mm: Avoid possible deadlock caused by too_many_isolated()
Subsystems (3)
PATCH 1/1 mm: Avoid possible deadlock caused by too_many_isolated()
Neil find that if too_many_isolated() returns true while performing direct reclaim we can end up waiting for other threads to complete their direct reclaim. If those threads are allowed to enter the FS or IO to free memory, but this thread is not, then it is possible that those threads will be waiting on this thread and so we get a circular deadlock.
some task enters direct reclaim with GFP_KERNEL
=> too_many_isolated() false
=> vmscan and run into dirty pages
=> pageout()
=> take some FS lock
=> fs/block code does GFP_NOIO allocation
=> enter direct reclaim again
=> too_many_isolated() true
=> waiting for others to progress, however the other
tasks may be circular waiting for the FS lock..
The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher priority than normal ones, by lowering the throttle threshold for the latter.
Allowing ~1/8 isolated pages in normal is large enough. For example, for a 1GB LRU list, that's ~128MB isolated pages, or 1k blocked tasks (each isolates 32 4KB pages), or 64 blocked tasks per logical CPU (assuming 16 logical CPUs per NUMA node). So it's not likely some CPU goes idle waiting (when it could make progress) because of this limit:
there are much more sleeping reclaim tasks than the number of CPU, so the task may well be blocked by some low level queue/lock anyway.
Now !GFP_IOFS reclaims won't be waiting for GFP_IOFS reclaims to progress. They will be blocked only when there are too many concurrent !GFP_IOFS reclaims, however that's very unlikely because the IO-less direct reclaims is able to progress much more faster, and they won't deadlock each other. The threshold is raised high enough for them, so that there can be sufficient parallel progress of !GFP_IOFS reclaims.
--- mm/vmscan.c | 7 +++++++ 1 file changed, 7 insertions(+)
mm/vmscan.c +7 -0
@@ -1163,6 +1163,13 @@ static int too_many_isolated(struct zone isolated =zone_page_state (zone,NR_ISOLATED_ANON ); } +/* +* GFP_NOIO/GFP_NOFS callers are allowed to isolate more pages, so that +* they won't get blocked by normal ones and form circular deadlock. +*/ +if ((sc->gfp_mask &GFP_IOFS ) ==GFP_IOFS ) + inactive >>=3 ; +return isolated > inactive; }
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PATCH 1/1 vmscan: comment too_many_isolated()
Comment "Why it's doing so" rather than "What it does" as proposed by Andrew Morton.
--- mm/vmscan.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)
mm/vmscan.c +5 -1
@@ -1142,7 +1142,11 @@ int isolate_lru_page(struct page *page) }/* -* Are there way too many processes in the direct reclaim path already? +* A direct reclaimer may isolate SWAP_CLUSTER_MAX pages from the LRU list and +* then get resheduled. When there are massive number of tasks doing page +* allocation, such sleeping direct reclaimers may keep piling up on each CPU, +* the LRU list will go small and be scanned faster than necessary, leading to +* unnecessary swapping, thrashing and OOM. */ static int too_many_isolated (struct zone *zone,int file,struct scan_control *sc)
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Re: [PATCH] vmscan: comment too_many_isolated()
Quoted 6 lines · Wu Fengguang
Comment "Why it's doing so" rather than "What it does" as proposed by Andrew Morton.
Reviewed-by KOSAKI Motohiro<[email protected]>Reviewed-by Minchan Kim<[email protected]>Signed-off-by Wu Fengguang<[email protected]>
Signature · 7 lines
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Quoted 7 lines · Wu Fengguang
Now !GFP_IOFS reclaims won't be waiting for
GFP_IOFSreclaims to progress. They will be blocked only when there are too many concurrent !GFP_IOFS reclaims, however that's very unlikely because the IO-less direct reclaims is able to progress much more faster, and they won't deadlock each other. The threshold is raised high enough for them, so that there can be sufficient parallel progress of !GFP_IOFS reclaims.
Quoted 5 lines
CC Torsten Kaiser<[email protected]>CC Minchan Kim<[email protected]>Tested-by NeilBrown<[email protected]>Acked-by KOSAKI Motohiro<[email protected]>Signed-off-by Wu Fengguang<[email protected]>
Signature · 7 lines
All rights reversed -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
Quoted 37 lines · Wu Fengguang
Neil find that if
too_many_isolated()returns true while performing direct reclaim we can end up waiting for other threads to complete their direct reclaim. If those threads are allowed to enter the FS or IO to free memory, but this thread is not, then it is possible that those threads will be waiting on this thread and so we get a circular deadlock.some task enters direct reclaim with
GFP_KERNEL
=>too_many_isolated()false
=> vmscan and run into dirty pages
=>pageout()
=> take some FS lock
=> fs/block code doesGFP_NOIOallocation
=> enter direct reclaim again
=>too_many_isolated()true
=> waiting for others to progress, however the other tasks may be circular waiting for the FS lock..The fix is to let !__GFP_IO and !__GFP_FS direct reclaims enjoy higher priority than normal ones, by lowering the throttle threshold for the latter.
Allowing ~1/8 isolated pages in normal is large enough. For example, for a 1GB LRU list, that's ~128MB isolated pages, or 1k blocked tasks (each isolates 32 4KB pages), or 64 blocked tasks per logical CPU (assuming 16 logical CPUs per NUMA node). So it's not likely some CPU goes idle waiting (when it could make progress) because of this limit:
there are much more sleeping reclaim tasks than the number of CPU, so the task may well be blocked by some low level queue/lock anyway.Now !GFP_IOFS reclaims won't be waiting for
GFP_IOFSreclaims to progress. They will be blocked only when there are too many concurrent !GFP_IOFS reclaims, however that's very unlikely because the IO-less direct reclaims is able to progress much more faster, and they won't deadlock each other. The threshold is raised high enough for them, so that there can be sufficient parallel progress of !GFP_IOFS reclaims.
Quoted 5 lines
CC Torsten Kaiser <[email protected]>CC Minchan Kim <[email protected]>Tested-by NeilBrown <[email protected]>Acked-by KOSAKI Motohiro <[email protected]>Signed-off-by Wu Fengguang <[email protected]>
Signature · 8 lines
Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"[email protected]"> [email protected] </a>