Re: [patch 06/12] mm: memcontrol: simplify move precharge function

Michal Hocko
2014-06-17T14:59:30Z

On Mon 16-06-14 15:54:26, Johannes Weiner wrote:
> The move precharge function does some baroque things: it tries raw
> res_counter charging of the entire amount first, and then falls back
> to a loop of one-by-one charges, with checks for pending signals and
> cond_resched() batching.
>
> Just use mem_cgroup_try_charge() without __GFP_WAIT for the first bulk
> charge attempt. In the one-by-one loop, remove the signal check (this
> is already checked in try_charge), and simply call cond_resched()
> after every charge - it's not that expensive.

Agreed. There shouldn't be any calls to res_counters for {un}charging
outside of mem_cgroup_try_charge and kmem variant.

> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> mm/memcontrol.c | 51 +++++++++++++++++----------------------------------
> 1 file changed, 17 insertions(+), 34 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9c646b9b56f4..3d9df94896a7 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -6372,55 +6372,38 @@ static void mem_cgroup_css_free(struct cgroup_subsys_state *css)
>
> #ifdef CONFIG_MMU
> /* Handlers for move charge at task migration. */
> -#define PRECHARGE_COUNT_AT_ONCE 256
> static int mem_cgroup_do_precharge(unsigned long count)
> {
> - int ret = 0;
> - int batch_count = PRECHARGE_COUNT_AT_ONCE;
> - struct mem_cgroup *memcg = mc.to;
> + int ret;
>
> - if (mem_cgroup_is_root(memcg)) {
> + if (mem_cgroup_is_root(mc.to)) {
> mc.precharge += count;
> /* we don't need css_get for root */
> return ret;
> }
> - /* try to charge at once */
> - if (count > 1) {
> - struct res_counter *dummy;
> - /*
> - * "memcg" cannot be under rmdir() because we've already checked
> - * by cgroup_lock_live_cgroup() that it is not removed and we
> - * are still under the same cgroup_mutex. So we can postpone
> - * css_get().
> - */
> - if (res_counter_charge(&memcg->res, PAGE_SIZE * count, &dummy))
> - goto one_by_one;
> - if (do_swap_account && res_counter_charge(&memcg->memsw,
> - PAGE_SIZE * count, &dummy)) {
> - res_counter_uncharge(&memcg->res, PAGE_SIZE * count);
> - goto one_by_one;
> - }
> +
> + /* Try a single bulk charge without reclaim first */
> + ret = mem_cgroup_try_charge(mc.to, GFP_KERNEL & ~__GFP_WAIT,
> + count, false);
> + if (!ret) {
> mc.precharge += count;
> return ret;
> }
> -one_by_one:
> - /* fall back to one by one charge */
> +
> + /* Try charges one by one with reclaim */
> while (count--) {
> - if (signal_pending(current)) {
> - ret = -EINTR;
> - break;
> - }
> - if (!batch_count--) {
> - batch_count = PRECHARGE_COUNT_AT_ONCE;
> - cond_resched();
> - }
> - ret = mem_cgroup_try_charge(memcg, GFP_KERNEL, 1, false);
> + ret = mem_cgroup_try_charge(mc.to, GFP_KERNEL, 1, false);
> + /*
> + * In case of failure, any residual charges against
> + * mc.to will be dropped by mem_cgroup_clear_mc()
> + * later on.
> + */
> if (ret)
> - /* mem_cgroup_clear_mc() will do uncharge later */
> return ret;
> mc.precharge++;
> + cond_resched();
> }
> - return ret;
> + return 0;
> }
>
> /**
> --
> 2.0.0
>

--
Michal Hocko
SUSE Labs