Re: [PATCH 0/3] OOM detection rework v4

[David Rientjes]

On Wed, 20 Jan 2016, Michal Hocko wrote:

> > That trigger was introduced by commit 97a16fc82a7c5b0c ("mm, page_alloc: only
> > enforce watermarks for order-0 allocations"), and "mm, oom: rework oom detection"
> > patch hits the trigger.
> [....]
> > [  154.829582] zone=DMA32 reclaimable=308907 available=312734 no_progress_loops=0 did_some_progress=50
> > [  154.831562] zone=DMA reclaimable=2 available=1728 no_progress_loops=0 did_some_progress=50
> > [  154.838499] fork invoked oom-killer: order=2, oom_score_adj=0, gfp_mask=0x27000c0(GFP_KERNEL|GFP_NOTRACK|0x100000)
> > [  154.841167] fork cpuset=/ mems_allowed=0
> [...]
> > [  154.917857] Node 0 DMA32 free:17996kB min:5172kB low:6464kB high:7756kB ....
> [...]
> > [  154.931918] Node 0 DMA: 107*4kB (UME) 72*8kB (ME) 47*16kB (UME) 19*32kB (UME) 9*64kB (ME) 1*128kB (M) 3*256kB (M) 2*512kB (E) 2*1024kB (UM) 0*2048kB 0*4096kB = 6908kB
> > [  154.937453] Node 0 DMA32: 1113*4kB (UME) 1400*8kB (UME) 116*16kB (UM) 15*32kB (UM) 1*64kB (M) 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 18052kB
> 
> It is really strange that __zone_watermark_ok claimed DMA32 unusable
> here. With the target of 312734 which should easilly pass the wmark
> check for the particular order and there are 116*16kB 15*32kB 1*64kB
> blocks "usable" for our request because GFP_KERNEL can use both
> Unmovable and Movable blocks. So it makes sense to wait for more order-0
> allocations to pass the basic (NR_FREE_MEMORY) watermark and continue
> with this particular allocation request.
> 
> The nr_reserved_highatomic might be too high to matter but then you see
> [1] the reserce being 0. So this doesn't make much sense to me. I will
> dig into it some more.
> 
> [1] http://lkml.kernel.org/r/201601161007.DDG56185.QOHMOFOLtSFJVF@xxxxxxxxxxxxxxxxxxx

There's another issue in the use of zone_reclaimable_pages().  I think 
should_reclaim_retry() using zone_page_state_snapshot() is approrpriate, 
as I indicated before, but notice that zone_reclaimable_pages() only uses 
zone_page_state().  It means that the heuristic is based on some 
up-to-date members and some stale members.  If we are relying on 
NR_ISOLATED_* to be accurate, for example, in zone_reclaimable_pages(), 
then it may take up to 1s for that to actually occur and may quickly 
exhaust the retry counter in should_reclaim_retry() before that happens.

This is the same issue that Joonsoo reported with the use of 
zone_page_state(NR_ISOLATED_*) in the too_many_isolated() loops of reclaim 
and compaction.