Re: [PATCH bpf-next v3 4/8] bpf: Introduce cgroup iter

[Hao Luo]

On Thu, Jul 21, 2022 at 11:16 AM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 7/21/22 10:21 AM, Hao Luo wrote:
> > On Thu, Jul 21, 2022 at 9:15 AM Yonghong Song <yhs@xxxxxx> wrote:
> >>
> >>
> >>
> >> On 7/20/22 5:40 PM, Hao Luo wrote:
> >>> On Mon, Jul 11, 2022 at 8:45 PM Yonghong Song <yhs@xxxxxx> wrote:
> >>>>
> >>>> On 7/11/22 5:42 PM, Hao Luo wrote:
> >>> [...]
> >>>>>>>> +
> >>>>>>>> +static void *cgroup_iter_seq_start(struct seq_file *seq, loff_t *pos)
> >>>>>>>> +{
> >>>>>>>> +    struct cgroup_iter_priv *p = seq->private;
> >>>>>>>> +
> >>>>>>>> +    mutex_lock(&cgroup_mutex);
> >>>>>>>> +
> >>>>>>>> +    /* support only one session */
> >>>>>>>> +    if (*pos > 0)
> >>>>>>>> +        return NULL;
> >>>>>>>
> >>>>>>> This might be okay. But want to check what is
> >>>>>>> the practical upper limit for cgroups in a system
> >>>>>>> and whether we may miss some cgroups. If this
> >>>>>>> happens, it will be a surprise to the user.
> >>>>>>>
> >>>>>
> >>>>> Ok. What's the max number of items supported in a single session?
> >>>>
> >>>> The max number of items (cgroups) in a single session is determined
> >>>> by kernel_buffer_size which equals to 8 * PAGE_SIZE. So it really
> >>>> depends on how much data bpf program intends to send to user space.
> >>>> If each bpf program run intends to send 64B to user space, e.g., for
> >>>> cpu, memory, cpu pressure, mem pressure, io pressure, read rate, write
> >>>> rate, read/write rate. Then each session can support 512 cgroups.
> >>>>
> >>>
> >>> Hi Yonghong,
> >>>
> >>> Sorry about the late reply. It's possible that the number of cgroup
> >>> can be large, 1000+, in our production environment. But that may not
> >>> be common. Would it be good to leave handling large number of cgroups
> >>> as follow up for this patch? If it turns out to be a problem, to
> >>> alleviate it, we could:
> >>>
> >>> 1. tell users to write program to skip a certain uninteresting cgroups.
> >>> 2. support requesting large kernel_buffer_size for bpf_iter, maybe as
> >>> a new bpf_iter flag.
> >>
> >> Currently if we intend to support multiple read() for cgroup_iter,
> >> the following is a very inefficient approach:
> >>
> >> in seq_file private data structure, remember the last cgroup visited
> >> and for the second read() syscall, do the traversal again (but not
> >> calling bpf program) until the last cgroup and proceed from there.
> >> This is inefficient and probably works. But if the last cgroup is
> >> gone from the hierarchy, that the above approach won't work. One
> >> possibility is to remember the last two cgroups. If the last cgroup
> >> is gone, check the 'next' cgroup based on the one before the last
> >> cgroup. If both are gone, we return NULL.
> >>
> >
> > I suspect in reality, just remembering the last cgroup (or two
> > cgroups) may not be sufficient. First, I don't want to hold
> > cgroup_mutex across multiple sessions. I assume it's also not safe to
> > release cgroup_mutex in the middle of walking cgroup hierarchy.
> > Supporting multiple read() can be nasty for cgroup_iter.
>
> Right, holding cgroup_mutex across sessions is not bad idea
> and I didn't recommend it either.
>
> I am aware of remembering last (one or two) cgroups are not
> 100% reliable. All other iters have similar issues w.r.t.
> across multiple sessions. But the idea is to find a
> *reasonable* start for the second and later session.
> For example, for task related iter, the previous session
> last tid can be remember and the next session starts
> with next tid after last tid based on idr. Some old
> processes might be gone, but we got a reasonable
> approximation. But cgroup is different, holding
> the cgroup pointer with an additional reference
> across sessions is not good. but holding cgroup
> id requires to traverse the cgroup hierarchy
> again and this is not efficient. Maybe other people
> has a better idea how to do this.
>

Makes sense.

> >
> >> But in any case, if there are additional cgroups not visited,
> >> in the second read(), we should not return NULL which indicates
> >> done with all cgroups. We may return EOPNOTSUPP to indicate there
> >> are missing cgroups due to not supported.
> >>
> >> Once users see EOPNOTSUPP which indicates there are missing
> >> cgroups, they can do more filtering in bpf program to avoid
> >> large data volume to user space.
> >>
> >
> > Makes sense. Yonghong, one question to confirm, if the first read()
> > overflows, does the user still get partial data?
>
> Yes, the first read() and subsequent read()'s will be okay
> until user space receives all 8KB data where 8KB is the
> kernel buffer size. For example, if user provides 1KB buffer
> size, the first 8 read() syscalls will return proper data
> to user space.
>
> After that, read() should return
> EOPNOTSUPP instead of return 0 where '0' indicates
> no more data.
>

Sounds good. Will do that.

>
> >
> > I'll change the return code to EOPNOTSUPP in v4 of this patchset.
> >
> >> To provide a way to truely visit *all* cgroups,
> >> we can either use bpf_iter link_create->flags
> >> to increase the buffer size as your suggested in the above so
> >> user can try to allocate more kernel buffer size. Or implement
> >> proper second read() traversal which I don't have a good idea
> >> how to do it efficiently.
> >
> > I will try the buffer size increase first. Looks more doable. Do you
> > mind putting this support as a follow-up?
>
> If we cannot finalize the solution to completely support
> arbitrary user output for cgroup_iter, we need to support
> EOPNOTSUPP so user knows it should adjust the bpf program
> to have less data to user space through seq_file. For example,
> they can put data into mmap-ed array map. Please explain
> such a limitation and how to workaround this in commit
> message clearly.
>

Acknowledged. I will put a comment in the code and also explain in the
commit message. Thanks!

> So yes, to support buffer size increase through link_create
> flags or to support a better way to start iteration after 8KB
> user data can be a followup.
>
> >
> >>>
> >>> Hao
> >>>
> >>>>>
> >>> [...]
> >>>>>>> [...]