[PATCH V2 4/9] perf tools: Stitch LBR call stack

kan.liang
2020-03-09T17:47:51Z

In LBR call stack mode, the depth of reconstructed LBR call stack limits
to the number of LBR registers.

For example, on skylake, the depth of reconstructed LBR call stack is
always <= 32.

# To display the perf.data header info, please use
# --header/--header-only options.
#
#
# Total Lost Samples: 0
#
# Samples: 6K of event 'cycles'
# Event count (approx.): 6487119731
#
# Children Self Command Shared Object Symbol
# ........ ........ ............... ..................
# ................................

99.97% 99.97% tchain_edit tchain_edit [.] f43
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For a call stack which is deeper than LBR limit, HW will overwrite the
LBR register with oldest branch. Only partial call stacks can be
reconstructed.

However, the overwritten LBRs may still be retrieved from previous
sample. At that moment, HW hasn't overwritten the LBR registers yet.
Perf tools can stitch those overwritten LBRs on current call stacks to
get a more complete call stack.

To determine if LBRs can be stitched, perf tools need to compare current
sample with previous sample.
- They should have identical LBR records (Same from, to and flags
values, and the same physical index of LBR registers).
- The searching starts from the base-of-stack of current sample.

In struct lbr_stitch, add 'prev_sample' to save the previous sample.
Add 'prev_lbr_cursor' to save all LBR cursor nodes from previous sample.
Once perf determines to stitch the previous LBRs, the corresponding LBR
cursor nodes will be copied to 'lists'.
The 'lists' is to track the LBR cursor nodes which are going to be
stitched.
When the stitching is over, the nodes will not be freed immediately.
They will be moved to 'free_lists'. Next stitching may reuse the space.
Both 'lists' and 'free_lists' will be freed when all samples are
processed.

The 'lbr_stitch_enable' is used to indicate whether enable LBR stitch
approach, which is disabled by default. The following patch will
introduce a new option to enable the LBR stitch approach.
This is because,
- The stitching approach base on LBR call stack technology. The known
limitations of LBR call stack technology still apply to the approach,
e.g. Exception handing such as setjmp/longjmp will have calls/returns
not match.
- This approach is not full proof. There can be cases where it creates
incorrect call stacks from incorrect matches. There is no attempt
to validate any matches in another way.

However in many common cases with call stack overflows it can recreate
better call stacks than the default lbr call stack output. So if there
are problems with LBR overflows, this is a possible workaround.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---
tools/perf/util/branch.h | 5 +-
tools/perf/util/callchain.h | 12 +-
tools/perf/util/machine.c | 235 +++++++++++++++++++++++++++++++++++-
tools/perf/util/thread.c | 2 +
tools/perf/util/thread.h | 34 ++++++
5 files changed, 283 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/branch.h b/tools/perf/util/branch.h
index 154a05cd03af..32c18003e6f9 100644
--- a/tools/perf/util/branch.h
+++ b/tools/perf/util/branch.h
@@ -35,7 +35,10 @@ struct branch_info {
struct branch_entry {
u64 from;
u64 to;
- struct branch_flags flags;
+ union {
+ struct branch_flags flags;
+ u64 flags_value;
+ };
};

struct branch_stack {
diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
index 706bb7bbe1e1..e599a23c0fdb 100644
--- a/tools/perf/util/callchain.h
+++ b/tools/perf/util/callchain.h
@@ -148,7 +148,17 @@ struct callchain_cursor_node {
u64 branch_from;
int nr_loop_iter;
u64 iter_cycles;
- struct callchain_cursor_node *next;
+ union {
+ struct callchain_cursor_node *next;
+
+ /* Indicate valid cursor node for LBR stitch */
+ bool valid;
+ };
+};
+
+struct stitch_list {
+ struct list_head node;
+ struct callchain_cursor_node cursor;
};

struct callchain_cursor {
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index 4687067d609b..62cc5ebc2dac 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -2192,6 +2192,31 @@ static int lbr_callchain_add_kernel_ip(struct thread *thread,
return 0;
}

