On Mon, 2 Jun 2014, David Rientjes wrote:
> mem_cgroup_force_empty_list() can iterate a large number of pages on an lru and
> mem_cgroup_move_parent() doesn't return an errno unless certain criteria, none
> of which indicate that the iteration may be taking too long, is met.
>
> We have encountered the following stack trace many times indicating
> "need_resched set for > 51000020 ns (51 ticks) without schedule", for example:
>
> scheduler_tick()
> <timer irq>
> mem_cgroup_move_account+0x4d/0x1d5
> mem_cgroup_move_parent+0x8d/0x109
> mem_cgroup_reparent_charges+0x149/0x2ba
> mem_cgroup_css_offline+0xeb/0x11b
> cgroup_offline_fn+0x68/0x16b
> process_one_work+0x129/0x350
>
> If this iteration is taking too long, indicated by need_resched(), then
> periodically schedule and continue from where we last left off.
>
> Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> ---
> mm/memcontrol.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4764,6 +4764,7 @@ static void mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
> do {
> struct page_cgroup *pc;
> struct page *page;
> + int ret;
>
> spin_lock_irqsave(&zone->lru_lock, flags);
> if (list_empty(list)) {
> @@ -4781,8 +4782,13 @@ static void mem_cgroup_force_empty_list(struct mem_cgroup *memcg,
>
> pc = lookup_page_cgroup(page);
>
> - if (mem_cgroup_move_parent(page, pc, memcg)) {
> - /* found lock contention or "pc" is obsolete. */
> + ret = mem_cgroup_move_parent(page, pc, memcg);
> + if (ret || need_resched()) {
> + /*
> + * Couldn't grab the page reference, isolate the page,
> + * there was a pc mismatch, or we simply need to
> + * schedule because this is taking too long.
> + */
> busy = page;
> cond_resched();
> } else
Why not just move that cond_resched() down below the if/else?
No need to test need_resched() separately, and this page is not busy.
Hugh
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