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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
