Re: [tip: sched/core] sched: Fix performance regression introduced by mm_cid

John Johansen
2023-06-22T00:01:48Z

On 6/21/23 14:41, Mathieu Desnoyers wrote:
> On 6/21/23 14:51, Mathieu Desnoyers wrote:
>> On 6/21/23 12:36, Swapnil Sapkal wrote:
>>> Hello Mathieu,
>>>
>> [...]
>>>>
>>>> I suspect the regression is caused by the mm_count cache line bouncing.
>>>>
>>>> Please try with this additional patch applied:
>>>>
>>>> https://lore.kernel.org/lkml/20230515143536.114960-1-mathieu.desnoyers@efficios.com/
>>>
>>> Thanks for the suggestion. I tried out with the patch you suggested. I am seeing
>>> improvement in hackbench numbers with mm_count padding. But this is not matching
>>> with what we achieved through reverting the new mm_cid patch.
>>>
>>> Below are the results on the 1 Socket 4th Generation EPYC Processor (1 x 96C/192T):
>>>
>>> Threads:
>>>
>>> Test:              Base (v6.4-rc1)   Base + new_mmcid_reverted  Base + mm_count_padding
>>>   1-groups:         5.23 (0.00 pct)         4.61 (11.85 pct) 5.11 (2.29 pct)
>>>   2-groups:         4.99 (0.00 pct)         4.72 (5.41 pct) 5.00 (-0.20 pct)
>>>   4-groups:         5.96 (0.00 pct)         4.87 (18.28 pct) 5.86 (1.67 pct)
>>>   8-groups:         6.58 (0.00 pct)         5.44 (17.32 pct) 6.20 (5.77 pct)
>>> 16-groups:        11.48 (0.00 pct)         8.07 (29.70 pct) 10.68 (6.96 pct)
>>>
>>> Processes:
>>>
>>> Test:              Base (v6.4-rc1)  Base + new_mmcid_reverted   Base + mm_count_padding
>>>   1-groups:         5.19 (0.00 pct)         4.90 (5.58 pct) 5.19 (0.00 pct)
>>>   2-groups:         5.44 (0.00 pct)         5.39 (0.91 pct) 5.39 (0.91 pct)
>>>   4-groups:         5.69 (0.00 pct)         5.64 (0.87 pct) 5.64 (0.87 pct)
>>>   8-groups:         6.08 (0.00 pct)         6.01 (1.15 pct) 6.04 (0.65 pct)
>>> 16-groups:        10.87 (0.00 pct)        10.83 (0.36 pct) 10.93 (-0.55 pct)
>>>
>>> The ibs profile shows that function __switch_to_asm() is coming at top in baseline
>>> run and is not seen with mm_count padding patch. Will be attaching full ibs profile
>>> data for all the 3 runs:
>>>
>>> # Base (v6.4-rc1)
>>> Threads:
>>> Total time: 11.486 [sec]
>>>
>>>     5.15%  sched-messaging  [kernel.vmlinux]      [k] __switch_to_asm
>>>     4.31%  sched-messaging  [kernel.vmlinux]      [k] copyout
>>>     4.29%  sched-messaging  [kernel.vmlinux]      [k] native_queued_spin_lock_slowpath
>>>     4.22%  sched-messaging  [kernel.vmlinux]      [k] copyin
>>>     3.92%  sched-messaging  [kernel.vmlinux]      [k] apparmor_file_permission
>>>     2.91%  sched-messaging  [kernel.vmlinux]      [k] __schedule
>>>     2.34%  swapper          [kernel.vmlinux]      [k] __switch_to_asm
>>>     2.10%  sched-messaging  [kernel.vmlinux]      [k] prepare_to_wait_event
>>>     2.10%  sched-messaging  [kernel.vmlinux]      [k] try_to_wake_up
>>>     2.07%  sched-messaging  [kernel.vmlinux]      [k] finish_task_switch.isra.0
>>>     2.00%  sched-messaging  [kernel.vmlinux]      [k] pipe_write
>>>     1.82%  sched-messaging  [kernel.vmlinux]      [k] check_preemption_disabled
>>>     1.73%  sched-messaging  [kernel.vmlinux]      [k] exit_to_user_mode_prepare
>>>     1.52%  sched-messaging  [kernel.vmlinux]      [k] __entry_text_start
>>>     1.49%  sched-messaging  [kernel.vmlinux]      [k] osq_lock
>>>     1.45%  sched-messaging  libc.so.6             [.] write
>>>     1.44%  swapper          [kernel.vmlinux]      [k] native_sched_clock
>>>     1.38%  sched-messaging  [kernel.vmlinux]      [k] psi_group_change
>>>     1.38%  sched-messaging  [kernel.vmlinux]      [k] pipe_read
>>>     1.37%  sched-messaging  libc.so.6             [.] read
>>>     1.06%  sched-messaging  [kernel.vmlinux]      [k] vfs_read
>>>     1.01%  swapper          [kernel.vmlinux]      [k] psi_group_change
>>>     1.00%  sched-messaging  [kernel.vmlinux]      [k] update_curr
>>>
>>> # Base + mm_count_padding
>>> Threads:
>>> Total time: 11.384 [sec]
>>>
>>>     4.43%  sched-messaging  [kernel.vmlinux]         [k] copyin
>>>     4.39%  sched-messaging  [kernel.vmlinux]         [k] native_queued_spin_lock_slowpath
>>>     4.07%  sched-messaging  [kernel.vmlinux]         [k] apparmor_file_permission
>>>     4.07%  sched-messaging  [kernel.vmlinux]         [k] copyout
>>>     2.49%  sched-messaging  [kernel.vmlinux]         [k] entry_SYSCALL_64
>>>     2.37%  sched-messaging  [kernel.vmlinux]         [k] update_cfs_group
>>>     2.19%  sched-messaging  [kernel.vmlinux]         [k] pipe_write
>>>     2.00%  sched-messaging  [kernel.vmlinux]         [k] check_preemption_disabled
>>>     1.93%  swapper          [kernel.vmlinux]         [k] update_load_avg
