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perf/hw_breakpoint: Optimize list of per-task breakpoints

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On a machine with 256 CPUs, running the recently added perf breakpoint
benchmark results in:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 236.418 [sec]
 |
 |   123134.794271 usecs/op
 |  7880626.833333 usecs/op/cpu

The benchmark tests inherited breakpoint perf events across many
threads.

Looking at a perf profile, we can see that the majority of the time is
spent in various hw_breakpoint.c functions, which execute within the
'nr_bp_mutex' critical sections which then results in contention on that
mutex as well:

    37.27%  [kernel]       [k] osq_lock
    34.92%  [kernel]       [k] mutex_spin_on_owner
    12.15%  [kernel]       [k] toggle_bp_slot
    11.90%  [kernel]       [k] __reserve_bp_slot

The culprit here is task_bp_pinned(), which has a runtime complexity of
O(#tasks) due to storing all task breakpoints in the same list and
iterating through that list looking for a matching task. Clearly, this
does not scale to thousands of tasks.

Instead, make use of the "rhashtable" variant "rhltable" which stores
multiple items with the same key in a list. This results in average
runtime complexity of O(1) for task_bp_pinned().

With the optimization, the benchmark shows:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.208 [sec]
 |
 |      108.422396 usecs/op
 |     6939.033333 usecs/op/cpu

On this particular setup that's a speedup of ~1135x.

While one option would be to make task_struct a breakpoint list node,
this would only further bloat task_struct for infrequently used data.
Furthermore, after all optimizations in this series, there's no evidence
it would result in better performance: later optimizations make the time
spent looking up entries in the hash table negligible (we'll reach the
theoretical ideal performance i.e. no constraints).

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20220829124719.675715-5-elver@google.com
This commit is contained in:
Marco Elver 2022-08-29 14:47:09 +02:00 committed by Peter Zijlstra
parent 089cdcb0cd
commit 0370dc314d
2 changed files with 37 additions and 22 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/perf_event.h
View File

@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
};
#ifdef CONFIG_HAVE_HW_BREAKPOINT
#include <linux/rhashtable-types.h>
#include <asm/hw_breakpoint.h>
#endif
@ -178,7 +179,7 @@ struct hw_perf_event {
* creation and event initalization.
*/
struct arch_hw_breakpoint info;
struct list_head bp_list;
struct rhlist_head bp_list;
};
#endif
struct { /* amd_iommu */

56
kernel/events/hw_breakpoint.c
View File

@ -26,10 +26,10 @@
#include <linux/irqflags.h>
#include <linux/kdebug.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/mutex.h>
#include <linux/notifier.h>
#include <linux/percpu.h>
#include <linux/rhashtable.h>
#include <linux/sched.h>
#include <linux/slab.h>
@ -54,7 +54,13 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
}
/* Keep track of the breakpoints attached to tasks */
static LIST_HEAD(bp_task_head);
static struct rhltable task_bps_ht;
static const struct rhashtable_params task_bps_ht_params = {
.head_offset = offsetof(struct hw_perf_event, bp_list),
.key_offset = offsetof(struct hw_perf_event, target),
.key_len = sizeof_field(struct hw_perf_event, target),
.automatic_shrinking = true,
};
static int constraints_initialized;
@ -103,17 +109,23 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
*/
static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
{
struct task_struct *tsk = bp->hw.target;
struct rhlist_head *head, *pos;
struct perf_event *iter;
int count = 0;
list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
if (iter->hw.target == tsk &&
find_slot_idx(iter->attr.bp_type) == type &&
rcu_read_lock();
head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
if (!head)
goto out;
rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
if (find_slot_idx(iter->attr.bp_type) == type &&
(iter->cpu < 0 || cpu == iter->cpu))
count += hw_breakpoint_weight(iter);
}
out:
rcu_read_unlock();
return count;
}
@ -186,7 +198,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
/*
* Add/remove the given breakpoint in our constraint table
*/
static void
static int
toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
int weight)
{
@ -199,7 +211,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
/* Pinned counter cpu profiling */
if (!bp->hw.target) {
get_bp_info(bp->cpu, type)->cpu_pinned += weight;
return;
return 0;
}
/* Pinned counter task profiling */
@ -207,9 +219,9 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
toggle_bp_task_slot(bp, cpu, type, weight);
if (enable)
list_add_tail(&bp->hw.bp_list, &bp_task_head);
return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
else
list_del(&bp->hw.bp_list);
return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
}
__weak int arch_reserve_bp_slot(struct perf_event *bp)
@ -307,9 +319,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
if (ret)
return ret;
toggle_bp_slot(bp, true, type, weight);
return 0;
return toggle_bp_slot(bp, true, type, weight);
}
int reserve_bp_slot(struct perf_event *bp)
@ -334,7 +344,7 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)
type = find_slot_idx(bp_type);
weight = hw_breakpoint_weight(bp);
toggle_bp_slot(bp, false, type, weight);
WARN_ON(toggle_bp_slot(bp, false, type, weight));
}
void release_bp_slot(struct perf_event *bp)
@ -707,7 +717,7 @@ static struct pmu perf_breakpoint = {
int __init init_hw_breakpoint(void)
{
int cpu, err_cpu;
int i;
int i, ret;
for (i = 0; i < TYPE_MAX; i++)
nr_slots[i] = hw_breakpoint_slots(i);
@ -718,18 +728,24 @@ int __init init_hw_breakpoint(void)
info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
GFP_KERNEL);
if (!info->tsk_pinned)
goto err_alloc;
if (!info->tsk_pinned) {
ret = -ENOMEM;
goto err;
}
}
}
ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
if (ret)
goto err;
constraints_initialized = 1;
perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);
return register_die_notifier(&hw_breakpoint_exceptions_nb);
err_alloc:
err:
for_each_possible_cpu(err_cpu) {
for (i = 0; i < TYPE_MAX; i++)
kfree(get_bp_info(err_cpu, i)->tsk_pinned);
@ -737,7 +753,5 @@ int __init init_hw_breakpoint(void)
break;
}
return -ENOMEM;
return ret;
}