On 4/19/21 2:06 AM, Muchun Song wrote:
On Mon, Apr 19, 2021 at 8:01 AM Waiman Long <longman@xxxxxxxxxx> wrote:There are two issues with the current refill_obj_stock() code. First of all, when nr_bytes reaches over PAGE_SIZE, it calls drain_obj_stock() to atomically flush out remaining bytes to obj_cgroup, clear cached_objcg and do a obj_cgroup_put(). It is likely that the same obj_cgroup will be used again which leads to another call to drain_obj_stock() and obj_cgroup_get() as well as atomically retrieve the available byte from obj_cgroup. That is costly. Instead, we should just uncharge the excess pages, reduce the stock bytes and be done with it. The drain_obj_stock() function should only be called when obj_cgroup changes. Secondly, when charging an object of size not less than a page in obj_cgroup_charge(), it is possible that the remaining bytes to be refilled to the stock will overflow a page and cause refill_obj_stock() to uncharge 1 page. To avoid the additional uncharge in this case, a new overfill flag is added to refill_obj_stock() which will be set when called from obj_cgroup_charge(). Signed-off-by: Waiman Long <longman@xxxxxxxxxx> --- mm/memcontrol.c | 23 +++++++++++++++++------ 1 file changed, 17 insertions(+), 6 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index a6dd18f6d8a8..d13961352eef 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -3357,23 +3357,34 @@ static bool obj_stock_flush_required(struct memcg_stock_pcp *stock, return false; } -static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes) +static void refill_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes, + bool overfill) { unsigned long flags; struct obj_stock *stock = get_obj_stock(&flags); + unsigned int nr_pages = 0; if (stock->cached_objcg != objcg) { /* reset if necessary */ - drain_obj_stock(stock); + if (stock->cached_objcg) + drain_obj_stock(stock); obj_cgroup_get(objcg); stock->cached_objcg = objcg; stock->nr_bytes = atomic_xchg(&objcg->nr_charged_bytes, 0); } stock->nr_bytes += nr_bytes; - if (stock->nr_bytes > PAGE_SIZE) - drain_obj_stock(stock); + if (!overfill && (stock->nr_bytes > PAGE_SIZE)) { + nr_pages = stock->nr_bytes >> PAGE_SHIFT; + stock->nr_bytes &= (PAGE_SIZE - 1); + } put_obj_stock(flags); + + if (nr_pages) { + rcu_read_lock(); + __memcg_kmem_uncharge(obj_cgroup_memcg(objcg), nr_pages); + rcu_read_unlock(); + }It is not safe to call __memcg_kmem_uncharge() under rcu lock and without holding a reference to memcg. More details can refer to the following link. https://lore.kernel.org/linux-mm/20210319163821.20704-2-songmuchun@xxxxxxxxxxxxx/ In the above patchset, we introduce obj_cgroup_uncharge_pages to uncharge some pages from object cgroup. You can use this safe API.
Thanks for the comment. Will update my patch with call to obj_cgroup_uncharge_pages().
Cheers, Longman