+static void save_lbr_cursor_node(struct thread *thread,
+ struct callchain_cursor *cursor,
+ int idx)
+{
+ struct lbr_stitch *lbr_stitch = thread->lbr_stitch;
+
+ if (!lbr_stitch)
+ return;
+
+ if (cursor->pos == cursor->nr) {
+ lbr_stitch->prev_lbr_cursor[idx].valid = false;
+ return;
+ }
+
+ if (!cursor->curr)
+ cursor->curr = cursor->first;
+ else
+ cursor->curr = cursor->curr->next;
+ memcpy(&lbr_stitch->prev_lbr_cursor[idx], cursor->curr,
+ sizeof(struct callchain_cursor_node));
+
+ lbr_stitch->prev_lbr_cursor[idx].valid = true;
+ cursor->pos++;
+}
+
static int lbr_callchain_add_lbr_ip(struct thread *thread,
struct callchain_cursor *cursor,
struct perf_sample *sample,
@@ -2207,6 +2232,21 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
u64 ip, branch_from = 0;
int err, i;

+ /*
+ * The curr and pos are not used in writing session. They are cleared
+ * in callchain_cursor_commit() when the writing session is closed.
+ * Using curr and pos to track the current cursor node.
+ */
+ if (thread->lbr_stitch) {
+ cursor->curr = NULL;
+ cursor->pos = cursor->nr;
+ if (cursor->nr) {
+ cursor->curr = cursor->first;
+ for (i = 0; i < (int)(cursor->nr - 1); i++)
+ cursor->curr = cursor->curr->next;
+ }
+ }
+
if (callee) {
ip = entries[0].to;
flags = &entries[0].flags;
@@ -2217,6 +2257,20 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
if (err)
return err;

+ /*
+ * The number of cursor node increases.
+ * Move the current cursor node.
+ * But does not need to save current cursor node for entry 0.
+ * It's impossible to stitch the whole LBRs of previous sample.
+ */
+ if (thread->lbr_stitch && (cursor->pos != cursor->nr)) {
+ if (!cursor->curr)
+ cursor->curr = cursor->first;
+ else
+ cursor->curr = cursor->curr->next;
+ cursor->pos++;
+ }
+
for (i = 0; i < lbr_nr; i++) {
ip = entries[i].from;
flags = &entries[i].flags;
@@ -2225,6 +2279,7 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
true, flags, NULL, branch_from);
if (err)
return err;
+ save_lbr_cursor_node(thread, cursor, i);
}
} else {
for (i = lbr_nr - 1; i >= 0; i--) {
@@ -2235,6 +2290,7 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
true, flags, NULL, branch_from);
if (err)
return err;
+ save_lbr_cursor_node(thread, cursor, i);
}

ip = entries[0].to;
@@ -2250,6 +2306,148 @@ static int lbr_callchain_add_lbr_ip(struct thread *thread,
return 0;
}

