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

[Michal Hocko]

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@xxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxx>

> ---
>  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
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