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

[Vladimir Davydov]

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.

> 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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