[PATCH v2 11/13] perf/hw_breakpoint: Introduce bp_slots_histogram

Marco Elver
2022-06-28T10:00:18Z

Factor out the existing `atomic_t count[N]` into its own struct called
'bp_slots_histogram', to generalize and make its intent clearer in
preparation of reusing elsewhere. The basic idea of bucketing "total
uses of N slots" resembles a histogram, so calling it such seems most
intuitive.

No functional change.

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* New patch.
---
kernel/events/hw_breakpoint.c | 94 +++++++++++++++++++++++------------
1 file changed, 62 insertions(+), 32 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 128ba3429223..18886f115abc 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -36,19 +36,27 @@
#include <linux/slab.h>

/*
- * Constraints data
+ * Datastructure to track the total uses of N slots across tasks or CPUs;
+ * bp_slots_histogram::count[N] is the number of assigned N+1 breakpoint slots.
*/
-struct bp_cpuinfo {
- /* Number of pinned cpu breakpoints in a cpu */
- unsigned int cpu_pinned;
- /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
+struct bp_slots_histogram {
#ifdef hw_breakpoint_slots
- atomic_t tsk_pinned[hw_breakpoint_slots(0)];
+ atomic_t count[hw_breakpoint_slots(0)];
#else
- atomic_t *tsk_pinned;
+ atomic_t *count;
#endif
};

+/*
+ * Per-CPU constraints data.
+ */
+struct bp_cpuinfo {
+ /* Number of pinned CPU breakpoints in a CPU. */
+ unsigned int cpu_pinned;
+ /* Histogram of pinned task breakpoints in a CPU. */
+ struct bp_slots_histogram tsk_pinned;
+};
+
static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);

static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
@@ -159,6 +167,18 @@ static inline int hw_breakpoint_slots_cached(int type)
return __nr_bp_slots[type];
}

+static __init bool
+bp_slots_histogram_alloc(struct bp_slots_histogram *hist, enum bp_type_idx type)
+{
+ hist->count = kcalloc(hw_breakpoint_slots_cached(type), sizeof(*hist->count), GFP_KERNEL);
+ return hist->count;
+}
+
+static __init void bp_slots_histogram_free(struct bp_slots_histogram *hist)
+{
+ kfree(hist->count);
+}
+
static __init int init_breakpoint_slots(void)
{
int i, cpu, err_cpu;
@@ -170,8 +190,7 @@ static __init int init_breakpoint_slots(void)
for (i = 0; i < TYPE_MAX; i++) {
struct bp_cpuinfo *info = get_bp_info(cpu, i);

- info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
- if (!info->tsk_pinned)
+ if (!bp_slots_histogram_alloc(&info->tsk_pinned, i))
goto err;
}
}
@@ -180,7 +199,7 @@ static __init int init_breakpoint_slots(void)
err:
for_each_possible_cpu(err_cpu) {
for (i = 0; i < TYPE_MAX; i++)
- kfree(get_bp_info(err_cpu, i)->tsk_pinned);
+ bp_slots_histogram_free(&get_bp_info(err_cpu, i)->tsk_pinned);
if (err_cpu == cpu)
break;
}
@@ -189,6 +208,34 @@ static __init int init_breakpoint_slots(void)
}
#endif

+static inline void
+bp_slots_histogram_add(struct bp_slots_histogram *hist, int old, int val)
+{
+ const int old_idx = old - 1;
+ const int new_idx = old_idx + val;
+
+ if (old_idx >= 0)
+ atomic_dec(&hist->count[old_idx]);
+ if (new_idx >= 0)
+ atomic_inc(&hist->count[new_idx]);
+}
+
+static int
+bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
+{
+ for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
+ const int count = atomic_read(&hist->count[i]);
+
+ /* Catch unexpected writers; we want a stable snapshot. */
+ ASSERT_EXCLUSIVE_WRITER(hist->count[i]);
+ if (count > 0)
+ return i + 1;
+ WARN(count < 0, "inconsistent breakpoint slots histogram");
+ }
+
+ return 0;
+}
+
#ifndef hw_breakpoint_weight
static inline int hw_breakpoint_weight(struct perf_event *bp)
{
@@ -205,13 +252,11 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
}

/*
- * Report the maximum number of pinned breakpoints a task
- * have in this cpu
+ * Return the maximum number of pinned breakpoints a task has in this CPU.
*/
static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
{
- atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
- int i;
+ struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;

/*
* At this point we want to have acquired the bp_cpuinfo_sem as a
@@ -219,14 +264,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
* toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
*/
lockdep_assert_held_write(&bp_cpuinfo_sem);
-
- for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
- ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
- if (atomic_read(&tsk_pinned[i]) > 0)
- return i + 1;
- }
-
- return 0;
+ return bp_slots_histogram_max(tsk_pinned, type);
}

/*
@@ -300,8 +338,7 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
enum bp_type_idx type, int weight)
{
- atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
- int old_idx, new_idx;
+ struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;

/*
* If bp->hw.target, tsk_pinned is only modified, but not used
@@ -311,14 +348,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
* bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
*/
lockdep_assert_held_read(&bp_cpuinfo_sem);
-
- old_idx = task_bp_pinned(cpu, bp, type) - 1;
- new_idx = old_idx + weight;
-
- if (old_idx >= 0)
- atomic_dec(&tsk_pinned[old_idx]);
- if (new_idx >= 0)
- atomic_inc(&tsk_pinned[new_idx]);
+ bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
}

/*
--
2.37.0.rc0.161.g10f37bed90-goog