* [PATCH 0/3 V2] ring-buffer: Restructure ftrace ring buffer time keeping to allow accurate nested timing
@ 2020-06-29 2:52 Steven Rostedt
2020-06-29 2:52 ` [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp Steven Rostedt
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Steven Rostedt @ 2020-06-29 2:52 UTC (permalink / raw)
To: linux-kernel
Cc: Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra, Thomas Gleixner,
Masami Hiramatsu, Arnaldo Carvalho de Melo, Jiri Olsa,
Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov, Tom Zanussi,
Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
V1 of this patch set can be found here:
https://lore.kernel.org/r/20200627010041.517736087@goodmis.org
Changes since V1 patch set:
- Fixed white space issues (kernel test robot)
- Fixed change log algorithm to match the code (Masami Hiramatsu)
- Removed next_write useless code. (Masami Hiramatsu)
Head SHA1: 72c27b3e97be0b2fca6b5518d299bccbc714a245
Steven Rostedt (VMware) (3):
ring-buffer: Have nested events still record running time stamp
ring-buffer: Incorporate absolute timestamp into add_timestamp logic
ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit
----
kernel/trace/ring_buffer.c | 498 ++++++++++++++++++++++++++++++++++++---------
1 file changed, 401 insertions(+), 97 deletions(-)
^ permalink raw reply [flat|nested] 6+ messages in thread
* [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp
2020-06-29 2:52 [PATCH 0/3 V2] ring-buffer: Restructure ftrace ring buffer time keeping to allow accurate nested timing Steven Rostedt
@ 2020-06-29 2:52 ` Steven Rostedt
2020-06-29 13:49 ` Masami Hiramatsu
2020-06-29 2:52 ` [PATCH 2/3 V2] ring-buffer: Incorporate absolute timestamp into add_timestamp logic Steven Rostedt
2020-06-29 2:52 ` [PATCH 3/3 V2] ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit Steven Rostedt
2 siblings, 1 reply; 6+ messages in thread
From: Steven Rostedt @ 2020-06-29 2:52 UTC (permalink / raw)
To: linux-kernel
Cc: Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra, Thomas Gleixner,
Masami Hiramatsu, Arnaldo Carvalho de Melo, Jiri Olsa,
Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov, Tom Zanussi,
Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Up until now, if an event is interrupted while it is recorded by an
interrupt, and that interrupt records events, the time of those events will
all be the same. This is because events only record the delta of the time
since the previous event (or beginning of a page), and to handle updating
the time keeping for that of nested events is extremely racy. After years of
thinking about this and several failed attempts, I finally have a solution
to solve this puzzle.
The problem is that you need to atomically calculate the delta and then
update the time stamp you made the delta from, as well as then record it
into the buffer, all this while at any time an interrupt can come in and
do the same thing. This is easy to solve with heavy weight atomics, but that
would be detrimental to the performance of the ring buffer. The current
state of affairs sacrificed the time deltas for nested events for
performance.
The reason for previous failed attempts at solving this puzzle was because I
was trying to completely avoid slow atomic operations like cmpxchg. I final
came to the conclusion to always avoid cmpxchg is not possible, which is why
those previous attempts always failed. But it is possible to pick one path
(the most common case) and avoid cmpxchg in that path, which is the "fast
path". The most common case is that an event will not be interrupted and
have other events added into it. An event can detect if it has
interrupted another event, and for these cases we can make it the slow
path and use the heavy operations like cmpxchg.
One more player was added to the game that made this possible, and that is
the "absolute timestamp" (by Tom Zanussi) that allows us to inject a full 59
bit time stamp. (Of course this breaks if a machine is running for more than
18 years without a reboot!).
There's barrier() placements around for being paranoid, even when they
are not needed because of other atomic functions near by. But those
should not hurt, as if they are not needed, they basically become a nop.
Note, this also makes the race window much smaller, which means there
are less slow paths to slow down the performance.
Here's the design of this solution:
All this is per cpu, and only needs to worry about nested events (not
parallel events).
The players:
write_tail: The index in the buffer where new events can be written to.
It is incremented via local_add() to reserve space for a new event.
before_stamp: A time stamp set by all events before reserving space.
write_stamp: A time stamp updated by events after it has successfully
reserved space.
/* Save the current position of write */
[A] w = local_read(write_tail);
barrier();
/* Read both before and write stamps before touching anything */
before = local_read(before_stamp);
after = local_read(write_stamp);
barrier();
/*
* If before and after are the same, then this event is not
* interrupting a time update. If it is, then reserve space for adding
* a full time stamp (this can turn into a time extend which is
* just an extended time delta but fill up the extra space).
*/
if (after != before)
abs = true;
ts = clock();
/* Now update the before_stamp (everyone does this!) */
[B] local_set(before_stamp, ts);
/* Now reserve space on the buffer */
[C] write = local_add_return(len, write_tail);
/* Set tail to be were this event's data is */
tail = write - len;
if (w == tail) {
/* Nothing interrupted this between A and C */
[D] local_set(write_stamp, ts);
barrier();
[E] save_before = local_read(before_stamp);
if (!abs) {
/* This did not interrupt a time update */
delta = ts - after;
} else {
delta = ts; /* The full time stamp will be in use */
}
if (ts != save_before) {
/* slow path - Was interrupted between C and E */
/* The update to write_stamp could have overwritten the update to
* it by the interrupting event, but before and after should be
* the same for all completed top events */
after = local_read(write_stamp);
if (save_before > after)
local_cmpxchg(write_stamp, after, save_before);
}
} else {
/* slow path - Interrupted between A and C */
after = local_read(write_stamp);
temp_ts = clock();
barrier();
[F] if (write == local_read(write_tail) && after < temp_ts) {
/* This was not interrupted since C and F
* The last write_stamp is still valid for the previous event
* in the buffer. */
delta = temp_ts - after;
/* OK to keep this new time stamp */
ts = temp_ts;
} else {
/* Interrupted between C and F
* Well, there's no use to try to know what the time stamp
* is for the previous event. Just set delta to zero and
* be the same time as that event that interrupted us before
* the reservation of the buffer. */
delta = 0;
}
/* No need to use full timestamps here */
abs = 0;
}
Link: https://lkml.kernel.org/r/20200625094454.732790f7@oasis.local.home
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 276 ++++++++++++++++++++++++-------------
1 file changed, 181 insertions(+), 95 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 00867ff82412..026238c55b0c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -27,6 +27,7 @@
#include <linux/oom.h>
#include <asm/local.h>
+#include <asm/local64.h>
static void update_pages_handler(struct work_struct *work);
@@ -418,6 +419,17 @@ struct rb_event_info {
int add_timestamp;
};
+/*
+ * Used for the add_timestamp
+ * NONE
+ * NORMAL - may be for either time extend or absolute
+ * FORCE - force a full time stamp.
+ */
+enum {
+ RB_ADD_STAMP_NONE,
+ RB_ADD_STAMP_NORMAL,
+ RB_ADD_STAMP_FORCE
+};
/*
* Used for which event context the event is in.
* NMI = 0
@@ -470,7 +482,8 @@ struct ring_buffer_per_cpu {
size_t shortest_full;
unsigned long read;
unsigned long read_bytes;
- u64 write_stamp;
+ local64_t write_stamp;
+ local64_t before_stamp;
u64 read_stamp;
/* ring buffer pages to update, > 0 to add, < 0 to remove */
long nr_pages_to_update;
@@ -2416,16 +2429,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
unsigned length = info->length;
u64 delta = info->delta;
- /* Only a commit updates the timestamp */
- if (unlikely(!rb_event_is_commit(cpu_buffer, event)))
- delta = 0;
-
/*
* If we need to add a timestamp, then we
* add it to the start of the reserved space.
*/
if (unlikely(info->add_timestamp)) {
- bool abs = ring_buffer_time_stamp_abs(cpu_buffer->buffer);
+ bool abs = info->add_timestamp == RB_ADD_STAMP_FORCE ||
+ ring_buffer_time_stamp_abs(cpu_buffer->buffer);
event = rb_add_time_stamp(event, abs ? info->delta : delta, abs);
length -= RB_LEN_TIME_EXTEND;
@@ -2480,6 +2490,39 @@ static inline bool sched_clock_stable(void)
}
#endif
+static __always_inline bool
+rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
+ struct ring_buffer_event *event)
+{
+ unsigned long addr = (unsigned long)event;
+ unsigned long index;
+
+ index = rb_event_index(event);
+ addr &= PAGE_MASK;
+
+ return cpu_buffer->commit_page->page == (void *)addr &&
+ rb_commit_index(cpu_buffer) == index;
+}
+
+static u64 rb_time_delta(struct ring_buffer_event *event)
+{
+ switch (event->type_len) {
+ case RINGBUF_TYPE_PADDING:
+ return 0;
+
+ case RINGBUF_TYPE_TIME_EXTEND:
+ return ring_buffer_event_time_stamp(event);
+
+ case RINGBUF_TYPE_TIME_STAMP:
+ return 0;
+
+ case RINGBUF_TYPE_DATA:
+ return event->time_delta;
+ default:
+ return 0;
+ }
+}
+
static inline int
rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
struct ring_buffer_event *event)
@@ -2488,6 +2531,8 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
struct buffer_page *bpage;
unsigned long index;
unsigned long addr;
+ u64 write_stamp;
+ u64 delta;
new_index = rb_event_index(event);
old_index = new_index + rb_event_ts_length(event);
@@ -2496,10 +2541,32 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
bpage = READ_ONCE(cpu_buffer->tail_page);
+ delta = rb_time_delta(event);
+
+ write_stamp = local64_read(&cpu_buffer->write_stamp);
+
+ /* Make sure the write stamp is read before testing the location */
+ barrier();
+
if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
unsigned long write_mask =
local_read(&bpage->write) & ~RB_WRITE_MASK;
unsigned long event_length = rb_event_length(event);
+ u64 ret;
+
+ ret = local64_cmpxchg(&cpu_buffer->write_stamp, write_stamp, write_stamp - delta);
+ /* Something came in, can't discard */
+ if (ret != write_stamp)
+ return 0;
+
+ /*
+ * If an event were to come in now, it would see that the
+ * write_stamp and the before_stamp are different, and assume
+ * that this event just added itself before updating
+ * the write stamp. The interrupting event will fix the
+ * write stamp for us, and use the before stamp as its delta.
