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Subject[RFC PATCH V2 4/7] perf/x86/lbr: Fix shorter LBRs call stacks for system-wide mode
Date
From: Kan Liang <kan.liang@linux.intel.com>

In system-wide mode, LBR callstacks are shorter in comparison to
per-process mode.

LBR MSRs are reset during context switch in system-wide mode. For LBR
call stack, the LBRs should be always saved/restored during context
switch.

Use the space in task_struct to save/restore the LBR call stack data.

For system-wide event, it's unnecessagy to update the
lbr_callstack_users for each threads. Add a variable in x86_pmu to
indicate if the system-wide event is active.

Fixes: 76cb2c617f12 ("perf/x86/intel: Save/restore LBR stack during context switch")
Reported-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Debugged-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
---

No changes since V1

arch/x86/events/intel/lbr.c | 57 ++++++++++++++++++++++++++++++++++----------
arch/x86/events/perf_event.h | 1 +
2 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
index dbf31f9..855628a 100644
--- a/arch/x86/events/intel/lbr.c
+++ b/arch/x86/events/intel/lbr.c
@@ -337,6 +337,12 @@ static inline u64 rdlbr_to(unsigned int idx)
return val;
}

+static bool has_lbr_callstack_users(struct x86_perf_task_context *task_ctx)
+{
+ return task_ctx->lbr_callstack_users ||
+ x86_pmu.lbr_callstack_users;
+}
+
static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
@@ -344,7 +350,7 @@ static void __intel_pmu_lbr_restore(struct x86_perf_task_context *task_ctx)
unsigned lbr_idx, mask;
u64 tos;

- if (task_ctx->lbr_callstack_users == 0 ||
+ if (!has_lbr_callstack_users(task_ctx) ||
task_ctx->lbr_stack_state == LBR_NONE) {
intel_pmu_lbr_reset();
return;
@@ -392,7 +398,7 @@ static void __intel_pmu_lbr_save(struct x86_perf_task_context *task_ctx)
u64 tos, from;
int i;

- if (task_ctx->lbr_callstack_users == 0) {
+ if (!has_lbr_callstack_users(task_ctx)) {
task_ctx->lbr_stack_state = LBR_NONE;
return;
}
@@ -445,6 +451,7 @@ void intel_pmu_lbr_sched_task(struct perf_event_context *ctx,
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
struct x86_perf_task_context *task_ctx;
+ struct perf_ctx_data *ctx_data;

if (!cpuc->lbr_users)
return;
@@ -454,15 +461,18 @@ void intel_pmu_lbr_sched_task(struct perf_event_context *ctx,
* the task was scheduled out, restore the stack. Otherwise flush
* the LBR stack.
*/
- task_ctx = ctx ? ctx->task_ctx_data : NULL;
+ rcu_read_lock();
+ ctx_data = rcu_dereference(task->perf_ctx_data);
+ task_ctx = ctx_data ? (struct x86_perf_task_context *) ctx_data->data : NULL;
if (task_ctx) {
if (sched_in)
__intel_pmu_lbr_restore(task_ctx);
else
__intel_pmu_lbr_save(task_ctx);
+ rcu_read_unlock();
return;
}
-
+ rcu_read_unlock();
/*
* Since a context switch can flip the address space and LBR entries
* are not tagged with an identifier, we need to wipe the LBR, even for
@@ -481,16 +491,27 @@ static inline bool branch_user_callstack(unsigned br_sel)
void intel_pmu_lbr_add(struct perf_event *event)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
- struct x86_perf_task_context *task_ctx;

if (!x86_pmu.lbr_nr)
return;

cpuc->br_sel = event->hw.branch_reg.reg;

- if (branch_user_callstack(cpuc->br_sel) && event->ctx->task_ctx_data) {
- task_ctx = event->ctx->task_ctx_data;
- task_ctx->lbr_callstack_users++;
+ if (branch_user_callstack(cpuc->br_sel)) {
+ if (event->attach_state & PERF_ATTACH_TASK) {
+ struct x86_perf_task_context *task_ctx;
+ struct task_struct *task = event->hw.target;
+ struct perf_ctx_data *ctx_data;
+
+ rcu_read_lock();
+ ctx_data = rcu_dereference(task->perf_ctx_data);
+ if (ctx_data) {
+ task_ctx = (struct x86_perf_task_context *)ctx_data->data;
+ task_ctx->lbr_callstack_users++;
+ }
+ rcu_read_unlock();
+ } else
+ x86_pmu.lbr_callstack_users++;
}

/*
@@ -522,15 +543,25 @@ void intel_pmu_lbr_add(struct perf_event *event)
void intel_pmu_lbr_del(struct perf_event *event)
{
struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
- struct x86_perf_task_context *task_ctx;

if (!x86_pmu.lbr_nr)
return;

- if (branch_user_callstack(cpuc->br_sel) &&
- event->ctx->task_ctx_data) {
- task_ctx = event->ctx->task_ctx_data;
- task_ctx->lbr_callstack_users--;
+ if (branch_user_callstack(cpuc->br_sel)) {
+ if (event->attach_state & PERF_ATTACH_TASK) {
+ struct task_struct *task = event->hw.target;
+ struct x86_perf_task_context *task_ctx;
+ struct perf_ctx_data *ctx_data;
+
+ rcu_read_lock();
+ ctx_data = rcu_dereference(task->perf_ctx_data);
+ if (ctx_data) {
+ task_ctx = (struct x86_perf_task_context *)ctx_data->data;
+ task_ctx->lbr_callstack_users--;
+ }
+ rcu_read_unlock();
+ } else
+ x86_pmu.lbr_callstack_users--;
}

if (x86_pmu.intel_cap.pebs_baseline && event->attr.precise_ip > 0)
diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
index 55c4812..b8b7280 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -674,6 +674,7 @@ struct x86_pmu {
int lbr_nr; /* hardware stack size */
u64 lbr_sel_mask; /* LBR_SELECT valid bits */
const int *lbr_sel_map; /* lbr_select mappings */
+ u64 lbr_callstack_users; /* lbr callstack system wide users */
bool lbr_double_abort; /* duplicated lbr aborts */
bool lbr_pt_coexist; /* (LBR|BTS) may coexist with PT */

--
2.7.4
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