On Fri, Mar 20, 2026 at 04:42:36PM -0400, Waiman Long wrote: > For a system with 4k page size, each percpu memcg_stock can hide up > to 256 kbytes of memory with the current MEMCG_CHARGE_BATCH value of > 64. For another system with 64k page size, that becomes 4 Mbytes. This > hidden charges will affect the accurary of the memory.current value. > > This MEMCG_CHARGE_BATCH value also controls how often should the > memcg vmstat values need flushing. As a result, the values reported > in memory.stat cgroup control files are less indicative of the actual > memory consumption of a particular memory cgroup when the page size > increases from 4k. > > This problem can be illustrated by running the test_memcontrol > selftest. Running a 4k page size kernel on a 128-core arm64 system, > the test_memcg_current_peak test which allocates a 50M anonymous memory > passed. With a 64k page size kernel on the same system, however, the > same test failed because the "anon" attribute of memory.stat file might > report a size of 0 depending on the number of CPUs the system has. > > To solve this inaccurate memory stats problem, we need to scale down > the amount of memory that can be hidden by reducing MEMCG_CHARGE_BATCH > when the page size increases. The same user application will likely > consume more memory on systems with larger page size and it is also > less efficient if we scale down MEMCG_CHARGE_BATCH by too much. So I > believe a good compromise is to scale down MEMCG_CHARGE_BATCH by 2 for > 16k page size and by 4 with 64k page size. > > With that change, the test_memcg_current_peak test passed again with > the modified 64k page size kernel. > > Signed-off-by: Waiman Long <[email protected]> > --- > include/linux/memcontrol.h | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index 70b685a85bf4..748cfd75d998 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -328,8 +328,14 @@ struct mem_cgroup { > * size of first charge trial. > * TODO: maybe necessary to use big numbers in big irons or dynamic based of > the > * workload. > + * > + * There are 3 common base page sizes - 4k, 16k & 64k. In order to limit the > + * amount of memory that can be hidden in each percpu memcg_stock for a given > + * memcg, we scale down MEMCG_CHARGE_BATCH by 2 for 16k and 4 for 64k. > */ > -#define MEMCG_CHARGE_BATCH 64U > +#define MEMCG_CHARGE_BATCH_BASE 64U > +#define MEMCG_CHARGE_BATCH_SHIFT ((PAGE_SHIFT <= 16) ? (PAGE_SHIFT - 12)/2 : > 2) > +#define MEMCG_CHARGE_BATCH (MEMCG_CHARGE_BATCH_BASE >> > MEMCG_CHARGE_BATCH_SHIFT) > > extern struct mem_cgroup *root_mem_cgroup;
Reviewed-by: Li Wang <[email protected]> -- Regards, Li Wang