>>>     1.81%  sched-messaging  [kernel.vmlinux]         [k] exit_to_user_mode_prepare
>>>     1.69%  sched-messaging  [kernel.vmlinux]         [k] try_to_wake_up
>>>     1.58%  sched-messaging  libc.so.6                [.] write
>>>     1.53%  sched-messaging  [kernel.vmlinux]         [k] psi_group_change
>>>     1.50%  sched-messaging  libc.so.6                [.] read
>>>     1.50%  sched-messaging  [kernel.vmlinux]         [k] pipe_read
>>>     1.39%  sched-messaging  [kernel.vmlinux]         [k] update_load_avg
>>>     1.39%  sched-messaging  [kernel.vmlinux]         [k] osq_lock
>>>     1.30%  sched-messaging  [kernel.vmlinux]         [k] update_curr
>>>     1.28%  swapper          [kernel.vmlinux]         [k] psi_group_change
>>>     1.16%  sched-messaging  [kernel.vmlinux]         [k] vfs_read
>>>     1.12%  sched-messaging  [kernel.vmlinux]         [k] vfs_write
>>>     1.10%  sched-messaging  [kernel.vmlinux]         [k] entry_SYSRETQ_unsafe_stack
>>>     1.09%  sched-messaging  [kernel.vmlinux]         [k] __switch_to_asm
>>>     1.08%  sched-messaging  [kernel.vmlinux]         [k] do_syscall_64
>>>     1.06%  sched-messaging  [kernel.vmlinux]         [k] select_task_rq_fair
>>>     1.03%  swapper          [kernel.vmlinux]         [k] update_cfs_group
>>>     1.00%  swapper          [kernel.vmlinux]         [k] rb_insert_color
>>>
>>> # Base + reverted_new_mm_cid
>>> Threads:
>>> Total time: 7.847 [sec]
>>>
>>>    12.14%  sched-messaging  [kernel.vmlinux]      [k] native_queued_spin_lock_slowpath
>>>     8.86%  swapper          [kernel.vmlinux]      [k] native_queued_spin_lock_slowpath
>>>     6.13%  sched-messaging  [kernel.vmlinux]      [k] copyin
>>>     5.54%  sched-messaging  [kernel.vmlinux]      [k] apparmor_file_permission
>>>     3.59%  sched-messaging  [kernel.vmlinux]      [k] copyout
>>>     2.61%  sched-messaging  [kernel.vmlinux]      [k] osq_lock
>>>     2.48%  sched-messaging  [kernel.vmlinux]      [k] pipe_write
>>>     2.33%  sched-messaging  [kernel.vmlinux]      [k] exit_to_user_mode_prepare
>>>     2.01%  sched-messaging  [kernel.vmlinux]      [k] check_preemption_disabled
>>>     1.96%  sched-messaging  [kernel.vmlinux]      [k] __entry_text_start
>>>     1.91%  sched-messaging  libc.so.6             [.] write
>>>     1.77%  sched-messaging  libc.so.6             [.] read
>>>     1.64%  sched-messaging  [kernel.vmlinux]      [k] mutex_spin_on_owner
>>>     1.58%  sched-messaging  [kernel.vmlinux]      [k] pipe_read
>>>     1.52%  sched-messaging  [kernel.vmlinux]      [k] try_to_wake_up
>>>     1.38%  sched-messaging  [kernel.vmlinux]      [k] ktime_get_coarse_real_ts64
>>>     1.35%  sched-messaging  [kernel.vmlinux]      [k] vfs_write
>>>     1.28%  sched-messaging  [kernel.vmlinux]      [k] entry_SYSRETQ_unsafe_stack
>>>     1.28%  sched-messaging  [kernel.vmlinux]      [k] vfs_read
>>>     1.25%  sched-messaging  [kernel.vmlinux]      [k] do_syscall_64
>>>     1.22%  sched-messaging  [kernel.vmlinux]      [k] __fget_light
>>>     1.18%  sched-messaging  [kernel.vmlinux]      [k] mutex_lock
>>>     1.12%  sched-messaging  [kernel.vmlinux]      [k] file_update_time
>>>     1.04%  sched-messaging  [kernel.vmlinux]      [k] _copy_from_iter
>>>     1.01%  sched-messaging  [kernel.vmlinux]      [k] current_time
>>>
>>> So with the reverted new_mm_cid patch, we are seeing a lot of time being spent in
>>> native_queued_spin_lock_slowpath and yet, hackbench finishes faster.
>>>
>>> I keep further digging into this please let me know if you have any pointers for me.
>>
>> Do you have CONFIG_SECURITY_APPARMOR=y ? Can you try without ?
>>
>> I notice that apparmor_file_permission appears near the top of your
>> profiles, and apparmor uses an internal aa_buffers_lock spinlock,
>> which could possibly explain the top hits for
>> native_queued_spin_lock_slowpath. My current suspicion is that
>> the raw spinlock that was taken by "Base + reverted_new_mm_cid"
>> changed the contention pattern on the apparmor lock enough to
>> speed things up by pure accident.
>
> If apparmor happens to be the culprit here, we should have a hard look at this commit:
>
> commit df323337e50 "apparmor: Use a memory pool instead per-CPU caches"
>
> Which turned a per-cpu cache into a global memory pool protected by a spinlock. It may benefit RT, but it does not appear to be so great at scaling.
>
it is not. And I have a patch that needs some more formal testing for some stats.
Ubuntu pulled it in last cycle so it has gotten a fair bit of use and is looking good
on that end. There are probably some tweaks that can be done to improve it. The
backoff in particular is something that has mostly been adjusted in response to some
basic benchmarking.

