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perf/hw_breakpoint: Optimize toggle_bp_slot() for CPU-independent task targets 

We can still see that a majority of the time is spent hashing task pointers:

    ...
    16.98%  [kernel]       [k] rhashtable_jhash2
    ...

Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
CPU-independent. The reason for this is to update the per-CPU
'tsk_pinned' histogram.

To optimize the CPU-independent case to O(1), keep a separate
CPU-independent 'tsk_pinned_all' histogram.

The major source of complexity are transitions between "all
CPU-independent task breakpoints" and "mixed CPU-independent and
CPU-dependent task breakpoints". The code comments list all cases that
require handling.

After this optimization:

 | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
 |      Total time: 1.758 [sec]
 |
 |       34.336621 usecs/op
 |     4395.087500 usecs/op/cpu

    38.08%  [kernel]       [k] queued_spin_lock_slowpath
    10.81%  [kernel]       [k] smp_cfm_core_cond
     3.01%  [kernel]       [k] update_sg_lb_stats
     2.58%  [kernel]       [k] osq_lock
     2.57%  [kernel]       [k] llist_reverse_order
     1.45%  [kernel]       [k] find_next_bit
     1.21%  [kernel]       [k] flush_tlb_func_common
     1.01%  [kernel]       [k] arch_install_hw_breakpoint

Showing that the time spent hashing keys has become insignificant.

With the given benchmark parameters, that's an improvement of 12%
compared with the old O(#cpus) version.

And finally, using the less aggressive parameters from the preceding
changes, we now observe:

 | $> 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: 0.067 [sec]
 |
 |       35.292187 usecs/op
 |     2258.700000 usecs/op/cpu

Which is an improvement of 12% compared to without the histogram
optimizations (baseline is 40 usecs/op). This is now on par with the
theoretical ideal (constraints disabled), and only 12% slower than no
breakpoints at all.

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/20220829124719.675715-15-elver@google.com
This commit is contained in:
Marco Elver 2022-08-29 14:47:19 +02:00 committed by Peter Zijlstra
parent 9b1933b864
commit ecdfb8896f
1 changed files with 125 additions and 32 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

