}
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5 changes: 4 additions & 1 deletion kernel/events/ring_buffer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -172,8 +172,10 @@ __perf_output_begin(struct perf_output_handle *handle,
goto out;

if (unlikely(rb->paused)) {
if (rb->nr_pages)
if (rb->nr_pages) {
local_inc(&rb->lost);
atomic64_inc(&event->lost_samples);
}
goto out;
}

Expand Down Expand Up @@ -254,6 +256,7 @@ __perf_output_begin(struct perf_output_handle *handle,

fail:
local_inc(&rb->lost);
atomic64_inc(&event->lost_samples);
perf_output_put_handle(handle);
out:
rcu_read_unlock();
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