Re: [PATCH RFC 2/3] mm, vmscan: limit kswapd loop if no progress is made

[hejianet]

On 25/01/2017 12:54 AM, Michal Hocko wrote:
> On Tue 24-01-17 15:49:03, Jia He wrote:
> > Currently there is no hard limitation for kswapd retry times if no progress
> > is made.
>
> Yes, because the main objective of the kswapd is to balance all memory
> zones. So having a hard limit on retries doesn't make any sense.
But do you think even when there is no any process, kswapd still need
to run and take the cpu usage uselessly?

> > Then kswapd will take 100% for a long time.
>
> Where it is spending time?
I've watched kswapd takes 100% cpu for a whole night.

> > In my test, I tried to allocate 4000 hugepages by:
> > echo 4000 > /proc/sys/vm/nr_hugepages
> >
> > Then,kswapd will take 100% cpu for a long time.
> >
> > The numa layout is:
> > available: 7 nodes (0-6)
> > node 0 cpus: 0 1 2 3 4 5 6 7
> > node 0 size: 6611 MB
> > node 0 free: 1103 MB
> > node 1 cpus:
> > node 1 size: 12527 MB
> > node 1 free: 8477 MB
> > node 2 cpus:
> > node 2 size: 15087 MB
> > node 2 free: 11037 MB
> > node 3 cpus:
> > node 3 size: 16111 MB
> > node 3 free: 12060 MB
> > node 4 cpus: 8 9 10 11 12 13 14 15
> > node 4 size: 24815 MB
> > node 4 free: 20704 MB
> > node 5 cpus:
> > node 5 size: 4095 MB
> > node 5 free: 61 MB
> > node 6 cpus:
> > node 6 size: 22750 MB
> > node 6 free: 18716 MB
> >
> > The cause is kswapd will loop for long time even if there is no progress in
> > balance_pgdat.
>
> How does this solve anything? If the kswapd just backs off then the more
> work has to be done in the direct reclaim context.
What if there is still no progress in direct context?

B.R.
Jia
> > Signed-off-by: Jia He <hejianet@xxxxxxxxx>
> > ---
> >  mm/vmscan.c | 25 ++++++++++++++++++++++---
> >  1 file changed, 22 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 532a2a7..7396a0a 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -59,6 +59,7 @@
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/vmscan.h>
> >
> > +#define MAX_KSWAPD_RECLAIM_RETRIES 16
> >  struct scan_control {
> >  	/* How many pages shrink_list() should reclaim */
> >  	unsigned long nr_to_reclaim;
> > @@ -3202,7 +3203,8 @@ static bool kswapd_shrink_node(pg_data_t *pgdat,
> >   * or lower is eligible for reclaim until at least one usable zone is
> >   * balanced.
> >   */
> > -static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
> > +static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx,
> > +						 int *did_some_progress)
> >  {
> >  	int i;
> >  	unsigned long nr_soft_reclaimed;
> > @@ -3322,6 +3324,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
> >  	 * entered the allocator slow path while kswapd was awake, order will
> >  	 * remain at the higher level.
> >  	 */
> > +	*did_some_progress = !!(sc.nr_scanned || sc.nr_reclaimed);
> >  	return sc.order;
> >  }
> >
> > @@ -3417,6 +3420,8 @@ static int kswapd(void *p)
> >  	unsigned int alloc_order, reclaim_order, classzone_idx;
> >  	pg_data_t *pgdat = (pg_data_t*)p;
> >  	struct task_struct *tsk = current;
> > +	int no_progress_loops = 0;
> > +	int did_some_progress = 0;
> >
> >  	struct reclaim_state reclaim_state = {
> >  		.reclaimed_slab = 0,
> > @@ -3480,9 +3485,23 @@ static int kswapd(void *p)
> >  		 */
> >  		trace_mm_vmscan_kswapd_wake(pgdat->node_id, classzone_idx,
> >  						alloc_order);
> > -		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx);
> > -		if (reclaim_order < alloc_order)
> > +		reclaim_order = balance_pgdat(pgdat, alloc_order, classzone_idx,
> > +						&did_some_progress);
> > +
> > +		if (reclaim_order < alloc_order) {
> > +			no_progress_loops = 0;
> >  			goto kswapd_try_sleep;
> > +		}
> > +
> > +		if (did_some_progress)
> > +			no_progress_loops = 0;
> > +		else
> > +			no_progress_loops++;
> > +
> > +		if (no_progress_loops >= MAX_KSWAPD_RECLAIM_RETRIES) {
> > +			no_progress_loops = 0;
> > +			goto kswapd_try_sleep;
> > +		}
> >
> >  		alloc_order = reclaim_order = pgdat->kswapd_order;
> >  		classzone_idx = pgdat->kswapd_classzone_idx;
> > --
> > 2.5.5