+static int lbr_callchain_add_stitched_lbr_ip(struct thread *thread,
+ struct callchain_cursor *cursor)
+{
+ struct lbr_stitch *lbr_stitch = thread->lbr_stitch;
+ struct stitch_list *stitch_node;
+ int err;
+
+ struct callchain_cursor_node *cnode;
+
+ list_for_each_entry(stitch_node, &lbr_stitch->lists, node) {
+ cnode = &stitch_node->cursor;
+
+ err = callchain_cursor_append(cursor, cnode->ip,
+ &cnode->ms,
+ cnode->branch,
+ &cnode->branch_flags,
+ cnode->nr_loop_iter,
+ cnode->iter_cycles,
+ cnode->branch_from,
+ cnode->srcline);
+ if (err)
+ return err;
+
+ }
+ return 0;
+}
+
+static struct stitch_list *get_stitch_node(struct thread *thread)
+{
+ struct lbr_stitch *lbr_stitch = thread->lbr_stitch;
+ struct stitch_list *stitch_node;
+
+ if (!list_empty(&lbr_stitch->free_lists)) {
+ stitch_node = list_first_entry(&lbr_stitch->free_lists,
+ struct stitch_list, node);
+ list_del(&stitch_node->node);
+
+ return stitch_node;
+ }
+
+ return malloc(sizeof(struct stitch_list));
+}
+
+static bool has_stitched_lbr(struct thread *thread,
+ struct perf_sample *cur,
+ struct perf_sample *prev,
+ unsigned int max_lbr,
+ bool callee)
+{
+ struct branch_stack *cur_stack = cur->branch_stack;
+ struct branch_entry *cur_entries = perf_sample__branch_entries(cur);
+ struct branch_stack *prev_stack = prev->branch_stack;
+ struct branch_entry *prev_entries = perf_sample__branch_entries(prev);
+ struct lbr_stitch *lbr_stitch = thread->lbr_stitch;
+ int i, j, nr_identical_branches = 0;
+ struct stitch_list *stitch_node;
+ u64 cur_base, distance;
+
+ if (!cur_stack || !prev_stack)
+ return false;
+
+ /* Find the physical index of the base-of-stack for current sample. */
+ cur_base = max_lbr - cur_stack->nr + cur_stack->hw_idx + 1;
+
+ distance = (prev_stack->hw_idx > cur_base) ? (prev_stack->hw_idx - cur_base) :
+ (max_lbr + prev_stack->hw_idx - cur_base);
+ /* Previous sample has shorter stack. Nothing can be stitched. */
+ if (distance + 1 > prev_stack->nr)
+ return false;
+
+ /*
+ * Check if there are identical LBRs between two samples.
+ * Identicall LBRs must have same from, to and flags values. Also,
+ * they have to be saved in the same LBR registers (same physical
+ * index).
+ *
+ * Starts from the base-of-stack of current sample.
+ */
+ for (i = distance, j = cur_stack->nr - 1; (i >= 0) && (j >= 0); i--, j--) {
+ if ((prev_entries[i].from != cur_entries[j].from) ||
+ (prev_entries[i].to != cur_entries[j].to) ||
+ (prev_entries[i].flags_value != cur_entries[j].flags_value))
+ break;
+
+ nr_identical_branches++;
+ }
+
+ if (!nr_identical_branches)
+ return false;
+
+ /*
+ * Save the LBRs between the base-of-stack of previous sample
+ * and the base-of-stack of current sample into lbr_stitch->lists.
+ * These LBRs will be stitched later.
+ */
+ for (i = prev_stack->nr - 1; i > (int)distance; i--) {
+
+ if (!lbr_stitch->prev_lbr_cursor[i].valid)
+ continue;
+
+ stitch_node = get_stitch_node(thread);
+ if (!stitch_node)
+ return false;
+
+ memcpy(&stitch_node->cursor, &lbr_stitch->prev_lbr_cursor[i],
+ sizeof(struct callchain_cursor_node));
+
+ if (callee)
+ list_add(&stitch_node->node, &lbr_stitch->lists);
+ else
+ list_add_tail(&stitch_node->node, &lbr_stitch->lists);
+ }
+
+ return true;
+}
+
+static bool alloc_lbr_stitch(struct thread *thread, unsigned int max_lbr)
+{
+ if (thread->lbr_stitch)
+ return true;
+
+ thread->lbr_stitch = calloc(1, sizeof(struct lbr_stitch));
+ if (!thread->lbr_stitch)
+ goto err;
+
+ thread->lbr_stitch->prev_lbr_cursor = calloc(max_lbr + 1, sizeof(struct callchain_cursor_node));
+ if (!thread->lbr_stitch->prev_lbr_cursor)
+ goto free_lbr_stitch;
+
+ INIT_LIST_HEAD(&thread->lbr_stitch->lists);
+ INIT_LIST_HEAD(&thread->lbr_stitch->free_lists);
+
+ return true;
+
+free_lbr_stitch:
+ free(thread->lbr_stitch);
+ thread->lbr_stitch = NULL;
+err:
+ pr_warning("Failed to allocate space for stitched LBRs. Disable LBR stitch\n");
+ thread->lbr_stitch_enable = false;
+ return false;
+}
/*
* Recolve LBR callstack chain sample
* Return:
@@ -2262,10 +2460,14 @@ static int resolve_lbr_callchain_sample(struct thread *thread,
struct perf_sample *sample,
struct symbol **parent,
struct addr_location *root_al,
- int max_stack)
+ int max_stack,
+ unsigned int max_lbr)
{
struct ip_callchain *chain = sample->callchain;
int chain_nr = min(max_stack, (int)chain->nr);
+ bool callee = (callchain_param.order == ORDER_CALLEE);
+ struct lbr_stitch *lbr_stitch;
+ bool stitched_lbr = false;
int i, err;

for (i = 0; i < chain_nr; i++) {
@@ -2280,7 +2482,21 @@ static int resolve_lbr_callchain_sample(struct thread *thread,
if (i == chain_nr)
return 0;