+ */
+
/*
* This is on the tail page. It is possible that
* a write could come in and move the tail page
@@ -2551,10 +2618,6 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
local_set(&cpu_buffer->commit_page->page->commit,
rb_page_write(cpu_buffer->commit_page));
rb_inc_page(cpu_buffer, &cpu_buffer->commit_page);
- /* Only update the write stamp if the page has an event */
- if (rb_page_write(cpu_buffer->commit_page))
- cpu_buffer->write_stamp =
- cpu_buffer->commit_page->page->time_stamp;
/* add barrier to keep gcc from optimizing too much */
barrier();
}
@@ -2626,54 +2689,10 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
event->time_delta = 1;
}
-static __always_inline bool
-rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
- struct ring_buffer_event *event)
-{
- unsigned long addr = (unsigned long)event;
- unsigned long index;
-
- index = rb_event_index(event);
- addr &= PAGE_MASK;
-
- return cpu_buffer->commit_page->page == (void *)addr &&
- rb_commit_index(cpu_buffer) == index;
-}
-
-static __always_inline void
-rb_update_write_stamp(struct ring_buffer_per_cpu *cpu_buffer,
- struct ring_buffer_event *event)
-{
- u64 delta;
-
- /*
- * The event first in the commit queue updates the
- * time stamp.
- */
- if (rb_event_is_commit(cpu_buffer, event)) {
- /*
- * A commit event that is first on a page
- * updates the write timestamp with the page stamp
- */
- if (!rb_event_index(event))
- cpu_buffer->write_stamp =
- cpu_buffer->commit_page->page->time_stamp;
- else if (event->type_len == RINGBUF_TYPE_TIME_EXTEND) {
- delta = ring_buffer_event_time_stamp(event);
- cpu_buffer->write_stamp += delta;
- } else if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
- delta = ring_buffer_event_time_stamp(event);
- cpu_buffer->write_stamp = delta;
- } else
- cpu_buffer->write_stamp += event->time_delta;
- }
-}
-
static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
struct ring_buffer_event *event)
{
local_inc(&cpu_buffer->entries);
- rb_update_write_stamp(cpu_buffer, event);
rb_end_commit(cpu_buffer);
}
@@ -2872,13 +2891,13 @@ rb_handle_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
(unsigned long long)info->delta,
(unsigned long long)info->ts,
- (unsigned long long)cpu_buffer->write_stamp,
+ (unsigned long long)local64_read(&cpu_buffer->write_stamp),
sched_clock_stable() ? "" :
"If you just came from a suspend/resume,\n"
"please switch to the trace global clock:\n"
" echo global > /sys/kernel/debug/tracing/trace_clock\n"
"or add trace_clock=global to the kernel command line\n");
- info->add_timestamp = 1;
+ info->add_timestamp = RB_ADD_STAMP_NORMAL;
}
static struct ring_buffer_event *
@@ -2887,8 +2906,36 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
{
struct ring_buffer_event *event;
struct buffer_page *tail_page;
- unsigned long tail, write;
+ unsigned long tail, write, w;
+ u64 before, after;
+ bool abs = false;
+
+ /* Don't let the compiler play games with cpu_buffer->tail_page */
+ tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
+
+ /*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
+ barrier();
+ before = local64_read(&cpu_buffer->before_stamp);
+ after = local64_read(&cpu_buffer->write_stamp);
+ barrier();
+ info->ts = rb_time_stamp(cpu_buffer->buffer);
+
+ if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
+ info->delta = info->ts;
+ abs = true;
+ } else {
+ info->delta = info->ts - after;
+ }
+
+ if (unlikely(test_time_stamp(info->delta)))
+ rb_handle_timestamp(cpu_buffer, info);
+ /*
+ * If interrupting an event time update, we may need an absolute timestamp.
+ * Don't bother if this is the start of a new page (w == 0).
+ */
+ if (unlikely(before != after && w))
+ info->add_timestamp = RB_ADD_STAMP_FORCE;
/*
* If the time delta since the last event is too big to
* hold in the time field of the event, then we append a
@@ -2897,25 +2944,88 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (unlikely(info->add_timestamp))
info->length += RB_LEN_TIME_EXTEND;
- /* Don't let the compiler play games with cpu_buffer->tail_page */
- tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
- write = local_add_return(info->length, &tail_page->write);
+ /*B*/ local64_set(&cpu_buffer->before_stamp, info->ts);
+
+ /*C*/ write = local_add_return(info->length, &tail_page->write);
/* set write to only the index of the write */
write &= RB_WRITE_MASK;
+
tail = write - info->length;
+ /* See if we shot pass the end of this buffer page */
+ if (unlikely(write > BUF_PAGE_SIZE)) {
+ if (tail != w) {
+ /* before and after may now different, fix it up*/
+ before = local64_read(&cpu_buffer->before_stamp);
+ after = local64_read(&cpu_buffer->write_stamp);
+ if (before != after)
+ (void)local64_cmpxchg(&cpu_buffer->before_stamp, before, after);
+ }
+ return rb_move_tail(cpu_buffer, tail, info);
+ }
+
+ if (likely(tail == w)) {
+ u64 save_before;
+
+ /* Nothing interrupted us between A and C */
+ /*D*/ local64_set(&cpu_buffer->write_stamp, info->ts);
+ barrier();
+ /*E*/ save_before = local64_read(&cpu_buffer->before_stamp);
+ if (likely(info->add_timestamp != RB_ADD_STAMP_FORCE))
+ /* This did not interrupt any time update */
+ info->delta = info->ts - after;
+ else
+ /* Just use full timestamp for inerrupting event */
+ info->delta = info->ts;
+ barrier();
+ if (unlikely(info->ts != save_before)) {
+ /* SLOW PATH - Interrupted between C and E */
+
+ after = local64_read(&cpu_buffer->write_stamp);
+ /* Write stamp must only go forward */
+ if (save_before > after) {
+ /*
+ * We do not care about the result, only that
+ * it gets updated atomically.
+ */
+ (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, save_before);
+ }
+ }
+ } else {
+ u64 ts;
+ /* SLOW PATH - Interrupted between A and C */
+ after = local64_read(&cpu_buffer->write_stamp);
+ ts = rb_time_stamp(cpu_buffer->buffer);
+ barrier();
+ /*E*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
+ after < ts) {
+ /* Nothing came after this event between C and E */
+ info->delta = ts - after;
+ (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, info->ts);
+ info->ts = ts;
+ } else {
+ /*
+ * Interrupted beween C and E:
+ * Lost the previous events time stamp. Just set the
+ * delta to zero, and this will be the same time as
+ * the event this event interrupted. And the events that
+ * came after this will still be correct (as they would
+ * have built their delta on the previous event.
+ */
+ info->delta = 0;
+ }
+ if (info->add_timestamp == RB_ADD_STAMP_FORCE)
+ info->add_timestamp = RB_ADD_STAMP_NORMAL;
+ }
+
/*
* If this is the first commit on the page, then it has the same
* timestamp as the page itself.
*/
- if (!tail && !ring_buffer_time_stamp_abs(cpu_buffer->buffer))
+ if (unlikely(!tail && info->add_timestamp != RB_ADD_STAMP_FORCE && !abs))
info->delta = 0;
- /* See if we shot pass the end of this buffer page */
- if (unlikely(write > BUF_PAGE_SIZE))
- return rb_move_tail(cpu_buffer, tail, info);
-
/* We reserved something on the buffer */
event = __rb_page_index(tail_page, tail);
@@ -2944,9 +3054,9 @@ rb_reserve_next_event(struct trace_buffer *buffer,
struct ring_buffer_event *event;
struct rb_event_info info;
int nr_loops = 0;
- u64 diff;
rb_start_commit(cpu_buffer);
+ /* The commit page can not change after this */
#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
/*
@@ -2965,7 +3075,7 @@ rb_reserve_next_event(struct trace_buffer *buffer,
info.length = rb_calculate_event_length(length);
again:
- info.add_timestamp = 0;
+ info.add_timestamp = RB_ADD_STAMP_NONE;
info.delta = 0;
/*
@@ -2980,22 +3090,6 @@ rb_reserve_next_event(struct trace_buffer *buffer,
if (RB_WARN_ON(cpu_buffer, ++nr_loops > 1000))
goto out_fail;
- info.ts = rb_time_stamp(cpu_buffer->buffer);
- diff = info.ts - cpu_buffer->write_stamp;
-
- /* make sure this diff is calculated here */
- barrier();
-
- if (ring_buffer_time_stamp_abs(buffer)) {
- info.delta = info.ts;
- rb_handle_timestamp(cpu_buffer, &info);
- } else /* Did the write stamp get updated already? */
- if (likely(info.ts >= cpu_buffer->write_stamp)) {
- info.delta = diff;
- if (unlikely(test_time_stamp(info.delta)))
- rb_handle_timestamp(cpu_buffer, &info);
- }
-
event = __rb_reserve_next(cpu_buffer, &info);
if (unlikely(PTR_ERR(event) == -EAGAIN)) {
@@ -3004,11 +3098,8 @@ rb_reserve_next_event(struct trace_buffer *buffer,
goto again;
}
- if (!event)
- goto out_fail;
-
- return event;
-
+ if (likely(event))
+ return event;
out_fail:
rb_end_commit(cpu_buffer);
return NULL;
@@ -3154,11 +3245,6 @@ void ring_buffer_discard_commit(struct trace_buffer *buffer,
if (rb_try_to_discard(cpu_buffer, event))
goto out;
- /*
- * The commit is still visible by the reader, so we
- * must still update the timestamp.
- */
- rb_update_write_stamp(cpu_buffer, event);
out:
rb_end_commit(cpu_buffer);
@@ -4475,8 +4561,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->read = 0;
cpu_buffer->read_bytes = 0;
- cpu_buffer->write_stamp = 0;
- cpu_buffer->read_stamp = 0;
+ local64_set(&cpu_buffer->write_stamp, 0);
+ local64_set(&cpu_buffer->before_stamp, 0);
cpu_buffer->lost_events = 0;
cpu_buffer->last_overrun = 0;
--
2.26.2
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH 2/3 V2] ring-buffer: Incorporate absolute timestamp into add_timestamp logic
2020-06-29 2:52 [PATCH 0/3 V2] ring-buffer: Restructure ftrace ring buffer time keeping to allow accurate nested timing Steven Rostedt
2020-06-29 2:52 ` [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp Steven Rostedt
@ 2020-06-29 2:52 ` Steven Rostedt
2020-06-29 2:52 ` [PATCH 3/3 V2] ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit Steven Rostedt
2 siblings, 0 replies; 6+ messages in thread
From: Steven Rostedt @ 2020-06-29 2:52 UTC (permalink / raw)
To: linux-kernel
Cc: Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra, Thomas Gleixner,
Masami Hiramatsu, Arnaldo Carvalho de Melo, Jiri Olsa,
Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov, Tom Zanussi,
Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Instead of calling out the absolute test for each time to check if the
ring buffer wants absolute time stamps for all its recording, incorporate it
with the add_timestamp field and turn it into flags for faster processing
between wanting a absolute tag and needing to force one.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 40 +++++++++++++++++++++-----------------
1 file changed, 22 insertions(+), 18 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 026238c55b0c..7ee6619951ea 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -422,13 +422,15 @@ struct rb_event_info {
/*
* Used for the add_timestamp
* NONE
- * NORMAL - may be for either time extend or absolute
+ * EXTEND - wants a time extend
+ * ABSOLUTE - the buffer requests all events to have absolute time stamps
* FORCE - force a full time stamp.
