On Mon, Feb 3, 2014 at 10:57 AM, Vladimir Davydov <vdavy...@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. >> 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. -- E Mare, Libertas -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
