Re: [PATCH 2/2] loop: charge i/o per cgroup

[Johannes Weiner]

Hello Dan,

On Wed, Feb 05, 2020 at 02:33:48PM -0800, Dan Schatzberg wrote:
> @@ -1925,14 +1990,13 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	}
>  
>  	/* always use the first bio's css */
> +	cmd->blk_css = NULL;
>  #ifdef CONFIG_BLK_CGROUP
> -	if (cmd->use_aio && rq->bio && rq->bio->bi_blkg) {
> -		cmd->css = &bio_blkcg(rq->bio)->css;
> -		css_get(cmd->css);
> -	} else
> +	if (rq->bio && rq->bio->bi_blkg)
> +		cmd->blk_css = &bio_blkcg(rq->bio)->css;
>  #endif
> -		cmd->css = NULL;
> -	kthread_queue_work(&lo->worker, &cmd->work);
> +
> +	loop_queue_work(lo, cmd);
>  
>  	return BLK_STS_OK;
>  }
> @@ -1942,6 +2006,9 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
>  	struct request *rq = blk_mq_rq_from_pdu(cmd);
>  	const bool write = op_is_write(req_op(rq));
>  	struct loop_device *lo = rq->q->queuedata;
> +#ifdef CONFIG_MEMCG
> +	struct cgroup_subsys_state *mem_css;
> +#endif
>  	int ret = 0;
>  
>  	if (write && (lo->lo_flags & LO_FLAGS_READ_ONLY)) {
> @@ -1949,8 +2016,24 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
>  		goto failed;
>  	}
>  
> +	if (cmd->blk_css) {
> +#ifdef CONFIG_MEMCG
> +		mem_css = cgroup_get_e_css(cmd->blk_css->cgroup,
> +					&memory_cgrp_subsys);
> +		memalloc_use_memcg(mem_cgroup_from_css(mem_css));
> +#endif
> +		kthread_associate_blkcg(cmd->blk_css);
> +	}
> +
>  	ret = do_req_filebacked(lo, rq);
> - failed:
> +
> +	if (cmd->blk_css) {
> +		kthread_associate_blkcg(NULL);
> +#ifdef CONFIG_MEMCG
> +		memalloc_unuse_memcg();
> +#endif

cgroup_get_e_css() acquires a reference, it looks like you're missing
a css_put() here.

I also wonder why you look up blk_css and mem_css in separate
places. Since you already renamed cmd->css to cmd->blk_css, can you
also add cmd->mem_css and pair up their lookup and refcounting?

This should make loop_handle_cmd() a bit more straight-forward:

	if (cmd->blk_css)
		kthread_associate_blkcg(cmd->blk_css);
	if (cmd->mem_css)
		memalloc_use_memcg(mem_cgroup_from_css(mem_css));

	ret = do_req_filebacked(lo, rq);

	memalloc_unuse_memcg();
	kthread_associate_blkcg(NULL);

All these functions have dummy implementations for !CONFIG_BLK_CGROUP,
!CONFIG_MEMCG etc., so it shouldn't require any additional ifdefs.