*/
enum {
- RB_ADD_STAMP_NONE,
- RB_ADD_STAMP_NORMAL,
- RB_ADD_STAMP_FORCE
+ RB_ADD_STAMP_NONE = 0,
+ RB_ADD_STAMP_EXTEND = BIT(1),
+ RB_ADD_STAMP_ABSOLUTE = BIT(2),
+ RB_ADD_STAMP_FORCE = BIT(3)
};
/*
* Used for which event context the event is in.
@@ -2434,8 +2436,8 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
* add it to the start of the reserved space.
*/
if (unlikely(info->add_timestamp)) {
- bool abs = info->add_timestamp == RB_ADD_STAMP_FORCE ||
- ring_buffer_time_stamp_abs(cpu_buffer->buffer);
+ bool abs = info->add_timestamp &
+ (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE);
event = rb_add_time_stamp(event, abs ? info->delta : delta, abs);
length -= RB_LEN_TIME_EXTEND;
@@ -2884,8 +2886,8 @@ int ring_buffer_unlock_commit(struct trace_buffer *buffer,
EXPORT_SYMBOL_GPL(ring_buffer_unlock_commit);
static noinline void
-rb_handle_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
- struct rb_event_info *info)
+rb_check_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
+ struct rb_event_info *info)
{
WARN_ONCE(info->delta > (1ULL << 59),
KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
@@ -2897,7 +2899,6 @@ rb_handle_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
"please switch to the trace global clock:\n"
" echo global > /sys/kernel/debug/tracing/trace_clock\n"
"or add trace_clock=global to the kernel command line\n");
- info->add_timestamp = RB_ADD_STAMP_NORMAL;
}
static struct ring_buffer_event *
@@ -2908,7 +2909,6 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
struct buffer_page *tail_page;
unsigned long tail, write, w;
u64 before, after;
- bool abs = false;
/* Don't let the compiler play games with cpu_buffer->tail_page */
tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
@@ -2922,20 +2922,23 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
info->delta = info->ts;
- abs = true;
+ info->add_timestamp = RB_ADD_STAMP_ABSOLUTE;
} else {
info->delta = info->ts - after;
}
- if (unlikely(test_time_stamp(info->delta)))
- rb_handle_timestamp(cpu_buffer, info);
+ if (unlikely(test_time_stamp(info->delta))) {
+ rb_check_timestamp(cpu_buffer, info);
+ info->add_timestamp |= RB_ADD_STAMP_EXTEND;
+ }
/*
* If interrupting an event time update, we may need an absolute timestamp.
* Don't bother if this is the start of a new page (w == 0).
*/
if (unlikely(before != after && w))
- info->add_timestamp = RB_ADD_STAMP_FORCE;
+ info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
+
/*
* If the time delta since the last event is too big to
* hold in the time field of the event, then we append a
@@ -2972,7 +2975,8 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
/*D*/ local64_set(&cpu_buffer->write_stamp, info->ts);
barrier();
/*E*/ save_before = local64_read(&cpu_buffer->before_stamp);
- if (likely(info->add_timestamp != RB_ADD_STAMP_FORCE))
+ if (likely(!(info->add_timestamp &
+ (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
/* This did not interrupt any time update */
info->delta = info->ts - after;
else
@@ -3015,15 +3019,15 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
*/
info->delta = 0;
}
- if (info->add_timestamp == RB_ADD_STAMP_FORCE)
- info->add_timestamp = RB_ADD_STAMP_NORMAL;
+ info->add_timestamp &= ~RB_ADD_STAMP_FORCE;
}
/*
* If this is the first commit on the page, then it has the same
* timestamp as the page itself.
*/
- if (unlikely(!tail && info->add_timestamp != RB_ADD_STAMP_FORCE && !abs))
+ if (unlikely(!tail && !(info->add_timestamp &
+ (RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
info->delta = 0;
/* We reserved something on the buffer */
--
2.26.2
^ permalink raw reply related [flat|nested] 6+ messages in thread
* [PATCH 3/3 V2] ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit
2020-06-29 2:52 [PATCH 0/3 V2] ring-buffer: Restructure ftrace ring buffer time keeping to allow accurate nested timing Steven Rostedt
2020-06-29 2:52 ` [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp Steven Rostedt
2020-06-29 2:52 ` [PATCH 2/3 V2] ring-buffer: Incorporate absolute timestamp into add_timestamp logic Steven Rostedt
@ 2020-06-29 2:52 ` Steven Rostedt
2 siblings, 0 replies; 6+ messages in thread
From: Steven Rostedt @ 2020-06-29 2:52 UTC (permalink / raw)
To: linux-kernel
Cc: Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra, Thomas Gleixner,
Masami Hiramatsu, Arnaldo Carvalho de Melo, Jiri Olsa,
Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov, Tom Zanussi,
Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
After a discussion with the new time algorithm to have nested events still
have proper time keeping but required using local64_t atomic operations.
Mathieu was concerned about the performance this would have on 32 bit
machines, as in most cases, atomic 64 bit operations on them can be
expensive.
As the ring buffer's timing needs do not require full features of local64_t,
a wrapper is made to implement a new rb_time_t operation that uses two longs
on 32 bit machines but still uses the local64_t operations on 64 bit
machines. There's a switch that can be made in the file to force 64 bit to
use the 32 bit version just for testing purposes.
All reads do not need to succeed if a read happened while the stamp being
read is in the process of being updated. The requirement is that all reads
must succed that were done by an interrupting event (where this event was
interrupted by another event that did the write). Or if the event itself did
the write first. That is: rb_time_set(t, x) followed by rb_time_read(t) will
always succeed (even if it gets interrupted by another event that writes to
t. The result of the read will be either the previous set, or a set
performed by an interrupting event.
If the read is done by an event that interrupted another event that was in
the process of setting the time stamp, and no other event came along to
write to that time stamp, it will fail and the rb_time_read() will return
that it failed (the value to read will be undefined).
A set will always write to the time stamp and return with a valid time
stamp, such that any read after it will be valid.
A cmpxchg may fail if it interrupted an event that was in the process of
updating the time stamp just like the reads do. Other than that, it will act
like a normal cmpxchg.
The way this works is that the rb_time_t is made of of three fields. A cnt,
that gets updated atomically everyting a modification is made. A top that
represents the most significant 30 bits of the time, and a bottom to
represent the least significant 30 bits of the time. Notice, that the time
values is only 60 bits long (where the ring buffer only uses 59 bits, which
gives us 18 years of nanoseconds!).
The top two bits of both the top and bottom is a 2 bit counter that gets set
by the value of the least two significant bits of the cnt. A read of the top
and the bottom where both the top and bottom have the same most significant
top 2 bits, are considered a match and a valid 60 bit number can be created
from it. If they do not match, then the number is considered invalid, and
this must only happen if an event interrupted another event in the midst of
updating the time stamp.
This is only used for 32 bits machines as 64 bit machines can get better
performance out of the local64_t. This has been tested heavily by forcing 64
bit to use this logic.
Link: https://lore.kernel.org/r/20200625225345.18cf5881@oasis.local.home
Inspired-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 262 +++++++++++++++++++++++++++++++++----
1 file changed, 238 insertions(+), 24 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 7ee6619951ea..350350254a8b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -27,7 +27,6 @@
#include <linux/oom.h>
#include <asm/local.h>
-#include <asm/local64.h>
static void update_pages_handler(struct work_struct *work);
@@ -449,6 +448,28 @@ enum {
RB_CTX_MAX
};
+#if BITS_PER_LONG == 32
+#define RB_TIME_32
+#endif
+
+/* To test on 64 bit machines */
+//#define RB_TIME_32
+
+#ifdef RB_TIME_32
+
+struct rb_time_struct {
+ local_t cnt;
+ local_t top;
+ local_t bottom;
+};
+#else
+#include <asm/local64.h>
+struct rb_time_struct {
+ local64_t time;
+};
+#endif
+typedef struct rb_time_struct rb_time_t;
+
/*
* head_page == tail_page && head == tail then buffer is empty.
*/
@@ -484,8 +505,8 @@ struct ring_buffer_per_cpu {
size_t shortest_full;
unsigned long read;
unsigned long read_bytes;
- local64_t write_stamp;
- local64_t before_stamp;
+ rb_time_t write_stamp;
+ rb_time_t before_stamp;
u64 read_stamp;
/* ring buffer pages to update, > 0 to add, < 0 to remove */
long nr_pages_to_update;
@@ -528,6 +549,189 @@ struct ring_buffer_iter {
int missed_events;
};
+#ifdef RB_TIME_32
+
+/*
+ * On 32 bit machines, local64_t is very expensive. As the ring
+ * buffer doesn't need all the features of a true 64 bit atomic,
+ * on 32 bit, it uses these functions (64 still uses local64_t).
+ *
+ * For the ring buffer, 64 bit required operations for the time is
+ * the following:
+ *
+ * - Only need 59 bits (uses 60 to make it even).
+ * - Reads may fail if it interrupted a modification of the time stamp.
+ * It will succeed if it did not interrupt another write even if
+ * the read itself is interrupted by a write.
+ * It returns whether it was successful or not.
+ *
+ * - Writes always succeed and will overwrite other writes and writes
+ * that were done by events interrupting the current write.
+ *
+ * - A write followed by a read of the same time stamp will always succeed,
+ * but may not contain the same value.
+ *
+ * - A cmpxchg will fail if it interrupted another write or cmpxchg.
+ * Other than that, it acts like a normal cmpxchg.
+ *
+ * The 60 bit time stamp is broken up by 30 bits in a top and bottom half
+ * (bottom being the least significant 30 bits of the 60 bit time stamp).
+ *
+ * The two most significant bits of each half holds a 2 bit counter (0-3).
+ * Each update will increment this counter by one.
+ * When reading the top and bottom, if the two counter bits match then the
+ * top and bottom together make a valid 60 bit number.
+ */
+#define RB_TIME_SHIFT 30
+#define RB_TIME_VAL_MASK ((1 << RB_TIME_SHIFT) - 1)
+
+static inline int rb_time_cnt(unsigned long val)
+{
+ return (val >> RB_TIME_SHIFT) & 3;
+}
+
+static inline u64 rb_time_val(unsigned long top, unsigned long bottom)
+{
+ u64 val;
+
+ val = top & RB_TIME_VAL_MASK;
+ val <<= RB_TIME_SHIFT;
+ val |= bottom & RB_TIME_VAL_MASK;
+
+ return val;
+}
+
+static inline bool __rb_time_read(rb_time_t *t, u64 *ret, unsigned long *cnt)
+{
+ unsigned long top, bottom;
+ unsigned long c;
+
+ /*
+ * If the read is interrupted by a write, then the cnt will
+ * be different. Loop until both top and bottom have been read
+ * without interruption.
