[RFC] memory controller : backgorund reclaim and avoid excessive locking [2/5] background reclaim.

[Balbir Singh]

KAMEZAWA Hiroyuki wrote:
> A patch for background reclaim based on high-low watermak in res_counter.
> The daemon is called as "memcontd", here.
> 
> Implements following:
>  * If res->usage is higher than res->hwmark, start memcontd.
>  * memcontd calls try_to_free_pages.
>  * memcontd stops if res->usage is lower than res->lwmark.
> 

Can we call the dameon memcgroupd?

> Maybe we can add more tunings but no extra params now.
> 
> ChangeLog:
>  - start "memcontd" at first change in hwmark.
>    (In old verion, it started at cgroup creation.)
>  - changed "relax" logic in memcontd daemon.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com>
> 
>  mm/memcontrol.c |  112 ++++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  1 files changed, 109 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6.24-mm1/mm/memcontrol.c
> ===================================================================
> --- linux-2.6.24-mm1.orig/mm/memcontrol.c
> +++ linux-2.6.24-mm1/mm/memcontrol.c
> @@ -30,6 +30,8 @@
>  #include <linux/spinlock.h>
>  #include <linux/fs.h>
>  #include <linux/seq_file.h>
> +#include <linux/kthread.h>
> +#include <linux/freezer.h>
> 
>  #include <asm/uaccess.h>
> 
> @@ -136,6 +138,13 @@ struct mem_cgroup {
>  	 * statistics.
>  	 */
>  	struct mem_cgroup_stat stat;
> +	/*
> +	 * background reclaim.
> +	 */
> +	struct {
> +		wait_queue_head_t	waitq;
> +		struct task_struct	*kthread;
> +	} daemon;
>  };
> 
>  /*
> @@ -504,6 +513,14 @@ long mem_cgroup_calc_reclaim_inactive(st
>  	return (nr_inactive >> priority);
>  }
> 
> +static inline void mem_cgroup_schedule_daemon(struct mem_cgroup *mem)
> +{
> +	if (likely(mem->daemon.kthread) && /* can be NULL at boot */
> +	    waitqueue_active(&mem->daemon.waitq))
> +		wake_up_interruptible(&mem->daemon.waitq);
> +}
> +
> +
>  unsigned long mem_cgroup_isolate_pages(unsigned long nr_to_scan,
>  					struct list_head *dst,
>  					unsigned long *scanned, int order,
> @@ -658,6 +675,9 @@ retry:
>  		congestion_wait(WRITE, HZ/10);
>  	}
> 
> +	if (res_counter_above_hwmark(&mem->res))
> +		mem_cgroup_schedule_daemon(mem);
> +

I don't understand this part, some comments might be good here. Why do we wake
up the dameon in mem_cgroup_isolate_pages? Why not do so under memory pressure
like the soft limit patches does? I suspect that these patches and soft limit
patches might have some overlap.

>  	atomic_set(&pc->ref_cnt, 1);
>  	pc->mem_cgroup = mem;
>  	pc->page = page;
> @@ -762,6 +782,50 @@ void mem_cgroup_uncharge_page(struct pag
>  }
> 
>  /*
> + * background page reclaim routine for cgroup.
> + */
> +static int mem_cgroup_reclaim_daemon(void *data)
> +{
> +	DEFINE_WAIT(wait);
> +	struct mem_cgroup *mem = data;
> +
> +	css_get(&mem->css);
> +	current->flags |= PF_SWAPWRITE;
> +	set_freezable();
> +
> +	while (!kthread_should_stop()) {
> +		prepare_to_wait(&mem->daemon.waitq, &wait, TASK_INTERRUPTIBLE);
> +		if (res_counter_below_lwmark(&mem->res)) {
> +			if (!kthread_should_stop()) {
> +				schedule();
> +				try_to_freeze();
> +			}
> +			finish_wait(&mem->daemon.waitq, &wait);
> +			continue;
> +		}
> +		finish_wait(&mem->daemon.waitq, &wait);
> +		try_to_free_mem_cgroup_pages(mem, GFP_HIGHUSER_MOVABLE);
> +		/* Am I in hurry ? */
> +		if (!res_counter_above_hwmark(&mem->res)) {
> +			/*
> +			 * Extra relaxing..memory reclaim is hevay work.
> +			 * we don't know there is I/O congestion or not.
> +			 * So use just relax rather than congesiton_wait().
> +			 * HZ/10 is widely used value under /mm.
> +			 */
> +			schedule_timeout(HZ/10);

Why not just yeild() here and let another iteration push us back to our low
water mark?