anyways patch below

commit e057e9b47f1749882ea0efb4427d6b9671c761ab
Author: John Johansen <john.johansen@canonical.com>
Date: Tue Oct 25 01:18:41 2022 -0700

apparmor: cache buffers on percpu list if there is lock contention

df323337e507 ("apparmor: Use a memory pool instead per-CPU caches")
changed buffer allocation to use a memory pool, however on a heavily
loaded machine there can be lock contention on the global buffers
lock. Add a percpu list to cache buffers on when lock contention is
encountered.

When allocating buffers attempt to use cached buffers first,
before taking the global buffers lock. When freeing buffers
try to put them back to the global list but if contention is
encountered, put the buffer on the percpu list.

The length of time a buffer is held on the percpu list is dynamically
adjusted based on lock contention. The amount of hold time is rapidly
increased and slow ramped down.

v4:
- fix percpu ->count buffer count which had been spliced across a
debug patch.
- introduce define for MAX_LOCAL_COUNT
- rework count check and locking around it.
- update commit message to reference commit that introduced the
memory pool.
v3:
- limit number of buffers that can be pushed onto the percpu
list. This avoids a problem on some kernels where one percpu
list can inherit buffers from another cpu after a reschedule,
causing more kernel memory to used than is necessary. Under
normal conditions this should eventually return to normal
but under pathelogical conditions the extra memory consumption
may have been unbouanded
v2:
- dynamically adjust buffer hold time on percpu list based on
lock contention.
v1:
- cache buffers on percpu list on lock contention

Signed-off-by: John Johansen <john.johansen@canonical.com>

diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
index e6671a4a89c4..ea3af769af5a 100644
--- a/security/apparmor/lsm.c
+++ b/security/apparmor/lsm.c
@@ -55,12 +55,21 @@ union aa_buffer {
char buffer[1];
};

+struct aa_local_cache {
+ unsigned int contention;
+ unsigned int hold;
+ unsigned int count;
+ struct list_head head;
+};
+
+#define MAX_LOCAL_COUNT 2
#define RESERVE_COUNT 2
static int reserve_count = RESERVE_COUNT;
static int buffer_count;

static LIST_HEAD(aa_global_buffers);
static DEFINE_SPINLOCK(aa_buffers_lock);
+static DEFINE_PER_CPU(struct aa_local_cache, aa_local_buffers);

struct kmem_cache *aa_audit_slab;