+ */
+ do {
+ c = local_read(&t->cnt);
+ top = local_read(&t->top);
+ bottom = local_read(&t->bottom);
+ } while (c != local_read(&t->cnt));
+
+ *cnt = rb_time_cnt(top);
+
+ /* If top and bottom counts don't match, this interrupted a write */
+ if (*cnt != rb_time_cnt(bottom))
+ return false;
+
+ *ret = rb_time_val(top, bottom);
+ return true;
+}
+
+static bool rb_time_read(rb_time_t *t, u64 *ret)
+{
+ unsigned long cnt;
+
+ return __rb_time_read(t, ret, &cnt);
+}
+
+static inline unsigned long rb_time_val_cnt(unsigned long val, unsigned long cnt)
+{
+ return (val & RB_TIME_VAL_MASK) | ((cnt & 3) << RB_TIME_SHIFT);
+}
+
+static inline void rb_time_split(u64 val, unsigned long *top, unsigned long *bottom)
+{
+ *top = (unsigned long)((val >> RB_TIME_SHIFT) & RB_TIME_VAL_MASK);
+ *bottom = (unsigned long)(val & RB_TIME_VAL_MASK);
+}
+
+static inline void rb_time_val_set(local_t *t, unsigned long val, unsigned long cnt)
+{
+ val = rb_time_val_cnt(val, cnt);
+ local_set(t, val);
+}
+
+static void rb_time_set(rb_time_t *t, u64 val)
+{
+ unsigned long cnt, top, bottom;
+
+ rb_time_split(val, &top, &bottom);
+
+ /* Writes always succeed with a valid number even if it gets interrupted. */
+ do {
+ cnt = local_inc_return(&t->cnt);
+ rb_time_val_set(&t->top, top, cnt);
+ rb_time_val_set(&t->bottom, bottom, cnt);
+ } while (cnt != local_read(&t->cnt));
+}
+
+static inline bool
+rb_time_read_cmpxchg(local_t *l, unsigned long expect, unsigned long set)
+{
+ unsigned long ret;
+
+ ret = local_cmpxchg(l, expect, set);
+ return ret == expect;
+}
+
+static int rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
+{
+ unsigned long cnt, top, bottom;
+ unsigned long cnt2, top2, bottom2;
+ u64 val;
+
+ /* The cmpxchg always fails if it interrupted an update */
+ if (!__rb_time_read(t, &val, &cnt2))
+ return false;
+
+ if (val != expect)
+ return false;
+
+ cnt = local_read(&t->cnt);
+ if ((cnt & 3) != cnt2)
+ return false;
+
+ cnt2 = cnt + 1;
+
+ rb_time_split(val, &top, &bottom);
+ top = rb_time_val_cnt(top, cnt);
+ bottom = rb_time_val_cnt(bottom, cnt);
+
+ rb_time_split(set, &top2, &bottom2);
+ top2 = rb_time_val_cnt(top2, cnt2);
+ bottom2 = rb_time_val_cnt(bottom2, cnt2);
+
+ if (!rb_time_read_cmpxchg(&t->cnt, cnt, cnt2))
+ return false;
+ if (!rb_time_read_cmpxchg(&t->top, top, top2))
+ return false;
+ if (!rb_time_read_cmpxchg(&t->bottom, bottom, bottom2))
+ return false;
+ return true;
+}
+
+#else /* 64 bits */
+
+/* local64_t always succeeds */
+
+static inline bool rb_time_read(rb_time_t *t, u64 *ret)
+{
+ *ret = local64_read(&t->time);
+ return true;
+}
+static void rb_time_set(rb_time_t *t, u64 val)
+{
+ local64_set(&t->time, val);
+}
+
+static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
+{
+ u64 val;
+ val = local64_cmpxchg(&t->time, expect, set);
+ return val == expect;
+}
+#endif
+
/**
* ring_buffer_nr_pages - get the number of buffer pages in the ring buffer
* @buffer: The ring_buffer to get the number of pages from
@@ -2545,7 +2749,8 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
delta = rb_time_delta(event);
- write_stamp = local64_read(&cpu_buffer->write_stamp);
+ if (!rb_time_read(&cpu_buffer->write_stamp, &write_stamp))
+ return 0;
/* Make sure the write stamp is read before testing the location */
barrier();
@@ -2554,11 +2759,10 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
unsigned long write_mask =
local_read(&bpage->write) & ~RB_WRITE_MASK;
unsigned long event_length = rb_event_length(event);
- u64 ret;
- ret = local64_cmpxchg(&cpu_buffer->write_stamp, write_stamp, write_stamp - delta);
/* Something came in, can't discard */
- if (ret != write_stamp)
+ if (!rb_time_cmpxchg(&cpu_buffer->write_stamp,
+ write_stamp, write_stamp - delta))
return 0;
/*
@@ -2889,11 +3093,13 @@ static noinline void
rb_check_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
struct rb_event_info *info)
{
+ u64 write_stamp;
+
WARN_ONCE(info->delta > (1ULL << 59),
KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
(unsigned long long)info->delta,
(unsigned long long)info->ts,
- (unsigned long long)local64_read(&cpu_buffer->write_stamp),
+ (unsigned long long)(rb_time_read(&cpu_buffer->write_stamp, &write_stamp) ? write_stamp : 0),
sched_clock_stable() ? "" :
"If you just came from a suspend/resume,\n"
"please switch to the trace global clock:\n"
@@ -2909,14 +3115,16 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
struct buffer_page *tail_page;
unsigned long tail, write, w;
u64 before, after;
+ bool a_ok;
+ bool b_ok;
/* Don't let the compiler play games with cpu_buffer->tail_page */
tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
/*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
barrier();
- before = local64_read(&cpu_buffer->before_stamp);
- after = local64_read(&cpu_buffer->write_stamp);
+ b_ok = rb_time_read(&cpu_buffer->before_stamp, &before);
+ a_ok = rb_time_read(&cpu_buffer->write_stamp, &after);
barrier();
info->ts = rb_time_stamp(cpu_buffer->buffer);
@@ -2936,7 +3144,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
* If interrupting an event time update, we may need an absolute timestamp.
* Don't bother if this is the start of a new page (w == 0).
*/
- if (unlikely(before != after && w))
+ if (unlikely(!a_ok || !b_ok || (before != after && w)))
info->add_timestamp |= RB_ADD_STAMP_FORCE | RB_ADD_STAMP_EXTEND;
/*
@@ -2947,7 +3155,7 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (unlikely(info->add_timestamp))
info->length += RB_LEN_TIME_EXTEND;
- /*B*/ local64_set(&cpu_buffer->before_stamp, info->ts);
+ /*B*/ rb_time_set(&cpu_buffer->before_stamp, info->ts);
/*C*/ write = local_add_return(info->length, &tail_page->write);
@@ -2960,21 +3168,23 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (unlikely(write > BUF_PAGE_SIZE)) {
if (tail != w) {
/* before and after may now different, fix it up*/
- before = local64_read(&cpu_buffer->before_stamp);
- after = local64_read(&cpu_buffer->write_stamp);
- if (before != after)
- (void)local64_cmpxchg(&cpu_buffer->before_stamp, before, after);
+ b_ok = rb_time_read(&cpu_buffer->before_stamp, &before);
+ a_ok = rb_time_read(&cpu_buffer->write_stamp, &after);
+ if (a_ok && b_ok && before != after)
+ (void)rb_time_cmpxchg(&cpu_buffer->before_stamp, before, after);
}
return rb_move_tail(cpu_buffer, tail, info);
}
if (likely(tail == w)) {
u64 save_before;
+ bool s_ok;
/* Nothing interrupted us between A and C */
- /*D*/ local64_set(&cpu_buffer->write_stamp, info->ts);
+ /*D*/ rb_time_set(&cpu_buffer->write_stamp, info->ts);
barrier();
- /*E*/ save_before = local64_read(&cpu_buffer->before_stamp);
+ /*E*/ s_ok = rb_time_read(&cpu_buffer->before_stamp, &save_before);
+ RB_WARN_ON(cpu_buffer, !s_ok);
if (likely(!(info->add_timestamp &
(RB_ADD_STAMP_FORCE | RB_ADD_STAMP_ABSOLUTE))))
/* This did not interrupt any time update */
@@ -2986,27 +3196,31 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (unlikely(info->ts != save_before)) {
/* SLOW PATH - Interrupted between C and E */
- after = local64_read(&cpu_buffer->write_stamp);
+ a_ok = rb_time_read(&cpu_buffer->write_stamp, &after);
+ RB_WARN_ON(cpu_buffer, !a_ok);
+
/* Write stamp must only go forward */
if (save_before > after) {
/*
* We do not care about the result, only that
* it gets updated atomically.
*/
- (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, save_before);
+ (void)rb_time_cmpxchg(&cpu_buffer->write_stamp, after, save_before);
}
}
} else {
u64 ts;
/* SLOW PATH - Interrupted between A and C */
- after = local64_read(&cpu_buffer->write_stamp);
+ a_ok = rb_time_read(&cpu_buffer->write_stamp, &after);
+ /* Was interrupted before here, write_stamp must be valid */
+ RB_WARN_ON(cpu_buffer, !a_ok);
ts = rb_time_stamp(cpu_buffer->buffer);
barrier();
/*E*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
after < ts) {
/* Nothing came after this event between C and E */
info->delta = ts - after;
- (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, info->ts);
+ (void)rb_time_cmpxchg(&cpu_buffer->write_stamp, after, info->ts);
info->ts = ts;
} else {
/*
@@ -4565,8 +4779,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->read = 0;
cpu_buffer->read_bytes = 0;
- local64_set(&cpu_buffer->write_stamp, 0);
- local64_set(&cpu_buffer->before_stamp, 0);
+ rb_time_set(&cpu_buffer->write_stamp, 0);
+ rb_time_set(&cpu_buffer->before_stamp, 0);
cpu_buffer->lost_events = 0;
cpu_buffer->last_overrun = 0;
--
2.26.2
^ permalink raw reply related [flat|nested] 6+ messages in thread
* Re: [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp
2020-06-29 2:52 ` [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp Steven Rostedt
@ 2020-06-29 13:49 ` Masami Hiramatsu
2020-06-29 16:25 ` Steven Rostedt
0 siblings, 1 reply; 6+ messages in thread
From: Masami Hiramatsu @ 2020-06-29 13:49 UTC (permalink / raw)
To: Steven Rostedt
Cc: linux-kernel, Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra,
Thomas Gleixner, Masami Hiramatsu, Arnaldo Carvalho de Melo,
Jiri Olsa, Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov,
Tom Zanussi, Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
On Sun, 28 Jun 2020 22:52:25 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> Up until now, if an event is interrupted while it is recorded by an
> interrupt, and that interrupt records events, the time of those events will
> all be the same. This is because events only record the delta of the time
> since the previous event (or beginning of a page), and to handle updating
> the time keeping for that of nested events is extremely racy. After years of
> thinking about this and several failed attempts, I finally have a solution
> to solve this puzzle.