- if (callchain_param.order == ORDER_CALLEE) {
+ if (thread->lbr_stitch_enable && !sample->no_hw_idx &&
+ (max_lbr > 0) && alloc_lbr_stitch(thread, max_lbr)) {
+ lbr_stitch = thread->lbr_stitch;
+
+ stitched_lbr = has_stitched_lbr(thread, sample,
+ &lbr_stitch->prev_sample,
+ max_lbr, callee);
+ if (!stitched_lbr) {
+ list_replace_init(&lbr_stitch->lists,
+ &lbr_stitch->free_lists);
+ }
+ memcpy(&lbr_stitch->prev_sample, sample, sizeof(*sample));
+ }
+
+ if (callee) {
err = lbr_callchain_add_kernel_ip(thread, cursor, sample,
parent, root_al, true, i);
if (err)
@@ -2289,7 +2505,17 @@ static int resolve_lbr_callchain_sample(struct thread *thread,
parent, root_al, true);
if (err)
goto error;
+ if (stitched_lbr) {
+ err = lbr_callchain_add_stitched_lbr_ip(thread, cursor);
+ if (err)
+ goto error;
+ }
} else {
+ if (stitched_lbr) {
+ err = lbr_callchain_add_stitched_lbr_ip(thread, cursor);
+ if (err)
+ goto error;
+ }
err = lbr_callchain_add_lbr_ip(thread, cursor, sample,
parent, root_al, false);
if (err)
@@ -2347,8 +2573,11 @@ static int thread__resolve_callchain_sample(struct thread *thread,
chain_nr = chain->nr;

if (perf_evsel__has_branch_callstack(evsel)) {
+ struct perf_env *env = perf_evsel__env(evsel);
+
err = resolve_lbr_callchain_sample(thread, cursor, sample, parent,
- root_al, max_stack);
+ root_al, max_stack,
+ !env ? 0 : env->max_branches);
if (err)
return (err < 0) ? err : 0;
}
diff --git a/tools/perf/util/thread.c b/tools/perf/util/thread.c
index 28b719388028..8d0da260c84c 100644
--- a/tools/perf/util/thread.c
+++ b/tools/perf/util/thread.c
@@ -47,6 +47,7 @@ struct thread *thread__new(pid_t pid, pid_t tid)
thread->tid = tid;
thread->ppid = -1;
thread->cpu = -1;
+ thread->lbr_stitch_enable = false;
INIT_LIST_HEAD(&thread->namespaces_list);
INIT_LIST_HEAD(&thread->comm_list);
init_rwsem(&thread->namespaces_lock);
@@ -110,6 +111,7 @@ void thread__delete(struct thread *thread)

exit_rwsem(&thread->namespaces_lock);
exit_rwsem(&thread->comm_lock);
+ thread__free_stitch_list(thread);
free(thread);
}

diff --git a/tools/perf/util/thread.h b/tools/perf/util/thread.h
index 20b96b5d1f15..58f0d23f471e 100644
--- a/tools/perf/util/thread.h
+++ b/tools/perf/util/thread.h
@@ -13,6 +13,8 @@
#include <strlist.h>
#include <intlist.h>
#include "rwsem.h"
+#include "event.h"
+#include "callchain.h"

struct addr_location;
struct map;
@@ -20,6 +22,13 @@ struct perf_record_namespaces;
struct thread_stack;
struct unwind_libunwind_ops;

+struct lbr_stitch {
+ struct list_head lists;
+ struct list_head free_lists;
+ struct perf_sample prev_sample;
+ struct callchain_cursor_node *prev_lbr_cursor;
+};
+
struct thread {
union {
struct rb_node rb_node;
@@ -46,6 +55,10 @@ struct thread {
struct srccode_state srccode_state;
bool filter;
int filter_entry_depth;
+
+ /* LBR call stack stitch */
+ bool lbr_stitch_enable;
+ struct lbr_stitch *lbr_stitch;
};

struct machine;
@@ -142,4 +155,25 @@ static inline bool thread__is_filtered(struct thread *thread)
return false;
}

+static inline void thread__free_stitch_list(struct thread *thread)
+{
+ struct lbr_stitch *lbr_stitch = thread->lbr_stitch;
+ struct stitch_list *pos, *tmp;
+
+ if (!lbr_stitch)
+ return;
+
+ list_for_each_entry_safe(pos, tmp, &lbr_stitch->lists, node) {
+ list_del_init(&pos->node);
+ free(pos);
+ }
+
+ list_for_each_entry_safe(pos, tmp, &lbr_stitch->free_lists, node) {
+ list_del_init(&pos->node);
+ free(pos);
+ }
+ free(lbr_stitch->prev_lbr_cursor);
+ free(thread->lbr_stitch);
+}
+
#endif /* __PERF_THREAD_H */
--
2.17.1