157
kernel/events/hw_breakpoint.c
View File

@ -66,6 +66,8 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
/* Number of pinned CPU breakpoints globally. */
static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
/* Number of pinned CPU-independent task breakpoints. */
static struct bp_slots_histogram tsk_pinned_all[TYPE_MAX];
/* Keep track of the breakpoints attached to tasks */
static struct rhltable task_bps_ht;
@ -200,6 +202,8 @@ static __init int init_breakpoint_slots(void)
for (i = 0; i < TYPE_MAX; i++) {
if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
goto err;
if (!bp_slots_histogram_alloc(&tsk_pinned_all[i], i))
goto err;
}
return 0;
@ -210,8 +214,10 @@ err:
if (err_cpu == cpu)
break;
}
for (i = 0; i < TYPE_MAX; i++)
for (i = 0; i < TYPE_MAX; i++) {
bp_slots_histogram_free(&cpu_pinned[i]);
bp_slots_histogram_free(&tsk_pinned_all[i]);
}
return -ENOMEM;
}
@ -245,6 +251,26 @@ bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
return 0;
}
static int
bp_slots_histogram_max_merge(struct bp_slots_histogram *hist1, struct bp_slots_histogram *hist2,
enum bp_type_idx type)
{
for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
const int count1 = atomic_read(&hist1->count[i]);
const int count2 = atomic_read(&hist2->count[i]);
/* Catch unexpected writers; we want a stable snapshot. */
ASSERT_EXCLUSIVE_WRITER(hist1->count[i]);
ASSERT_EXCLUSIVE_WRITER(hist2->count[i]);
if (count1 + count2 > 0)
return i + 1;
WARN(count1 < 0, "inconsistent breakpoint slots histogram");
WARN(count2 < 0, "inconsistent breakpoint slots histogram");
}
return 0;
}
#ifndef hw_breakpoint_weight
static inline int hw_breakpoint_weight(struct perf_event *bp)
{
@ -273,7 +299,7 @@ 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);
return bp_slots_histogram_max(tsk_pinned, type);
return bp_slots_histogram_max_merge(tsk_pinned, &tsk_pinned_all[type], type);
}
/*
@ -367,12 +393,28 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
}
/*
* Add a pinned breakpoint for the given task in our constraint table
* Add/remove the given breakpoint in our constraint table
*/
static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
enum bp_type_idx type, int weight)
static int
toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, int weight)
{
struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
int cpu, next_tsk_pinned;
if (!enable)
weight = -weight;
if (!bp->hw.target) {
/*
* Update the pinned CPU slots, in per-CPU bp_cpuinfo and in the
* global histogram.
*/
struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
lockdep_assert_held_write(&bp_cpuinfo_sem);
bp_slots_histogram_add(&cpu_pinned[type], info->cpu_pinned, weight);
info->cpu_pinned += weight;
return 0;
}
/*
* If bp->hw.target, tsk_pinned is only modified, but not used
@ -382,35 +424,83 @@ 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);
bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
}
/*
* Add/remove the given breakpoint in our constraint table
*/
static int
toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
int weight)
{
const struct cpumask *cpumask = cpumask_of_bp(bp);
int cpu;
/*
* Update the pinned task slots, in per-CPU bp_cpuinfo and in the global
* histogram. We need to take care of 4 cases:
*
* 1. This breakpoint targets all CPUs (cpu < 0), and there may only
* exist other task breakpoints targeting all CPUs. In this case we
* can simply update the global slots histogram.
*
* 2. This breakpoint targets a specific CPU (cpu >= 0), but there may
* only exist other task breakpoints targeting all CPUs.
*
* a. On enable: remove the existing breakpoints from the global
* slots histogram and use the per-CPU histogram.
*
* b. On disable: re-insert the existing breakpoints into the global
* slots histogram and remove from per-CPU histogram.
*
* 3. Some other existing task breakpoints target specific CPUs. Only
* update the per-CPU slots histogram.
*/
if (!enable)
weight = -weight;
if (!enable) {
/*
* Remove before updating histograms so we can determine if this
* was the last task breakpoint for a specific CPU.
*/
int ret = rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
/* Pinned counter cpu profiling */
if (!bp->hw.target) {
struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
lockdep_assert_held_write(&bp_cpuinfo_sem);
bp_slots_histogram_add(&cpu_pinned[type], info->cpu_pinned, weight);
info->cpu_pinned += weight;
return 0;
if (ret)
return ret;
}
/*
* Note: If !enable, next_tsk_pinned will not count the to-be-removed breakpoint.
*/
next_tsk_pinned = task_bp_pinned(-1, bp, type);
/* Pinned counter task profiling */
for_each_cpu(cpu, cpumask)
toggle_bp_task_slot(bp, cpu, type, weight);
if (next_tsk_pinned >= 0) {
if (bp->cpu < 0) { /* Case 1: fast path */
if (!enable)
next_tsk_pinned += hw_breakpoint_weight(bp);
bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned, weight);
} else if (enable) { /* Case 2.a: slow path */
/* Add existing to per-CPU histograms. */
for_each_possible_cpu(cpu) {
bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
0, next_tsk_pinned);
}
/* Add this first CPU-pinned task breakpoint. */
bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
next_tsk_pinned, weight);
/* Rebalance global task pinned histogram. */
bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned,
-next_tsk_pinned);
} else { /* Case 2.b: slow path */
/* Remove this last CPU-pinned task breakpoint. */
bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
next_tsk_pinned + hw_breakpoint_weight(bp), weight);
/* Remove all from per-CPU histograms. */
for_each_possible_cpu(cpu) {
bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
next_tsk_pinned, -next_tsk_pinned);
}
/* Rebalance global task pinned histogram. */
bp_slots_histogram_add(&tsk_pinned_all[type], 0, next_tsk_pinned);
}
} else { /* Case 3: slow path */
const struct cpumask *cpumask = cpumask_of_bp(bp);
for_each_cpu(cpu, cpumask) {
next_tsk_pinned = task_bp_pinned(cpu, bp, type);
if (!enable)
next_tsk_pinned += hw_breakpoint_weight(bp);
bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
next_tsk_pinned, weight);
}
}
/*
* Readers want a stable snapshot of the per-task breakpoint list.
@ -419,8 +509,8 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
if (enable)
return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
else
return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
return 0;
}
__weak int arch_reserve_bp_slot(struct perf_event *bp)
@ -850,6 +940,9 @@ bool hw_breakpoint_is_used(void)
*/
if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
return true;
if (atomic_read(&tsk_pinned_all[type].count[slot]))
return true;
}
}