On Thu, Apr 15, 2021 at 01:08:29PM -0400, Waiman Long wrote: > On 4/15/21 12:50 PM, Johannes Weiner wrote: > > On Tue, Apr 13, 2021 at 09:20:25PM -0400, Waiman Long wrote: > > > Before the new slab memory controller with per object byte charging, > > > charging and vmstat data update happen only when new slab pages are > > > allocated or freed. Now they are done with every kmem_cache_alloc() > > > and kmem_cache_free(). This causes additional overhead for workloads > > > that generate a lot of alloc and free calls. > > > > > > The memcg_stock_pcp is used to cache byte charge for a specific > > > obj_cgroup to reduce that overhead. To further reducing it, this patch > > > makes the vmstat data cached in the memcg_stock_pcp structure as well > > > until it accumulates a page size worth of update or when other cached > > > data change. > > > > > > On a 2-socket Cascade Lake server with instrumentation enabled and this > > > patch applied, it was found that about 17% (946796 out of 5515184) of the > > > time when __mod_obj_stock_state() is called leads to an actual call to > > > mod_objcg_state() after initial boot. When doing parallel kernel build, > > > the figure was about 16% (21894614 out of 139780628). So caching the > > > vmstat data reduces the number of calls to mod_objcg_state() by more > > > than 80%. > > Right, but mod_objcg_state() is itself already percpu-cached. What's > > the benefit of avoiding calls to it with another percpu cache? > > > There are actually 2 set of vmstat data that have to be updated. One is > associated with the memcg and other one is for each lruvec within the > cgroup. Caching it in obj_stock, we replace 2 writes to two colder > cachelines with one write to a hot cacheline. If you look at patch 5, I > break obj_stock into two - one for task context and one for irq context. > Interrupt disable is no longer needed in task context, but that is not > possible when writing to the actual vmstat data arrays.
Ah, thanks for the explanation. Both of these points are worth mentioning in the changelog of this patch.