>
> The problem is that you need to atomically calculate the delta and then
> update the time stamp you made the delta from, as well as then record it
> into the buffer, all this while at any time an interrupt can come in and
> do the same thing. This is easy to solve with heavy weight atomics, but that
> would be detrimental to the performance of the ring buffer. The current
> state of affairs sacrificed the time deltas for nested events for
> performance.
>
> The reason for previous failed attempts at solving this puzzle was because I
> was trying to completely avoid slow atomic operations like cmpxchg. I final
> came to the conclusion to always avoid cmpxchg is not possible, which is why
> those previous attempts always failed. But it is possible to pick one path
> (the most common case) and avoid cmpxchg in that path, which is the "fast
> path". The most common case is that an event will not be interrupted and
> have other events added into it. An event can detect if it has
> interrupted another event, and for these cases we can make it the slow
> path and use the heavy operations like cmpxchg.
>
> One more player was added to the game that made this possible, and that is
> the "absolute timestamp" (by Tom Zanussi) that allows us to inject a full 59
> bit time stamp. (Of course this breaks if a machine is running for more than
> 18 years without a reboot!).
>
> There's barrier() placements around for being paranoid, even when they
> are not needed because of other atomic functions near by. But those
> should not hurt, as if they are not needed, they basically become a nop.
>
> Note, this also makes the race window much smaller, which means there
> are less slow paths to slow down the performance.
>
> Here's the design of this solution:
>
> All this is per cpu, and only needs to worry about nested events (not
> parallel events).
>
> The players:
>
> write_tail: The index in the buffer where new events can be written to.
> It is incremented via local_add() to reserve space for a new event.
>
> before_stamp: A time stamp set by all events before reserving space.
>
> write_stamp: A time stamp updated by events after it has successfully
> reserved space.
>
> /* Save the current position of write */
> [A] w = local_read(write_tail);
> barrier();
> /* Read both before and write stamps before touching anything */
> before = local_read(before_stamp);
> after = local_read(write_stamp);
> barrier();
>
> /*
> * If before and after are the same, then this event is not
> * interrupting a time update. If it is, then reserve space for adding
> * a full time stamp (this can turn into a time extend which is
> * just an extended time delta but fill up the extra space).
> */
> if (after != before)
> abs = true;
>
> ts = clock();
>
> /* Now update the before_stamp (everyone does this!) */
> [B] local_set(before_stamp, ts);
>
> /* Now reserve space on the buffer */
> [C] write = local_add_return(len, write_tail);
>
> /* Set tail to be were this event's data is */
> tail = write - len;
>
> if (w == tail) {
>
> /* Nothing interrupted this between A and C */
> [D] local_set(write_stamp, ts);
> barrier();
> [E] save_before = local_read(before_stamp);
>
> if (!abs) {
> /* This did not interrupt a time update */
> delta = ts - after;
> } else {
> delta = ts; /* The full time stamp will be in use */
> }
> if (ts != save_before) {
> /* slow path - Was interrupted between C and E */
> /* The update to write_stamp could have overwritten the update to
> * it by the interrupting event, but before and after should be
> * the same for all completed top events */
> after = local_read(write_stamp);
> if (save_before > after)
> local_cmpxchg(write_stamp, after, save_before);
> }
> } else {
> /* slow path - Interrupted between A and C */
>
> after = local_read(write_stamp);
> temp_ts = clock();
> barrier();
> [F] if (write == local_read(write_tail) && after < temp_ts) {
> /* This was not interrupted since C and F
> * The last write_stamp is still valid for the previous event
> * in the buffer. */
> delta = temp_ts - after;
> /* OK to keep this new time stamp */
> ts = temp_ts;
> } else {
> /* Interrupted between C and F
> * Well, there's no use to try to know what the time stamp
> * is for the previous event. Just set delta to zero and
> * be the same time as that event that interrupted us before
> * the reservation of the buffer. */
>
> delta = 0;
> }
> /* No need to use full timestamps here */
> abs = 0;
> }
>
> Link: https://lkml.kernel.org/r/20200625094454.732790f7@oasis.local.home
>
Looks good to me :)
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Thank you!
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> kernel/trace/ring_buffer.c | 276 ++++++++++++++++++++++++-------------
> 1 file changed, 181 insertions(+), 95 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 00867ff82412..026238c55b0c 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -27,6 +27,7 @@
> #include <linux/oom.h>
>
> #include <asm/local.h>
> +#include <asm/local64.h>
>
> static void update_pages_handler(struct work_struct *work);
>
> @@ -418,6 +419,17 @@ struct rb_event_info {
> int add_timestamp;
> };
>
> +/*
> + * Used for the add_timestamp
> + * NONE
> + * NORMAL - may be for either time extend or absolute
> + * FORCE - force a full time stamp.
> + */
> +enum {
> + RB_ADD_STAMP_NONE,
> + RB_ADD_STAMP_NORMAL,
> + RB_ADD_STAMP_FORCE
> +};
> /*
> * Used for which event context the event is in.
> * NMI = 0
> @@ -470,7 +482,8 @@ struct ring_buffer_per_cpu {
> size_t shortest_full;
> unsigned long read;
> unsigned long read_bytes;
> - u64 write_stamp;
> + local64_t write_stamp;
> + local64_t before_stamp;
> u64 read_stamp;
> /* ring buffer pages to update, > 0 to add, < 0 to remove */
> long nr_pages_to_update;
> @@ -2416,16 +2429,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
> unsigned length = info->length;
> u64 delta = info->delta;
>
> - /* Only a commit updates the timestamp */
> - if (unlikely(!rb_event_is_commit(cpu_buffer, event)))
> - delta = 0;
> -
> /*
> * If we need to add a timestamp, then we
> * add it to the start of the reserved space.
> */
> if (unlikely(info->add_timestamp)) {
> - bool abs = ring_buffer_time_stamp_abs(cpu_buffer->buffer);
> + bool abs = info->add_timestamp == RB_ADD_STAMP_FORCE ||
> + ring_buffer_time_stamp_abs(cpu_buffer->buffer);
>
> event = rb_add_time_stamp(event, abs ? info->delta : delta, abs);
> length -= RB_LEN_TIME_EXTEND;
> @@ -2480,6 +2490,39 @@ static inline bool sched_clock_stable(void)
> }
> #endif
>
> +static __always_inline bool
> +rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
> + struct ring_buffer_event *event)
> +{
> + unsigned long addr = (unsigned long)event;
> + unsigned long index;
> +
> + index = rb_event_index(event);
> + addr &= PAGE_MASK;
> +
> + return cpu_buffer->commit_page->page == (void *)addr &&
> + rb_commit_index(cpu_buffer) == index;
> +}
> +
> +static u64 rb_time_delta(struct ring_buffer_event *event)
> +{
> + switch (event->type_len) {
> + case RINGBUF_TYPE_PADDING:
> + return 0;
> +
> + case RINGBUF_TYPE_TIME_EXTEND:
> + return ring_buffer_event_time_stamp(event);
> +
> + case RINGBUF_TYPE_TIME_STAMP:
> + return 0;
> +
> + case RINGBUF_TYPE_DATA:
> + return event->time_delta;
> + default:
> + return 0;
> + }
> +}
> +
> static inline int
> rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
> struct ring_buffer_event *event)
> @@ -2488,6 +2531,8 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
> struct buffer_page *bpage;
> unsigned long index;
> unsigned long addr;
> + u64 write_stamp;
> + u64 delta;
>
> new_index = rb_event_index(event);
> old_index = new_index + rb_event_ts_length(event);
> @@ -2496,10 +2541,32 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
>
> bpage = READ_ONCE(cpu_buffer->tail_page);
>
> + delta = rb_time_delta(event);
> +
> + write_stamp = local64_read(&cpu_buffer->write_stamp);
> +
> + /* Make sure the write stamp is read before testing the location */
> + barrier();
> +
> if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
> unsigned long write_mask =
> local_read(&bpage->write) & ~RB_WRITE_MASK;
> unsigned long event_length = rb_event_length(event);
> + u64 ret;
> +
> + ret = local64_cmpxchg(&cpu_buffer->write_stamp, write_stamp, write_stamp - delta);
> + /* Something came in, can't discard */
> + if (ret != write_stamp)
> + return 0;
> +
> + /*
> + * If an event were to come in now, it would see that the
> + * write_stamp and the before_stamp are different, and assume
> + * that this event just added itself before updating
> + * the write stamp. The interrupting event will fix the
> + * write stamp for us, and use the before stamp as its delta.
> + */
> +
> /*
> * This is on the tail page. It is possible that
> * a write could come in and move the tail page
> @@ -2551,10 +2618,6 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
> local_set(&cpu_buffer->commit_page->page->commit,
> rb_page_write(cpu_buffer->commit_page));
> rb_inc_page(cpu_buffer, &cpu_buffer->commit_page);
> - /* Only update the write stamp if the page has an event */
> - if (rb_page_write(cpu_buffer->commit_page))
> - cpu_buffer->write_stamp =
> - cpu_buffer->commit_page->page->time_stamp;
> /* add barrier to keep gcc from optimizing too much */
> barrier();
> }
> @@ -2626,54 +2689,10 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
> event->time_delta = 1;
> }
>
> -static __always_inline bool
> -rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
> - struct ring_buffer_event *event)
> -{
> - unsigned long addr = (unsigned long)event;
> - unsigned long index;
> -
> - index = rb_event_index(event);
> - addr &= PAGE_MASK;
> -
> - return cpu_buffer->commit_page->page == (void *)addr &&
> - rb_commit_index(cpu_buffer) == index;
> -}
> -
> -static __always_inline void
> -rb_update_write_stamp(struct ring_buffer_per_cpu *cpu_buffer,
> - struct ring_buffer_event *event)
> -{
> - u64 delta;
> -
> - /*
> - * The event first in the commit queue updates the
> - * time stamp.
