On Sun, Jul 15, 2018 at 6:50 PM Yafang Shao <laoar.s...@gmail.com> wrote: > > On Sun, Jul 15, 2018 at 11:04 PM, Shakeel Butt <shake...@google.com> wrote: > > On Sun, Jul 15, 2018 at 1:02 AM Yafang Shao <laoar.s...@gmail.com> wrote: > >> > >> On Sun, Jul 15, 2018 at 2:34 PM, Shakeel Butt <shake...@google.com> wrote: > >> > On Sat, Jul 14, 2018 at 10:26 PM Yafang Shao <laoar.s...@gmail.com> > >> > wrote: > >> >> > >> >> On Sun, Jul 15, 2018 at 12:25 PM, Shakeel Butt <shake...@google.com> > >> >> wrote: > >> >> > On Sat, Jul 14, 2018 at 7:10 PM Yafang Shao <laoar.s...@gmail.com> > >> >> > wrote: > >> >> >> > >> >> >> On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt <shake...@google.com> > >> >> >> wrote: > >> >> >> > On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao <laoar.s...@gmail.com> > >> >> >> > wrote: > >> >> >> >> > >> >> >> >> try_charge maybe executed in packet receive path, which is in > >> >> >> >> interrupt > >> >> >> >> context. > >> >> >> >> In this situation, the 'current' is the interrupted task, which > >> >> >> >> may has > >> >> >> >> no relation to the rx softirq, So it is nonsense to use 'current'. > >> >> >> >> > >> >> >> > > >> >> >> > Have you actually seen this occurring? > >> >> >> > >> >> >> Hi Shakeel, > >> >> >> > >> >> >> I'm trying to produce this issue, but haven't find it occur yet. > >> >> >> > >> >> >> > I am not very familiar with the > >> >> >> > network code but I can think of two ways try_charge() can be called > >> >> >> > from network code. Either through kmem charging or through > >> >> >> > mem_cgroup_charge_skmem() and both locations correctly handle > >> >> >> > interrupt context. > >> >> >> > > >> >> >> > >> >> >> Why do you say that mem_cgroup_charge_skmem() correctly hanle > >> >> >> interrupt context ? > >> >> >> > >> >> >> Let me show you why mem_cgroup_charge_skmem isn't hanling interrupt > >> >> >> context correctly. > >> >> >> > >> >> >> mem_cgroup_charge_skmem() is calling try_charge() twice. > >> >> >> The first one is with GFP_NOWAIT as the gfp_mask, and the second one > >> >> >> is with (GFP_NOWAIT | __GFP_NOFAIL) as the gfp_mask. > >> >> >> > >> >> >> If page_counter_try_charge() failes at the first time, -ENOMEM is > >> >> >> returned. > >> >> >> Then mem_cgroup_charge_skmem() will call try_charge() once more with > >> >> >> __GFP_NOFAIL set, and this time if If page_counter_try_charge() > >> >> >> failes > >> >> >> again the ' > >> >> >> force' label in try_charge() will be executed and 0 is returned. > >> >> >> > >> >> >> No matter what, the 'current' will be used and touched, that is > >> >> >> meaning mem_cgroup_charge_skmem() isn't hanling the interrupt context > >> >> >> correctly. > >> >> >> > >> >> > > >> >> > Hi Yafang, > >> >> > > >> >> > If you check mem_cgroup_charge_skmem(), the memcg passed is not > >> >> > 'current' but is from the sock object i.e. sk->sk_memcg for which the > >> >> > network buffer is allocated for. > >> >> > > >> >> > >> >> That's correct, the memcg if from the sock object. > >> >> But the point is, in this situation why 'current' is used in > >> >> try_charge() ? > >> >> As 'current' is not related with the memcg, which is just a interrupted > >> >> task. > >> >> > >> > > >> > Hmm so you mean the behavior of memcg charging in the interrupt > >> > context depends on the state of the interrupted task. > >> > >> Yes. > >> > >> > As you have > >> > noted, mem_cgroup_charge_skmem() tries charging again with > >> > __GFP_NOFAIL and the charge succeeds. Basically the memcg charging by > >> > mem_cgroup_charge_skmem() will always succeed irrespective of the > >> > state of the interrupted task. However mem_cgroup_charge_skmem() can > >> > return true if the interrupted task was exiting or a fatal signal is > >> > pending or oom victim or reclaiming memory. Can you please explain why > >> > this is bad? > >> > > >> > >> Let me show you the possible issues cause by this behavoir. > >> 1. In mem_cgroup_oom(), some members in 'current' is set. > >> That means an innocent task will be in task_in_memcg_oom state. > >> But this task may be in a different memcg, I mean the memcg of > >> the 'current' may be differenct with the sk->sk_memcg. > >> Then when this innocent 'current' do try_charge it will hit "if > >> (unlikely(task_in_memcg_oom(current)))" and -ENOMEM is returned, > >> While there're maybe some free memory (or some memory could be freed ) > >> in the memcg of the innocent 'task'. > >> > > > > No memory will be freed as try_charge() is in interrupt context. > > > > I mean when this interrupted 'current' is running, that's in process context. > In process context it should call try_to_free_mem_cgroup_pages() to > free some memory, > but it will hit "if (unlikely(task_in_memcg_oom(current)))" before as > it is set in the interrupt context. > > That's an obviously issue. Do you understand ? >
Not really. I couldn't find where current->memcg_in_oom can be set in the interrupt context. > >> 2. If the interrupted task was exiting or a fatal signal is pending > >> or oom victim, > >> it will directly goto force and 0 is returned, and then > >> mem_cgroup_charge_skmem() will return true. > >> But mem_cgroup_charge_skmem() maybe need to try the second time > >> and return false. > >> > >> That are all unexpected behavoir. > >> > > > > Yes, this is inconsistent behavior. Can you explain how this will > > affect network traffic? Basically mem_cgroup_charge_skmem() was > > supposed to return false but sometime based on the interrupted task, > > mem_cgroup_charge_skmem() returns true. How is this behavior bad for > > network traffic? > > > > You could see the funtion __sk_mem_raise_allocated(). > If mem_cgroup_charge_skmem() return false, it will goto > suppress_allocation and uncharge skmem, > while when mem_cgroup_charge_skmem() return true, it will charge skmem > sucessfully. > > The consequence behavior is sk_rmem_schedule may fail while it should sucess. > And then it will call tcp_prune_queue() and tcp collapse may take a long > time. > Is that a good thing or bad? From what I understand with your change if charge fails, sk_rmem_schedule will always fail. However without your change the interrupted task's state might help sk_rmem_schedule to pass. I am all for consistent behavior but I wanted to make sure if that is what you are aiming for. Anyways, from what I remember Facebook is using the cgroup-v2's tcpmem accounting. Johannes or Roman can shed some light if they have observed this issue in production and might have opinion on how to solve it. thanks, Shakeel
