Messages in this thread | | | Date | Tue, 1 Sep 2015 12:25:20 +0300 | From | Vladimir Davydov <> | Subject | Re: [PATCH 0/2] Fix memcg/memory.high in case kmem accounting is enabled |
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On Mon, Aug 31, 2015 at 03:22:22PM -0500, Christoph Lameter wrote: > On Mon, 31 Aug 2015, Vladimir Davydov wrote: > > > I totally agree that we should strive to make a kmem user feel roughly > > the same in memcg as if it were running on a host with equal amount of > > RAM. There are two ways to achieve that: > > > > 1. Make the API functions, i.e. kmalloc and friends, behave inside > > memcg roughly the same way as they do in the root cgroup. > > 2. Make the internal memcg functions, i.e. try_charge and friends, > > behave roughly the same way as alloc_pages. > > > > I find way 1 more flexible, because we don't have to blindly follow > > heuristics used on global memory reclaim and therefore have more > > opportunities to achieve the same goal. > > The heuristics need to integrate well if its in a cgroup or not. In > general make use of cgroups as transparent as possible to the rest of the > code.
Half of kmem accounting implementation resides in SLAB/SLUB. We can't just make use of cgroups there transparent. For the rest of the code using kmalloc, cgroups are transparent.
Indeed, we can make memcg_charge_slab behave exactly like alloc_pages, we can even put it to alloc_pages (where it used to be), but why if the only user of memcg_charge_slab is SLAB/SLUB core?
I think we'd have more space to manoeuvre if we just taught SLAB/SLUB to use memcg_charge_slab wisely (as it used to until recently), because memcg charge/reclaim is quite different from global alloc/reclaim:
- it isn't aware of NUMA nodes, so trying to charge w/o __GFP_WAIT while inspecting nodes, like in case of SLAB, is meaningless
- it isn't aware of high order page allocations, so trying to charge w/o __GFP_WAIT while trying optimistically to get a high order page, like in case of SLUB, is meaningless too
- it can always let a high prio allocation go unaccounted, so IMO there is no point in introducing emergency reserves (__GFP_MEMALLOC handling)
- it can always charge a GFP_NOWAIT allocation even if it exceeds the limit, issuing direct reclaim when a GFP_KERNEL allocation comes or from a task work, because there is no risk of depleting memory reserves; so it isn't obvious to me whether we really need an aync thread handling memcg reclaim like kswapd
Thanks, Vladimir
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