> - */
> - if (rb_event_is_commit(cpu_buffer, event)) {
> - /*
> - * A commit event that is first on a page
> - * updates the write timestamp with the page stamp
> - */
> - if (!rb_event_index(event))
> - cpu_buffer->write_stamp =
> - cpu_buffer->commit_page->page->time_stamp;
> - else if (event->type_len == RINGBUF_TYPE_TIME_EXTEND) {
> - delta = ring_buffer_event_time_stamp(event);
> - cpu_buffer->write_stamp += delta;
> - } else if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
> - delta = ring_buffer_event_time_stamp(event);
> - cpu_buffer->write_stamp = delta;
> - } else
> - cpu_buffer->write_stamp += event->time_delta;
> - }
> -}
> -
> static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
> struct ring_buffer_event *event)
> {
> local_inc(&cpu_buffer->entries);
> - rb_update_write_stamp(cpu_buffer, event);
> rb_end_commit(cpu_buffer);
> }
>
> @@ -2872,13 +2891,13 @@ rb_handle_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
> KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
> (unsigned long long)info->delta,
> (unsigned long long)info->ts,
> - (unsigned long long)cpu_buffer->write_stamp,
> + (unsigned long long)local64_read(&cpu_buffer->write_stamp),
> sched_clock_stable() ? "" :
> "If you just came from a suspend/resume,\n"
> "please switch to the trace global clock:\n"
> " echo global > /sys/kernel/debug/tracing/trace_clock\n"
> "or add trace_clock=global to the kernel command line\n");
> - info->add_timestamp = 1;
> + info->add_timestamp = RB_ADD_STAMP_NORMAL;
> }
>
> static struct ring_buffer_event *
> @@ -2887,8 +2906,36 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> {
> struct ring_buffer_event *event;
> struct buffer_page *tail_page;
> - unsigned long tail, write;
> + unsigned long tail, write, w;
> + u64 before, after;
> + bool abs = false;
> +
> + /* Don't let the compiler play games with cpu_buffer->tail_page */
> + tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
> +
> + /*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
> + barrier();
> + before = local64_read(&cpu_buffer->before_stamp);
> + after = local64_read(&cpu_buffer->write_stamp);
> + barrier();
> + info->ts = rb_time_stamp(cpu_buffer->buffer);
> +
> + if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
> + info->delta = info->ts;
> + abs = true;
> + } else {
> + info->delta = info->ts - after;
> + }
> +
> + if (unlikely(test_time_stamp(info->delta)))
> + rb_handle_timestamp(cpu_buffer, info);
>
> + /*
> + * If interrupting an event time update, we may need an absolute timestamp.
> + * Don't bother if this is the start of a new page (w == 0).
> + */
> + if (unlikely(before != after && w))
> + info->add_timestamp = RB_ADD_STAMP_FORCE;
> /*
> * If the time delta since the last event is too big to
> * hold in the time field of the event, then we append a
> @@ -2897,25 +2944,88 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
> if (unlikely(info->add_timestamp))
> info->length += RB_LEN_TIME_EXTEND;
>
> - /* Don't let the compiler play games with cpu_buffer->tail_page */
> - tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
> - write = local_add_return(info->length, &tail_page->write);
> + /*B*/ local64_set(&cpu_buffer->before_stamp, info->ts);
> +
> + /*C*/ write = local_add_return(info->length, &tail_page->write);
>
> /* set write to only the index of the write */
> write &= RB_WRITE_MASK;
> +
> tail = write - info->length;
>
> + /* See if we shot pass the end of this buffer page */
> + if (unlikely(write > BUF_PAGE_SIZE)) {
> + if (tail != w) {
> + /* before and after may now different, fix it up*/
> + before = local64_read(&cpu_buffer->before_stamp);
> + after = local64_read(&cpu_buffer->write_stamp);
> + if (before != after)
> + (void)local64_cmpxchg(&cpu_buffer->before_stamp, before, after);
> + }
> + return rb_move_tail(cpu_buffer, tail, info);
> + }
> +
> + if (likely(tail == w)) {
> + u64 save_before;
> +
> + /* Nothing interrupted us between A and C */
> + /*D*/ local64_set(&cpu_buffer->write_stamp, info->ts);
> + barrier();
> + /*E*/ save_before = local64_read(&cpu_buffer->before_stamp);
> + if (likely(info->add_timestamp != RB_ADD_STAMP_FORCE))
> + /* This did not interrupt any time update */
> + info->delta = info->ts - after;
> + else
> + /* Just use full timestamp for inerrupting event */
> + info->delta = info->ts;
> + barrier();
> + if (unlikely(info->ts != save_before)) {
> + /* SLOW PATH - Interrupted between C and E */
> +
> + after = local64_read(&cpu_buffer->write_stamp);
> + /* Write stamp must only go forward */
> + if (save_before > after) {
> + /*
> + * We do not care about the result, only that
> + * it gets updated atomically.
> + */
> + (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, save_before);
> + }
> + }
> + } else {
> + u64 ts;
> + /* SLOW PATH - Interrupted between A and C */
> + after = local64_read(&cpu_buffer->write_stamp);
> + ts = rb_time_stamp(cpu_buffer->buffer);
> + barrier();
> + /*E*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
> + after < ts) {
> + /* Nothing came after this event between C and E */
> + info->delta = ts - after;
> + (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, info->ts);
> + info->ts = ts;
> + } else {
> + /*
> + * Interrupted beween C and E:
> + * Lost the previous events time stamp. Just set the
> + * delta to zero, and this will be the same time as
> + * the event this event interrupted. And the events that
> + * came after this will still be correct (as they would
> + * have built their delta on the previous event.
> + */
> + info->delta = 0;
> + }
> + if (info->add_timestamp == RB_ADD_STAMP_FORCE)
> + info->add_timestamp = RB_ADD_STAMP_NORMAL;
> + }
> +
> /*
> * If this is the first commit on the page, then it has the same
> * timestamp as the page itself.
> */
> - if (!tail && !ring_buffer_time_stamp_abs(cpu_buffer->buffer))
> + if (unlikely(!tail && info->add_timestamp != RB_ADD_STAMP_FORCE && !abs))
> info->delta = 0;
>
> - /* See if we shot pass the end of this buffer page */
> - if (unlikely(write > BUF_PAGE_SIZE))
> - return rb_move_tail(cpu_buffer, tail, info);
> -
> /* We reserved something on the buffer */
>
> event = __rb_page_index(tail_page, tail);
> @@ -2944,9 +3054,9 @@ rb_reserve_next_event(struct trace_buffer *buffer,
> struct ring_buffer_event *event;
> struct rb_event_info info;
> int nr_loops = 0;
> - u64 diff;
>
> rb_start_commit(cpu_buffer);
> + /* The commit page can not change after this */
>
> #ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
> /*
> @@ -2965,7 +3075,7 @@ rb_reserve_next_event(struct trace_buffer *buffer,
>
> info.length = rb_calculate_event_length(length);
> again:
> - info.add_timestamp = 0;
> + info.add_timestamp = RB_ADD_STAMP_NONE;
> info.delta = 0;
>
> /*
> @@ -2980,22 +3090,6 @@ rb_reserve_next_event(struct trace_buffer *buffer,
> if (RB_WARN_ON(cpu_buffer, ++nr_loops > 1000))
> goto out_fail;
>
> - info.ts = rb_time_stamp(cpu_buffer->buffer);
> - diff = info.ts - cpu_buffer->write_stamp;
> -
> - /* make sure this diff is calculated here */
> - barrier();
> -
> - if (ring_buffer_time_stamp_abs(buffer)) {
> - info.delta = info.ts;
> - rb_handle_timestamp(cpu_buffer, &info);
> - } else /* Did the write stamp get updated already? */
> - if (likely(info.ts >= cpu_buffer->write_stamp)) {
> - info.delta = diff;
> - if (unlikely(test_time_stamp(info.delta)))
> - rb_handle_timestamp(cpu_buffer, &info);
> - }
> -
> event = __rb_reserve_next(cpu_buffer, &info);
>
> if (unlikely(PTR_ERR(event) == -EAGAIN)) {
> @@ -3004,11 +3098,8 @@ rb_reserve_next_event(struct trace_buffer *buffer,
> goto again;
> }
>
> - if (!event)
> - goto out_fail;
> -
> - return event;
> -
> + if (likely(event))
> + return event;
> out_fail:
> rb_end_commit(cpu_buffer);
> return NULL;
> @@ -3154,11 +3245,6 @@ void ring_buffer_discard_commit(struct trace_buffer *buffer,
> if (rb_try_to_discard(cpu_buffer, event))
> goto out;
>
> - /*
> - * The commit is still visible by the reader, so we
> - * must still update the timestamp.
> - */
> - rb_update_write_stamp(cpu_buffer, event);
> out:
> rb_end_commit(cpu_buffer);
>
> @@ -4475,8 +4561,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> cpu_buffer->read = 0;
> cpu_buffer->read_bytes = 0;
>
> - cpu_buffer->write_stamp = 0;
> - cpu_buffer->read_stamp = 0;
> + local64_set(&cpu_buffer->write_stamp, 0);
> + local64_set(&cpu_buffer->before_stamp, 0);
>
> cpu_buffer->lost_events = 0;
> cpu_buffer->last_overrun = 0;
> --
> 2.26.2
>
>
--
Masami Hiramatsu <mhiramat@kernel.org>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp
2020-06-29 13:49 ` Masami Hiramatsu
@ 2020-06-29 16:25 ` Steven Rostedt
0 siblings, 0 replies; 6+ messages in thread
From: Steven Rostedt @ 2020-06-29 16:25 UTC (permalink / raw)
To: Masami Hiramatsu
Cc: linux-kernel, Ingo Molnar, Mathieu Desnoyers, Peter Zijlstra,
Thomas Gleixner, Arnaldo Carvalho de Melo, Jiri Olsa,
Namhyung Kim, Yordan Karadzhov, Tzvetomir Stoyanov, Tom Zanussi,
Jason Behmer, Julia Lawall, Clark Williams, bristot,
Daniel Wagner, Darren Hart, Jonathan Corbet, Suresh E. Warrier
On Mon, 29 Jun 2020 22:49:19 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:
> Looks good to me :)
>
> Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Thanks Masami!
I added a more general description to the design which makes more
sense in English terms, before going into the more detailed
pseudo implementation of the design.
Look at where it states:
"The basic idea is that there's two main paths taken."
The patch is the same:
-- Steve
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
Date: Sun, 28 Jun 2020 22:52:25 -0400
Subject: [PATCH] ring-buffer: Have nested events still record running time
stamp
Up until now, if an event is interrupted while it is recorded by an
interrupt, and that interrupt records events, the time of those events will
all be the same. This is because events only record the delta of the time
since the previous event (or beginning of a page), and to handle updating
the time keeping for that of nested events is extremely racy. After years of
thinking about this and several failed attempts, I finally have a solution
to solve this puzzle.
