Re: [PATCH] mm: avoid bothering interrupted task when charge memcg in softirq

[Shakeel Butt <shakeelb@google.com>]

On Sun, Jul 15, 2018 at 1:02 AM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Sun, Jul 15, 2018 at 2:34 PM, Shakeel Butt <shakeelb@google.com> wrote:
> > On Sat, Jul 14, 2018 at 10:26 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >>
> >> On Sun, Jul 15, 2018 at 12:25 PM, Shakeel Butt <shakeelb@google.com> wrote:
> >> > On Sat, Jul 14, 2018 at 7:10 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >> >>
> >> >> On Sat, Jul 14, 2018 at 11:38 PM, Shakeel Butt <shakeelb@google.com> wrote:
> >> >> > On Sat, Jul 14, 2018 at 1:32 AM Yafang Shao <laoar.shao@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.

> 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?

Please note that I am not against this patch. I just want that the
motivation/reason behind it is very clear.

thanks,
Shakeel