On Wed 02-01-19 17:56:38, Shakeel Butt wrote:
> If a memcg is over high limit, memory reclaim is scheduled to run on
> return-to-userland. However it is assumed that the memcg is the current
> process's memcg. With remote memcg charging for kmem or swapping in a
> page charged to remote memcg, current process can trigger reclaim on
> remote memcg. So, schduling reclaim on return-to-userland for remote
> memcgs will ignore the high reclaim altogether. So, punt the high
> reclaim of remote memcgs to high_work.
Have you seen this happening in real life workloads? And is this
offloading what we really want to do? I mean it is clearly the current
task that has triggered the remote charge so why should we offload that
work to a system? Is there any reason we cannot reclaim on the remote
memcg from the return-to-userland path?
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
> mm/memcontrol.c | 20 ++++++++++++--------
> 1 file changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index e9db1160ccbc..47439c84667a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2302,19 +2302,23 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> * reclaim on returning to userland. We can perform reclaim here
> * if __GFP_RECLAIM but let's always punt for simplicity and so that
> * GFP_KERNEL can consistently be used during reclaim. @memcg is
> - * not recorded as it most likely matches current's and won't
> - * change in the meantime. As high limit is checked again before
> - * reclaim, the cost of mismatch is negligible.
> + * not recorded as the return-to-userland high reclaim will only reclaim
> + * from current's memcg (or its ancestor). For other memcgs we punt them
> + * to work queue.
> */
> do {
> if (page_counter_read(&memcg->memory) > memcg->high) {
> - /* Don't bother a random interrupted task */
> - if (in_interrupt()) {
> + /*
> + * Don't bother a random interrupted task or if the
> + * memcg is not current's memcg's ancestor.
> + */
> + if (in_interrupt() ||
> + !mm_match_cgroup(current->mm, memcg)) {
> schedule_work(&memcg->high_work);
> - break;
> + } else {
> + current->memcg_nr_pages_over_high += batch;
> + set_notify_resume(current);
> }
> - current->memcg_nr_pages_over_high += batch;
> - set_notify_resume(current);
> break;
> }
> } while ((memcg = parent_mem_cgroup(memcg)));
> --
> 2.20.1.415.g653613c723-goog
>
--
Michal Hocko
SUSE Labs