The problem is that you need to atomically calculate the delta and then
update the time stamp you made the delta from, as well as then record it
into the buffer, all this while at any time an interrupt can come in and
do the same thing. This is easy to solve with heavy weight atomics, but that
would be detrimental to the performance of the ring buffer. The current
state of affairs sacrificed the time deltas for nested events for
performance.
The reason for previous failed attempts at solving this puzzle was because I
was trying to completely avoid slow atomic operations like cmpxchg. I final
came to the conclusion to always avoid cmpxchg is not possible, which is why
those previous attempts always failed. But it is possible to pick one path
(the most common case) and avoid cmpxchg in that path, which is the "fast
path". The most common case is that an event will not be interrupted and
have other events added into it. An event can detect if it has
interrupted another event, and for these cases we can make it the slow
path and use the heavy operations like cmpxchg.
One more player was added to the game that made this possible, and that is
the "absolute timestamp" (by Tom Zanussi) that allows us to inject a full 59
bit time stamp. (Of course this breaks if a machine is running for more than
18 years without a reboot!).
There's barrier() placements around for being paranoid, even when they
are not needed because of other atomic functions near by. But those
should not hurt, as if they are not needed, they basically become a nop.
Note, this also makes the race window much smaller, which means there
are less slow paths to slow down the performance.
The basic idea is that there's two main paths taken.
1) Not being interrupted between time stamps and reserving buffer space.
In this case, the time stamps taken are true to the location in the
buffer.
2) Was interrupted by another path between taking time stamps and reserving
buffer space.
The objective is to know what the delta is from the last reserved location
in the buffer.
As it is possible to detect if an event is interrupting another event before
reserving data, space is added to the length to be reserved to inject a full
time stamp along with the event being reserved.
When an event is not interrupted, the write stamp is always the time of the
last event written to the buffer.
In path 1, there's two sub paths we care about:
a) The event did not interrupt another event.
b) The event interrupted another event.
In case a, as the write stamp was read and known to be correct, the delta
between the current time stamp and the write stamp is the delta between the
current event and the previously recorded event.
In case b, extra space was reserved to just put the full time stamp into the
buffer. Which is done, as stated, in this path the time stamp taken is known
to match the location in the buffer.
In path 2, there's also two sub paths we care about:
a) The event was not interrupted by another event since it reserved space
on the buffer and re-reading the write stamp.
b) The event was interrupted by another event.
In case a, the write stamp is that of the last event that interrupted this
event between taking the time stamps and reserving. As no event came in
after re-reading the write stamp, that event is known to be the time of the
event directly before this event and the delta can be the new time stamp and
the write stamp.
In case b, one or more events came in between reserving the event and
re-reading he write stamp. Since this event's buffer reservation is between
other events at this path, there's no way to know what the delta is. But
because an event interrupted this event after it started, its fine to just
give a zero delta, and take the same time stamp as the events that happened
within the event being recorded.
Here's the implementation of the design of this solution:
All this is per cpu, and only needs to worry about nested events (not
parallel events).
The players:
write_tail: The index in the buffer where new events can be written to.
It is incremented via local_add() to reserve space for a new event.
before_stamp: A time stamp set by all events before reserving space.
write_stamp: A time stamp updated by events after it has successfully
reserved space.
/* Save the current position of write */
[A] w = local_read(write_tail);
barrier();
/* Read both before and write stamps before touching anything */
before = local_read(before_stamp);
after = local_read(write_stamp);
barrier();
/*
* If before and after are the same, then this event is not
* interrupting a time update. If it is, then reserve space for adding
* a full time stamp (this can turn into a time extend which is
* just an extended time delta but fill up the extra space).
*/
if (after != before)
abs = true;
ts = clock();
/* Now update the before_stamp (everyone does this!) */
[B] local_set(before_stamp, ts);
/* Now reserve space on the buffer */
[C] write = local_add_return(len, write_tail);
/* Set tail to be were this event's data is */
tail = write - len;
if (w == tail) {
/* Nothing interrupted this between A and C */
[D] local_set(write_stamp, ts);
barrier();
[E] save_before = local_read(before_stamp);
if (!abs) {
/* This did not interrupt a time update */
delta = ts - after;
} else {
delta = ts; /* The full time stamp will be in use */
}
if (ts != save_before) {
/* slow path - Was interrupted between C and E */
/* The update to write_stamp could have overwritten the update to
* it by the interrupting event, but before and after should be
* the same for all completed top events */
after = local_read(write_stamp);
if (save_before > after)
local_cmpxchg(write_stamp, after, save_before);
}
} else {
/* slow path - Interrupted between A and C */
after = local_read(write_stamp);
temp_ts = clock();
barrier();
[F] if (write == local_read(write_tail) && after < temp_ts) {
/* This was not interrupted since C and F
* The last write_stamp is still valid for the previous event
* in the buffer. */
delta = temp_ts - after;
/* OK to keep this new time stamp */
ts = temp_ts;
} else {
/* Interrupted between C and F
* Well, there's no use to try to know what the time stamp
* is for the previous event. Just set delta to zero and
* be the same time as that event that interrupted us before
* the reservation of the buffer. */
delta = 0;
}
/* No need to use full timestamps here */
abs = 0;
}
Link: https://lkml.kernel.org/r/20200625094454.732790f7@oasis.local.home
Link: https://lore.kernel.org/r/20200627010041.517736087@goodmis.org
Link: http://lkml.kernel.org/r/20200629025258.957440797@goodmis.org
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
kernel/trace/ring_buffer.c | 276 ++++++++++++++++++++++++-------------
1 file changed, 181 insertions(+), 95 deletions(-)
diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 00867ff82412..026238c55b0c 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -27,6 +27,7 @@
#include <linux/oom.h>
#include <asm/local.h>
+#include <asm/local64.h>
static void update_pages_handler(struct work_struct *work);
@@ -418,6 +419,17 @@ struct rb_event_info {
int add_timestamp;
};
+/*
+ * Used for the add_timestamp
+ * NONE
+ * NORMAL - may be for either time extend or absolute
+ * FORCE - force a full time stamp.
+ */
+enum {
+ RB_ADD_STAMP_NONE,
+ RB_ADD_STAMP_NORMAL,
+ RB_ADD_STAMP_FORCE
+};
/*
* Used for which event context the event is in.
* NMI = 0
@@ -470,7 +482,8 @@ struct ring_buffer_per_cpu {
size_t shortest_full;
unsigned long read;
unsigned long read_bytes;
- u64 write_stamp;
+ local64_t write_stamp;
+ local64_t before_stamp;
u64 read_stamp;
/* ring buffer pages to update, > 0 to add, < 0 to remove */
long nr_pages_to_update;
@@ -2416,16 +2429,13 @@ rb_update_event(struct ring_buffer_per_cpu *cpu_buffer,
unsigned length = info->length;
u64 delta = info->delta;
- /* Only a commit updates the timestamp */
- if (unlikely(!rb_event_is_commit(cpu_buffer, event)))
- delta = 0;
-
/*
* If we need to add a timestamp, then we
* add it to the start of the reserved space.
*/
if (unlikely(info->add_timestamp)) {
- bool abs = ring_buffer_time_stamp_abs(cpu_buffer->buffer);
+ bool abs = info->add_timestamp == RB_ADD_STAMP_FORCE ||
+ ring_buffer_time_stamp_abs(cpu_buffer->buffer);
event = rb_add_time_stamp(event, abs ? info->delta : delta, abs);
length -= RB_LEN_TIME_EXTEND;
@@ -2480,6 +2490,39 @@ static inline bool sched_clock_stable(void)
}
#endif
+static __always_inline bool
+rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
+ struct ring_buffer_event *event)
+{
+ unsigned long addr = (unsigned long)event;
+ unsigned long index;
+
+ index = rb_event_index(event);
+ addr &= PAGE_MASK;
+
+ return cpu_buffer->commit_page->page == (void *)addr &&
+ rb_commit_index(cpu_buffer) == index;
+}
+
+static u64 rb_time_delta(struct ring_buffer_event *event)
+{
+ switch (event->type_len) {
+ case RINGBUF_TYPE_PADDING:
+ return 0;
+
+ case RINGBUF_TYPE_TIME_EXTEND:
+ return ring_buffer_event_time_stamp(event);
+
+ case RINGBUF_TYPE_TIME_STAMP:
+ return 0;
+
+ case RINGBUF_TYPE_DATA:
+ return event->time_delta;
+ default:
+ return 0;
+ }
+}
+
static inline int
rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
struct ring_buffer_event *event)
@@ -2488,6 +2531,8 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
struct buffer_page *bpage;
unsigned long index;
unsigned long addr;
+ u64 write_stamp;
+ u64 delta;
new_index = rb_event_index(event);
old_index = new_index + rb_event_ts_length(event);
@@ -2496,10 +2541,32 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
bpage = READ_ONCE(cpu_buffer->tail_page);
+ delta = rb_time_delta(event);
+
+ write_stamp = local64_read(&cpu_buffer->write_stamp);
+
+ /* Make sure the write stamp is read before testing the location */
+ barrier();
+
if (bpage->page == (void *)addr && rb_page_write(bpage) == old_index) {
unsigned long write_mask =
local_read(&bpage->write) & ~RB_WRITE_MASK;
unsigned long event_length = rb_event_length(event);
+ u64 ret;
+
+ ret = local64_cmpxchg(&cpu_buffer->write_stamp, write_stamp, write_stamp - delta);
+ /* Something came in, can't discard */
+ if (ret != write_stamp)
+ return 0;
+
+ /*
+ * If an event were to come in now, it would see that the
+ * write_stamp and the before_stamp are different, and assume
+ * that this event just added itself before updating
+ * the write stamp. The interrupting event will fix the
+ * write stamp for us, and use the before stamp as its delta.
+ */
+
/*
* This is on the tail page. It is possible that
* a write could come in and move the tail page
@@ -2551,10 +2618,6 @@ rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
local_set(&cpu_buffer->commit_page->page->commit,
rb_page_write(cpu_buffer->commit_page));
rb_inc_page(cpu_buffer, &cpu_buffer->commit_page);
- /* Only update the write stamp if the page has an event */
- if (rb_page_write(cpu_buffer->commit_page))
- cpu_buffer->write_stamp =
- cpu_buffer->commit_page->page->time_stamp;
/* add barrier to keep gcc from optimizing too much */
barrier();
}
@@ -2626,54 +2689,10 @@ static inline void rb_event_discard(struct ring_buffer_event *event)
event->time_delta = 1;
}
-static __always_inline bool
-rb_event_is_commit(struct ring_buffer_per_cpu *cpu_buffer,
- struct ring_buffer_event *event)
-{
- unsigned long addr = (unsigned long)event;
- unsigned long index;
-
- index = rb_event_index(event);
- addr &= PAGE_MASK;
-
- return cpu_buffer->commit_page->page == (void *)addr &&
- rb_commit_index(cpu_buffer) == index;
-}
-
-static __always_inline void
-rb_update_write_stamp(struct ring_buffer_per_cpu *cpu_buffer,
- struct ring_buffer_event *event)
-{
- u64 delta;
-
- /*
- * The event first in the commit queue updates the
- * time stamp.
