Re: [PATCH 1/4] mm: vmscan: Correct check for kswapd sleeping in sleeping_prematurely
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2011-06-28 21:49:54
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On Fri, 24 Jun 2011 15:44:54 +0100 Mel Gorman [off-list ref] wrote:
During allocator-intensive workloads, kswapd will be woken frequently
causing free memory to oscillate between the high and min watermark.
This is expected behaviour.
A problem occurs if the highest zone is small. balance_pgdat()
only considers unreclaimable zones when priority is DEF_PRIORITY
but sleeping_prematurely considers all zones. It's possible for this
sequence to occur
1. kswapd wakes up and enters balance_pgdat()
2. At DEF_PRIORITY, marks highest zone unreclaimable
3. At DEF_PRIORITY-1, ignores highest zone setting end_zone
4. At DEF_PRIORITY-1, calls shrink_slab freeing memory from
highest zone, clearing all_unreclaimable. Highest zone
is still unbalanced
5. kswapd returns and calls sleeping_prematurely
6. sleeping_prematurely looks at *all* zones, not just the ones
being considered by balance_pgdat. The highest small zone
has all_unreclaimable cleared but but the zone is not
balanced. all_zones_ok is false so kswapd stays awake
This patch corrects the behaviour of sleeping_prematurely to check
the zones balance_pgdat() checked.But kswapd is making progress: it's reclaiming slab. Eventually that won't work any more and all_unreclaimable will not be cleared and the condition will fix itself up? btw, if (!sleeping_prematurely(...)) sleep(); hurts my brain. My brain would prefer if (kswapd_should_sleep(...)) sleep(); no?
Reported-and-tested-by: Pádraig Brady <redacted>
But what were the before-and-after observations? I don't understand how this can cause a permanent cpuchew by kswapd.
quoted hunk ↗ jump to hunk
--- a/mm/vmscan.c +++ b/mm/vmscan.c@@ -2323,7 +2323,7 @@ static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining, return true; /* Check the watermark levels */ - for (i = 0; i < pgdat->nr_zones; i++) { + for (i = 0; i <= classzone_idx; i++) { struct zone *zone = pgdat->node_zones + i; if (!populated_zone(zone))
The patch looks sensible. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>