Re: [PATCH 1/8] memcg: export kmemcg cache id via cgroup fs

[Vladimir Davydov <vdavydov@parallels.com>]

On 02/03/2014 02:05 PM, Glauber Costa wrote:
> On Mon, Feb 3, 2014 at 10:57 AM, Vladimir Davydov
> <vdavydov@parallels.com> wrote:
>> On 02/03/2014 10:21 AM, David Rientjes wrote:
>>> On Sun, 2 Feb 2014, Vladimir Davydov wrote:
>>>
>>>> Per-memcg kmem caches are named as follows:
>>>>
>>>>   <global-cache-name>(<cgroup-kmem-id>:<cgroup-name>)
>>>>
>>>> where <cgroup-kmem-id> is the unique id of the memcg the cache belongs
>>>> to, <cgroup-name> is the relative name of the memcg on the cgroup fs.
>>>> Cache names are exposed to userspace for debugging purposes (e.g. via
>>>> sysfs in case of slub or via dmesg).
>>>>
>>>> Using relative names makes it impossible in general (in case the cgroup
>>>> hierarchy is not flat) to find out which memcg a particular cache
>>>> belongs to, because <cgroup-kmem-id> is not known to the user. Since
>>>> using absolute cgroup names would be an overkill, let's fix this by
>>>> exporting the id of kmem-active memcg via cgroup fs file
>>>> "memory.kmem.id".
>>>>
>>> Hmm, I'm not sure exporting additional information is the best way to do
>>> it only for this purpose.  I do understand the problem in naming
>>> collisions if the hierarchy isn't flat and we typically work around that
>>> by ensuring child memcgs still have a unique memcg.  This isn't only a
>>> problem in slab cache naming, me also avoid printing the entire absolute
>>> names for things like the oom killer.
>> AFAIU, cgroup identifiers dumped on oom (cgroup paths, currently) and
>> memcg slab cache names serve for different purposes. The point is oom is
>> a perfectly normal situation for the kernel, and info dumped to dmesg is
>> for admin to find out the cause of the problem (a greedy user or
>> cgroup). On the other hand, slab cache names are dumped to dmesg only on
>> extraordinary situations - like bugs in slab implementation, or double
>> free, or detected memory leaks - where we usually do not need the name
>> of the memcg that triggered the problem, because the bug is likely to be
>> in the kernel subsys using the cache. Plus, the names are exported to
>> sysfs in case of slub, again for debugging purposes, AFAIK. So IMO the
>> use cases for oom vs slab names are completely different - information
>> vs debugging - and I want to export kmem.id only for the ability of
>> debugging kmemcg and slab subsystems.
>>
> Then maybe it is better to wrap it into some kind of CONFIG_DEBUG wrap.
> We already have other files like that.

May be. However, kmemcg ids are actually exposed to userspace even on
non-debug kernels (for instance, through /sys/kernel/slub), so I guess
it's worth having this always enabled - the overhead of this is
negligible anyway.

Thanks.

>
>>> So it would be nice to have
>>> consensus on how people are supposed to identify memcgs with a hierarchy:
>>> either by exporting information like the id like you do here (but leave
>>> the oom killer still problematic) or by insisting people name their memcgs
>>> with unique names if they care to differentiate them.
>> Anyway, I agree with you that this needs a consensus, because this is a
>> functional change.
>>
>> Thanks.
>
>

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