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

[hejianet <hejianet@gmail.com>]

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@gmail.com>
>> ---
>>  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
>>
>

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