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Date: Thu, 22 Jun 2023 09:09:10 -0700
From: John Johansen <john.johansen@...onical.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Swapnil Sapkal <Swapnil.Sapkal@....com>,
Peter Zijlstra <peterz@...radead.org>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
Aaron Lu <aaron.lu@...el.com>, x86@...nel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [tip: sched/core] sched: Fix performance regression introduced by
mm_cid
On 6/22/23 07:33, Mathieu Desnoyers wrote:
> On 6/21/23 19:59, John Johansen wrote:
>> 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
>
> I don't understand why all these heuristics are needed at all ?
>
it can be simplified. The scaling heuristic came out of playing with it, doing some testing to reduce lock contention. Like I said it needs work. There is a reason its not landed anywhere yet. And yes it may not even be the right approach
> What was fundamentally wrong with the per-cpu caches before commit df323337e50 other than being non-RT friendly ? Was the only purpose of that commit to reduce the duration of preempt-off critical sections, or is there a bigger picture concern it was taking care of by introducing a global pool ?
>
- reducing preempt critical sections for rt.
- need to use GFP_ATOMIC within those large critical sections, there are audit allocations, etc.
- those large critical sections also introduce other locking constraints, that we wanted/needed to get rid of. There are places now that might sleep within the bounds of the old large critical section.
- reduce memory overhead on large multi-core systems
> Introducing per-cpu memory pools, dealing with migration by giving entries back to the right cpu's pool, taking into account the cpu the entry belongs to, and use a per-cpu/lock-free data structure allowing lock-free push to give back an entry on a remote cpu should do the trick without locking, and without long preempt-off critical sections.
I am not opposed to doing that, but we will need to be able to deal with preemption making it so we might need additional buffers for a given cpu, which either means allocating them, or having an even larger per cpu pool preallocated.
The other issue is the overhead on large multi-core systems, which makes going with a large preallocated set.
>
> The only downside I see for per-cpu memory pools is a slightly larger memory overhead on large multi-core systems. But is that really a concern ?
>
I was told it is
> What am I missing here ?
>
different people/groups having conflicting constraints.
- make it work with RT
- reduce large system overhead
- make it fast
...
> Thanks,
>
> Mathieu
>
>>
>> commit e057e9b47f1749882ea0efb4427d6b9671c761ab
>> Author: John Johansen <john.johansen@...onical.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@...onical.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
>>
>>
>>
>>
>
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