Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change

[Eric Dumazet]

On 5/5/20 9:25 AM, Eric Dumazet wrote:
> 
> 
> On 5/5/20 9:13 AM, SeongJae Park wrote:
>> On Tue, 5 May 2020 09:00:44 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>>
>>> On Tue, May 5, 2020 at 8:47 AM SeongJae Park <sjpark@xxxxxxxxxx> wrote:
>>>>
>>>> On Tue, 5 May 2020 08:20:50 -0700 Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
>>>>
>>>>>
>>>>>
>>>>> On 5/5/20 8:07 AM, SeongJae Park wrote:
>>>>>> On Tue, 5 May 2020 07:53:39 -0700 Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>>>>>>
>>>>>
>>>>>>> Why do we have 10,000,000 objects around ? Could this be because of
>>>>>>> some RCU problem ?
>>>>>>
>>>>>> Mainly because of a long RCU grace period, as you guess.  I have no idea how
>>>>>> the grace period became so long in this case.
>>>>>>
>>>>>> As my test machine was a virtual machine instance, I guess RCU readers
>>>>>> preemption[1] like problem might affected this.
>>>>>>
>>>>>> [1] https://www.usenix.org/system/files/conference/atc17/atc17-prasad.pdf
>>>>>>
>>>>>>>
>>>>>>> Once Al patches reverted, do you have 10,000,000 sock_alloc around ?
>>>>>>
>>>>>> Yes, both the old kernel that prior to Al's patches and the recent kernel
>>>>>> reverting the Al's patches didn't reproduce the problem.
>>>>>>
>>>>>
>>>>> I repeat my question : Do you have 10,000,000 (smaller) objects kept in slab caches ?
>>>>>
>>>>> TCP sockets use the (very complex, error prone) SLAB_TYPESAFE_BY_RCU, but not the struct socket_wq
>>>>> object that was allocated in sock_alloc_inode() before Al patches.
>>>>>
>>>>> These objects should be visible in kmalloc-64 kmem cache.
>>>>
>>>> Not exactly the 10,000,000, as it is only the possible highest number, but I
>>>> was able to observe clear exponential increase of the number of the objects
>>>> using slabtop.  Before the start of the problematic workload, the number of
>>>> objects of 'kmalloc-64' was 5760, but I was able to observe the number increase
>>>> to 1,136,576.
>>>>
>>>>           OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
>>>> before:   5760   5088  88%    0.06K     90       64       360K kmalloc-64
>>>> after:  1136576 1136576 100%    0.06K  17759       64     71036K kmalloc-64
>>>>
>>>
>>> Great, thanks.
>>>
>>> How recent is the kernel you are running for your experiment ?
>>
>> It's based on 5.4.35.
>>
>>>
>>> Let's make sure the bug is not in RCU.
>>
>> One thing I can currently say is that the grace period passes at last.  I
>> modified the benchmark to repeat not 10,000 times but only 5,000 times to run
>> the test without OOM but easily observable memory pressure.  As soon as the
>> benchmark finishes, the memory were freed.
>>
>> If you need more tests, please let me know.
>>
> 
> I would ask Paul opinion on this issue, because we have many objects
> being freed after RCU grace periods.
> 
> If RCU subsystem can not keep-up, I guess other workloads will also suffer.
> 
> Sure, we can revert patches there and there trying to work around the issue,
> but for objects allocated from process context, we should not have these problems.
> 

I wonder if simply adjusting rcu_divisor to 6 or 5 would help 

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d9a49cd6065a20936edbda1b334136ab597cde52..fde833bac0f9f81e8536211b4dad6e7575c1219a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -427,7 +427,7 @@ module_param(qovld, long, 0444);
 static ulong jiffies_till_first_fqs = ULONG_MAX;
 static ulong jiffies_till_next_fqs = ULONG_MAX;
 static bool rcu_kick_kthreads;
-static int rcu_divisor = 7;
+static int rcu_divisor = 6;
 module_param(rcu_divisor, int, 0644);
 
 /* Force an exit from rcu_do_batch() after 3 milliseconds. */