[PATCH] powerpc/mm: Fix RECLAIM_DISTANCE

[Gavin Shan]

On Tue, Jan 31, 2017 at 04:01:05PM +1100, Gavin Shan wrote:
>On Tue, Jan 31, 2017 at 08:11:39AM +1100, Michael Ellerman wrote:
>>Gavin Shan <gwshan at linux.vnet.ibm.com> writes:
>>
>>I'd like to see some test results from multi-node systems.
>>
>>I'd also like to understand what has changed since we changed
>>RECLAIM_DISTANCE in the first place, ie. why did it used to work and now
>>doesn't?
>>
>
>[Ccing Mel]
>
>Michael, thanks for review. I would like to explain a bit more. The issue
>addressed by the patch is irrelevant to the number of NUMA nodes. There
>is one procfs entry ("/proc/sys/vm/zone_reclaim_mode") which corresponds
>to variable @node_reclaim_mode (their names don't match!). it can have
>belowing bits or any combination of them. Its default value is RECLAIM_OFF (0).
>Note RECLAIM_ZONE was obsoleted and I will send one patch to remove it
>later.
>
>#define RECLAIM_OFF 0
>#define RECLAIM_ZONE (1<<0)     /* Run shrink_inactive_list on the zone */
>#define RECLAIM_WRITE (1<<1)    /* Writeout pages during reclaim */
>#define RECLAIM_UNMAP (1<<2)    /* Unmap pages during reclaim */
>
>When @node_reclaim_mode is set to (RECLAIM_WRITE | RECLAIM_UNMAP), node_reclaim()
>isn't called on the preferred node as the condition is false: zone_allows_reclaim(
>node-A, node-A). As I observed, the distance from node-A to node-A is 10, equal to
>RECLAIM_DISTANCE.
>
>static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>{
>        return node_distance(zone_to_nid(local_zone), zone_to_nid(zone)) <
>                                RECLAIM_DISTANCE;
>}
>
>
>__alloc_pages_nodemask
>   get_page_from_freelist     <- WATERMARK_LOW
>      zone_watermark_fast     <- Assume the allocation is breaking WATERMARK_LOW
>         node_reclaim         <- @node_reclaim_node isn't 0 and
>                                 zone_allows_reclaim(preferred_zone, current_zone) returns true
>         __node_reclaim       <- SWAP, WRITEPAGE and UNMAP setting from @node_reclaim_node
>         shrink_node
>      buffered_rmqueue
>   __alloc_pages_slowpath
>      get_page_from_freelist        <- WATERMARK_MIN
>      __alloc_pages_direct_compact  <- If it's costly allocation (order > 3)
>      wake_all_kswapds
>      get_page_from_freelist        <- NO_WATERMARK, CPU local node is set to preferred one
>      __alloc_pages_direct_reclaim
>         __perform_reclaim
>         try_to_free_pages          <- WRITEPAGE + UNMAP + SWAP
>         do_try_to_free_pages
>         shrink_zones               <- Stop until priority (12) reaches to 0 or reclaimed enough
>         shrink_node
>      __alloc_pages_direct_compact
>
>
>Also, RECLAIM_DISTANCE is set to 30 in include/linux/topology.h. It's used when arch
>doesn't provide one. It's why I set this macro to 30 in this patch. This issue is 
>introduced by commit 5f7a75acdb2 ("mm: page_alloc: do not cache reclaim distances").
>In the patch, it had wrong replacement. So I would correct the wrong replacement
>alternatively. Or both of them. Which way do you think is the best? Maybe Mel also
>has thoughts.
>
>     39  static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>     40  {
>     41 -       return node_isset(local_zone->node, zone->zone_pgdat->reclaim_nodes);
>     42 -}
>     43 -
>     44 -static void __paginginit init_zone_allows_reclaim(int nid)
>     45 -{
>     46 -       int i;
>     47 -
>     48 -       for_each_node_state(i, N_MEMORY)
>     49 -               if (node_distance(nid, i) <= RECLAIM_DISTANCE)
>     50 -                       node_set(i, NODE_DATA(nid)->reclaim_nodes);
>     51 +       return node_distance(zone_to_nid(local_zone), zone_to_nid(zone)) <
>     52 +                               RECLAIM_DISTANCE;
>     53  }
>

Michael, I guess Mel might have missed this thread. I will send one patch to linux-mm
to address "<" and "<=" issue. If the replacement was intentional in Mel's patch, the
patch will introduce more discussion there.

On the other hand, I will repost this patch with upated changelog, meaning to set
RECLAIM_DISTANCE to 30. Its value matches with that in include/linux/topology.h.
The value 10 for RECLAIM_DISTANCE should work if "<" is changed to "<=". However,
30 isn't harmful because there is subsequent check to limit reclaiming on CPU's
local node or those who aren't associated with any CPUs.

Thanks,
Gavin