@@ -2029,14 +2038,45 @@ static int param_set_mode(const char *val, const struct kernel_param *kp)
return 0;
}

+static void update_contention(struct aa_local_cache *cache)
+{
+ cache->contention += 3;
+ if (cache->contention > 9)
+ cache->contention = 9;
+ cache->hold += 1 << cache->contention; /* 8, 64, 512 */
+}
+
char *aa_get_buffer(bool in_atomic)
{
union aa_buffer *aa_buf;
+ struct aa_local_cache *cache;
bool try_again = true;
gfp_t flags = (GFP_KERNEL | __GFP_RETRY_MAYFAIL | __GFP_NOWARN);

+ /* use per cpu cached buffers first */
+ cache = get_cpu_ptr(&aa_local_buffers);
+ if (!list_empty(&cache->head)) {
+ aa_buf = list_first_entry(&cache->head, union aa_buffer, list);
+ list_del(&aa_buf->list);
+ cache->hold--;
+ cache->count--;
+ put_cpu_ptr(&aa_local_buffers);
+ return &aa_buf->buffer[0];
+ }
+ put_cpu_ptr(&aa_local_buffers);
+
+ if (!spin_trylock(&aa_buffers_lock)) {
+ cache = get_cpu_ptr(&aa_local_buffers);
+ update_contention(cache);
+ put_cpu_ptr(&aa_local_buffers);
+ spin_lock(&aa_buffers_lock);
+ } else {
+ cache = get_cpu_ptr(&aa_local_buffers);
+ if (cache->contention)
+ cache->contention--;
+ put_cpu_ptr(&aa_local_buffers);
+ }
retry:
- spin_lock(&aa_buffers_lock);
if (buffer_count > reserve_count ||
(in_atomic && !list_empty(&aa_global_buffers))) {
aa_buf = list_first_entry(&aa_global_buffers, union aa_buffer,
@@ -2062,6 +2102,7 @@ char *aa_get_buffer(bool in_atomic)
if (!aa_buf) {
if (try_again) {
try_again = false;
+ spin_lock(&aa_buffers_lock);
goto retry;
}
pr_warn_once("AppArmor: Failed to allocate a memory buffer.\n");
@@ -2073,15 +2114,42 @@ char *aa_get_buffer(bool in_atomic)
void aa_put_buffer(char *buf)
{
union aa_buffer *aa_buf;
+ struct aa_local_cache *cache;

if (!buf)
return;
aa_buf = container_of(buf, union aa_buffer, buffer[0]);

- spin_lock(&aa_buffers_lock);
- list_add(&aa_buf->list, &aa_global_buffers);
- buffer_count++;
- spin_unlock(&aa_buffers_lock);
+ cache = get_cpu_ptr(&aa_local_buffers);
+ if (!cache->hold) {
+ bool must_lock = cache->count >= MAX_LOCAL_COUNT;
+
+ put_cpu_ptr(&aa_local_buffers);
+
+ if (must_lock) {
+ spin_lock(&aa_buffers_lock);
+ goto locked;
+ } else if (spin_trylock(&aa_buffers_lock)) {
+ locked:
+ /* put back on global list */
+ list_add(&aa_buf->list, &aa_global_buffers);
+ buffer_count++;
+ spin_unlock(&aa_buffers_lock);
+ cache = get_cpu_ptr(&aa_local_buffers);
+ if (cache->contention)
+ cache->contention--;
+ put_cpu_ptr(&aa_local_buffers);
+ return;
+ }
+ /* contention on global list, fallback to percpu */
+ cache = get_cpu_ptr(&aa_local_buffers);
+ update_contention(cache);
+ }
+
+ /* cache in percpu list */
+ list_add(&aa_buf->list, &cache->head);
+ cache->count++;
+ put_cpu_ptr(&aa_local_buffers);
}

/*
@@ -2123,6 +2191,16 @@ static int __init alloc_buffers(void)
union aa_buffer *aa_buf;
int i, num;

+ /*
+ * per cpu set of cached allocated buffers used to help reduce
+ * lock contention
+ */
+ for_each_possible_cpu(i) {
+ per_cpu(aa_local_buffers, i).contention = 0;
+ per_cpu(aa_local_buffers, i).hold = 0;
+ per_cpu(aa_local_buffers, i).count = 0;
+ INIT_LIST_HEAD(&per_cpu(aa_local_buffers, i).head);
+ }
/*
* A function may require two buffers at once. Usually the buffers are
* used for a short period of time and are shared. On UP kernel buffers