- */
- if (rb_event_is_commit(cpu_buffer, event)) {
- /*
- * A commit event that is first on a page
- * updates the write timestamp with the page stamp
- */
- if (!rb_event_index(event))
- cpu_buffer->write_stamp =
- cpu_buffer->commit_page->page->time_stamp;
- else if (event->type_len == RINGBUF_TYPE_TIME_EXTEND) {
- delta = ring_buffer_event_time_stamp(event);
- cpu_buffer->write_stamp += delta;
- } else if (event->type_len == RINGBUF_TYPE_TIME_STAMP) {
- delta = ring_buffer_event_time_stamp(event);
- cpu_buffer->write_stamp = delta;
- } else
- cpu_buffer->write_stamp += event->time_delta;
- }
-}
-
static void rb_commit(struct ring_buffer_per_cpu *cpu_buffer,
struct ring_buffer_event *event)
{
local_inc(&cpu_buffer->entries);
- rb_update_write_stamp(cpu_buffer, event);
rb_end_commit(cpu_buffer);
}
@@ -2872,13 +2891,13 @@ rb_handle_timestamp(struct ring_buffer_per_cpu *cpu_buffer,
KERN_WARNING "Delta way too big! %llu ts=%llu write stamp = %llu\n%s",
(unsigned long long)info->delta,
(unsigned long long)info->ts,
- (unsigned long long)cpu_buffer->write_stamp,
+ (unsigned long long)local64_read(&cpu_buffer->write_stamp),
sched_clock_stable() ? "" :
"If you just came from a suspend/resume,\n"
"please switch to the trace global clock:\n"
" echo global > /sys/kernel/debug/tracing/trace_clock\n"
"or add trace_clock=global to the kernel command line\n");
- info->add_timestamp = 1;
+ info->add_timestamp = RB_ADD_STAMP_NORMAL;
}
static struct ring_buffer_event *
@@ -2887,8 +2906,36 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
{
struct ring_buffer_event *event;
struct buffer_page *tail_page;
- unsigned long tail, write;
+ unsigned long tail, write, w;
+ u64 before, after;
+ bool abs = false;
+
+ /* Don't let the compiler play games with cpu_buffer->tail_page */
+ tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
+
+ /*A*/ w = local_read(&tail_page->write) & RB_WRITE_MASK;
+ barrier();
+ before = local64_read(&cpu_buffer->before_stamp);
+ after = local64_read(&cpu_buffer->write_stamp);
+ barrier();
+ info->ts = rb_time_stamp(cpu_buffer->buffer);
+
+ if (ring_buffer_time_stamp_abs(cpu_buffer->buffer)) {
+ info->delta = info->ts;
+ abs = true;
+ } else {
+ info->delta = info->ts - after;
+ }
+
+ if (unlikely(test_time_stamp(info->delta)))
+ rb_handle_timestamp(cpu_buffer, info);
+ /*
+ * If interrupting an event time update, we may need an absolute timestamp.
+ * Don't bother if this is the start of a new page (w == 0).
+ */
+ if (unlikely(before != after && w))
+ info->add_timestamp = RB_ADD_STAMP_FORCE;
/*
* If the time delta since the last event is too big to
* hold in the time field of the event, then we append a
@@ -2897,25 +2944,88 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
if (unlikely(info->add_timestamp))
info->length += RB_LEN_TIME_EXTEND;
- /* Don't let the compiler play games with cpu_buffer->tail_page */
- tail_page = info->tail_page = READ_ONCE(cpu_buffer->tail_page);
- write = local_add_return(info->length, &tail_page->write);
+ /*B*/ local64_set(&cpu_buffer->before_stamp, info->ts);
+
+ /*C*/ write = local_add_return(info->length, &tail_page->write);
/* set write to only the index of the write */
write &= RB_WRITE_MASK;
+
tail = write - info->length;
+ /* See if we shot pass the end of this buffer page */
+ if (unlikely(write > BUF_PAGE_SIZE)) {
+ if (tail != w) {
+ /* before and after may now different, fix it up*/
+ before = local64_read(&cpu_buffer->before_stamp);
+ after = local64_read(&cpu_buffer->write_stamp);
+ if (before != after)
+ (void)local64_cmpxchg(&cpu_buffer->before_stamp, before, after);
+ }
+ return rb_move_tail(cpu_buffer, tail, info);
+ }
+
+ if (likely(tail == w)) {
+ u64 save_before;
+
+ /* Nothing interrupted us between A and C */
+ /*D*/ local64_set(&cpu_buffer->write_stamp, info->ts);
+ barrier();
+ /*E*/ save_before = local64_read(&cpu_buffer->before_stamp);
+ if (likely(info->add_timestamp != RB_ADD_STAMP_FORCE))
+ /* This did not interrupt any time update */
+ info->delta = info->ts - after;
+ else
+ /* Just use full timestamp for inerrupting event */
+ info->delta = info->ts;
+ barrier();
+ if (unlikely(info->ts != save_before)) {
+ /* SLOW PATH - Interrupted between C and E */
+
+ after = local64_read(&cpu_buffer->write_stamp);
+ /* Write stamp must only go forward */
+ if (save_before > after) {
+ /*
+ * We do not care about the result, only that
+ * it gets updated atomically.
+ */
+ (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, save_before);
+ }
+ }
+ } else {
+ u64 ts;
+ /* SLOW PATH - Interrupted between A and C */
+ after = local64_read(&cpu_buffer->write_stamp);
+ ts = rb_time_stamp(cpu_buffer->buffer);
+ barrier();
+ /*E*/ if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
+ after < ts) {
+ /* Nothing came after this event between C and E */
+ info->delta = ts - after;
+ (void)local64_cmpxchg(&cpu_buffer->write_stamp, after, info->ts);
+ info->ts = ts;
+ } else {
+ /*
+ * Interrupted beween C and E:
+ * Lost the previous events time stamp. Just set the
+ * delta to zero, and this will be the same time as
+ * the event this event interrupted. And the events that
+ * came after this will still be correct (as they would
+ * have built their delta on the previous event.
+ */
+ info->delta = 0;
+ }
+ if (info->add_timestamp == RB_ADD_STAMP_FORCE)
+ info->add_timestamp = RB_ADD_STAMP_NORMAL;
+ }
+
/*
* If this is the first commit on the page, then it has the same
* timestamp as the page itself.
*/
- if (!tail && !ring_buffer_time_stamp_abs(cpu_buffer->buffer))
+ if (unlikely(!tail && info->add_timestamp != RB_ADD_STAMP_FORCE && !abs))
info->delta = 0;
- /* See if we shot pass the end of this buffer page */
- if (unlikely(write > BUF_PAGE_SIZE))
- return rb_move_tail(cpu_buffer, tail, info);
-
/* We reserved something on the buffer */
event = __rb_page_index(tail_page, tail);
@@ -2944,9 +3054,9 @@ rb_reserve_next_event(struct trace_buffer *buffer,
struct ring_buffer_event *event;
struct rb_event_info info;
int nr_loops = 0;
- u64 diff;
rb_start_commit(cpu_buffer);
+ /* The commit page can not change after this */
#ifdef CONFIG_RING_BUFFER_ALLOW_SWAP
/*
@@ -2965,7 +3075,7 @@ rb_reserve_next_event(struct trace_buffer *buffer,
info.length = rb_calculate_event_length(length);
again:
- info.add_timestamp = 0;
+ info.add_timestamp = RB_ADD_STAMP_NONE;
info.delta = 0;
/*
@@ -2980,22 +3090,6 @@ rb_reserve_next_event(struct trace_buffer *buffer,
if (RB_WARN_ON(cpu_buffer, ++nr_loops > 1000))
goto out_fail;
- info.ts = rb_time_stamp(cpu_buffer->buffer);
- diff = info.ts - cpu_buffer->write_stamp;
-
- /* make sure this diff is calculated here */
- barrier();
-
- if (ring_buffer_time_stamp_abs(buffer)) {
- info.delta = info.ts;
- rb_handle_timestamp(cpu_buffer, &info);
- } else /* Did the write stamp get updated already? */
- if (likely(info.ts >= cpu_buffer->write_stamp)) {
- info.delta = diff;
- if (unlikely(test_time_stamp(info.delta)))
- rb_handle_timestamp(cpu_buffer, &info);
- }
-
event = __rb_reserve_next(cpu_buffer, &info);
if (unlikely(PTR_ERR(event) == -EAGAIN)) {
@@ -3004,11 +3098,8 @@ rb_reserve_next_event(struct trace_buffer *buffer,
goto again;
}
- if (!event)
- goto out_fail;
-
- return event;
-
+ if (likely(event))
+ return event;
out_fail:
rb_end_commit(cpu_buffer);
return NULL;
@@ -3154,11 +3245,6 @@ void ring_buffer_discard_commit(struct trace_buffer *buffer,
if (rb_try_to_discard(cpu_buffer, event))
goto out;
- /*
- * The commit is still visible by the reader, so we
- * must still update the timestamp.
- */
- rb_update_write_stamp(cpu_buffer, event);
out:
rb_end_commit(cpu_buffer);
@@ -4475,8 +4561,8 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
cpu_buffer->read = 0;
cpu_buffer->read_bytes = 0;
- cpu_buffer->write_stamp = 0;
- cpu_buffer->read_stamp = 0;
+ local64_set(&cpu_buffer->write_stamp, 0);
+ local64_set(&cpu_buffer->before_stamp, 0);
cpu_buffer->lost_events = 0;
cpu_buffer->last_overrun = 0;
--
2.25.4
^ permalink raw reply related [flat|nested] 6+ messages in thread
end of thread, other threads:[~2020-06-29 21:41 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-06-29 2:52 [PATCH 0/3 V2] ring-buffer: Restructure ftrace ring buffer time keeping to allow accurate nested timing Steven Rostedt
2020-06-29 2:52 ` [PATCH 1/3 V2] ring-buffer: Have nested events still record running time stamp Steven Rostedt
2020-06-29 13:49 ` Masami Hiramatsu
2020-06-29 16:25 ` Steven Rostedt
2020-06-29 2:52 ` [PATCH 2/3 V2] ring-buffer: Incorporate absolute timestamp into add_timestamp logic Steven Rostedt
2020-06-29 2:52 ` [PATCH 3/3 V2] ring-buffer: Add rb_time_t 64 bit operations for speeding up 32 bit Steven Rostedt
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