Re: [PATCH 1/1] kasan: fix livelock in qlist_move_cache

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Nov 28, 2017 at 9:00 AM, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
> Thanks for reviewing
>    My machine has 128G of RAM, and runs many KVM virtual machines.
> libvirtd always
> report "internal error: received hangup / error event on socket" under
> heavy memory load.
>    Then I use perf top -g, qlist_move_cache consumes 100% cpu for
> several minutes.

For 128GB of RAM, batch size is 4MB. Processing such batch should not
take more than few ms. So I am still struggling  to understand how/why
your change helps and why there are issues in the first place...



> On Tue, Nov 28, 2017 at 3:45 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
>> On Tue, Nov 28, 2017 at 5:05 AM, Zhouyi Zhou <zhouzhouyi@gmail.com> wrote:
>>> When there are huge amount of quarantined cache allocates in system,
>>> number of entries in global_quarantine[i] will be great. Meanwhile,
>>> there is no relax in while loop in function qlist_move_cache which
>>> hold quarantine_lock. As a result, some userspace programs for example
>>> libvirt will complain.
>>
>> Hi,
>>
>> The QUARANTINE_BATCHES thing was supposed to fix this problem, see
>> quarantine_remove_cache() function.
>> What is the amount of RAM and number of CPUs in your system?
>> If system has 4GB of RAM, quarantine size is 128MB and that's split
>> into 1024 batches. Batch size is 128KB. Even if that's filled with the
>> smallest objects of size 32, that's only 4K objects. And there is a
>> cond_resched() between processing of every batch.
>> I don't understand why it causes problems in your setup. We use KASAN
>> extremely heavily on hundreds of machines 24x7 and we have not seen
>> any single report from this code...
>>
>>
>>> On Tue, Nov 28, 2017 at 12:04 PM,  <zhouzhouyi@gmail.com> wrote:
>>>> From: Zhouyi Zhou <zhouzhouyi@gmail.com>
>>>>
>>>> This patch fix livelock by conditionally release cpu to let others
>>>> has a chance to run.
>>>>
>>>> Tested on x86_64.
>>>> Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
>>>> ---
>>>>  mm/kasan/quarantine.c | 12 +++++++++++-
>>>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
>>>> index 3a8ddf8..33eeff4 100644
>>>> --- a/mm/kasan/quarantine.c
>>>> +++ b/mm/kasan/quarantine.c
>>>> @@ -265,10 +265,13 @@ static void qlist_move_cache(struct qlist_head *from,
>>>>                                    struct kmem_cache *cache)
>>>>  {
>>>>         struct qlist_node *curr;
>>>> +       struct qlist_head tmp_head;
>>>> +       unsigned long flags;
>>>>
>>>>         if (unlikely(qlist_empty(from)))
>>>>                 return;
>>>>
>>>> +       qlist_init(&tmp_head);
>>>>         curr = from->head;
>>>>         qlist_init(from);
>>>>         while (curr) {
>>>> @@ -278,10 +281,17 @@ static void qlist_move_cache(struct qlist_head *from,
>>>>                 if (obj_cache == cache)
>>>>                         qlist_put(to, curr, obj_cache->size);
>>>>                 else
>>>> -                       qlist_put(from, curr, obj_cache->size);
>>>> +                       qlist_put(&tmp_head, curr, obj_cache->size);
>>>>
>>>>                 curr = next;
>>>> +
>>>> +               if (need_resched()) {
>>>> +                       spin_unlock_irqrestore(&quarantine_lock, flags);
>>>> +                       cond_resched();
>>>> +                       spin_lock_irqsave(&quarantine_lock, flags);
>>>> +               }
>>>>         }
>>>> +       qlist_move_all(&tmp_head, from);
>>>>  }
>>>>
>>>>  static void per_cpu_remove_cache(void *arg)
>>>> --
>>>> 2.1.4
>>>>
>>>
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