> +		} else {
> +			/* Avoid occupation */
> +			yield();
> +		}
> +	}
> +
> +	css_put(&mem->css);
> +	return 0;
> +}
> +
> +
> +/*
>   * Returns non-zero if a page (under migration) has valid page_cgroup member.
>   * Refcnt of page_cgroup is incremented.
>   */
> @@ -931,15 +995,40 @@ static ssize_t mem_cgroup_read(struct cg
>  				NULL);
>  }
> 
> +static DEFINE_MUTEX(modify_param_mutex);
>  static ssize_t mem_cgroup_write(struct cgroup *cont, struct cftype *cft,
>  				struct file *file, const char __user *userbuf,
>  				size_t nbytes, loff_t *ppos)
>  {
> -	return res_counter_write(&mem_cgroup_from_cont(cont)->res,
> -				cft->private, userbuf, nbytes, ppos,
> +	int ret;
> +	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
> +
> +	mutex_lock(&modify_param_mutex);
> +	/* Attach new background reclaim daemon.
> +	   This must be done before change values (for easy error handling */
> +
> +	if (cft->private == RES_HWMARK &&
> +	    !mem->daemon.kthread) {
> +		struct task_struct *thr;
> +		thr = kthread_run(mem_cgroup_reclaim_daemon, mem, "memcontd");
> +		if (IS_ERR(thr)) {
> +			ret = PTR_ERR(thr);
> +			goto out;
> +		}
> +		mem->daemon.kthread = thr;
> +	}
> +	ret =  res_counter_write(&mem->res, cft->private, userbuf, nbytes, ppos,
>  				mem_cgroup_write_strategy);
> +
> +	/* Even on error, don't stop reclaim daemon here. not so problematic. */
> +
> +out:
> +	mutex_unlock(&modify_param_mutex);
> +	return ret;
>  }
> 

Could we document the changes to this function. It looks like writing a value to
the watermarks can cause the dameon to be scheduled.


> +
> +
>  static ssize_t mem_force_empty_write(struct cgroup *cont,
>  				struct cftype *cft, struct file *file,
>  				const char __user *userbuf,
> @@ -1032,6 +1121,20 @@ static struct cftype mem_cgroup_files[] 
>  		.write = mem_cgroup_write,
>  		.read = mem_cgroup_read,
>  	},
> +
> +	{
> +		.name = "lwmark_in_bytes",
> +		.private = RES_LWMARK,
> +		.write = mem_cgroup_write,
> +		.read = mem_cgroup_read,
> +	},
> +	{
> +		.name = "hwmark_in_bytes",
> +		.private = RES_HWMARK,
> +		.write = mem_cgroup_write,
> +		.read = mem_cgroup_read,
> +	},
> +
>  	{
>  		.name = "failcnt",
>  		.private = RES_FAILCNT,
> @@ -1110,7 +1213,8 @@ mem_cgroup_create(struct cgroup_subsys *
>  	for_each_node_state(node, N_POSSIBLE)
>  		if (alloc_mem_cgroup_per_zone_info(mem, node))
>  			goto free_out;
> -
> +	init_waitqueue_head(&mem->daemon.waitq);
> +	mem->daemon.kthread = NULL;
>  	return &mem->css;
>  free_out:
>  	for_each_node_state(node, N_POSSIBLE)
> @@ -1125,6 +1229,8 @@ static void mem_cgroup_pre_destroy(struc
>  {
>  	struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
>  	mem_cgroup_force_empty(mem);
> +	if (mem->daemon.kthread)
> +		kthread_stop(mem->daemon.kthread);
>  }
> 
>  static void mem_cgroup_destroy(struct cgroup_subsys *ss,
> 


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL