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Date: Thu, 25 Nov 2010 16:12:38 +0000
From: Mel Gorman <mel@....ul.ie>
To: Simon Kirby <sim@...tway.ca>
Cc: Shaohua Li <shaohua.li@...el.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: Free memory never fully used, swapping
On Thu, Nov 25, 2010 at 01:03:28AM -0800, Simon Kirby wrote:
> > > <SNIP>
> > >
> > > This x86_64 box has 4 GB of RAM; zones are set up as follows:
> > >
> > > [ 0.000000] Zone PFN ranges:
> > > [ 0.000000] DMA 0x00000001 -> 0x00001000
> > > [ 0.000000] DMA32 0x00001000 -> 0x00100000
> > > [ 0.000000] Normal 0x00100000 -> 0x00130000
> > > ...
> > > [ 0.000000] On node 0 totalpages: 1047279
> > > [ 0.000000] DMA zone: 56 pages used for memmap
> > > [ 0.000000] DMA zone: 0 pages reserved
> > > [ 0.000000] DMA zone: 3943 pages, LIFO batch:0
> > > [ 0.000000] DMA32 zone: 14280 pages used for memmap
> > > [ 0.000000] DMA32 zone: 832392 pages, LIFO batch:31
> > > [ 0.000000] Normal zone: 2688 pages used for memmap
> > > [ 0.000000] Normal zone: 193920 pages, LIFO batch:31
> > >
> > > So, "Normal" is relatively small, and DMA32 contains most of the RAM.
Ok. A consequence of this is that kswapd balancing a node will still try
to balance Normal even if DMA32 has enough memory. This could account
for some of kswapd being mean.
> > > Watermarks from /proc/zoneinfo are:
> > >
> > > Node 0, zone DMA
> > > min 7
> > > low 8
> > > high 10
> > > protection: (0, 3251, 4009, 4009)
> > > Node 0, zone DMA32
> > > min 1640
> > > low 2050
> > > high 2460
> > > protection: (0, 0, 757, 757)
> > > Node 0, zone Normal
> > > min 382
> > > low 477
> > > high 573
> > > protection: (0, 0, 0, 0)
> > >
> > > This box has a couple bnx2 NICs, which do about 60 Mbps each. Jumbo
> > > frames were disabled for now (to try to stop big order allocations), but
> > > this did not stop atomic allocations of order 3 coming in, as found with:
> > >
> > > perf record --event kmem:mm_page_alloc --filter 'order>=3' -a --call-graph -c 1 -a sleep 10
> > > perf report
> > >
> > > __alloc_pages_nodemask
> > > alloc_pages_current
> > > new_slab
> > > __slab_alloc
> > > __kmalloc_node_track_caller
> > > __alloc_skb
> > > __netdev_alloc_skb
> > > bnx2_poll_work
> > >
> > > From my reading of this, it seems like __alloc_skb uses kmalloc(), and
> > > kmalloc uses the kmalloc slab unless (unlikely(size > SLUB_MAX_SIZE)),
> > > where SLUB_MAX_SIZE is 2 * PAGE_SIZE, in which case kmalloc_large is
> > > called which allocates pages directly. This means that reception of
> > > jumbo frames probably actually results in (consistent) smaller order
> > > allocations! Anyway, these GFP_ATOMIC allocations don't seem to be
> > > failing, BUT...
> > >
It's possible to reduce the maximum order that SLUB uses but lets not
resort to that as a workaround just yet. In case it needs to be
elminiated as a source of problems later, the relevant kernel parameter
is slub_max_order=.
> > > Right after kswapd goes to sleep, we're left with DMA32 with 421k or so
> > > free pages, and Normal with 20k or so free pages (about 1.8 GB free).
> > >
> > > Immediately, zone Normal starts being used until it reaches about 468
> > > pages free in order 0, nothing else free. kswapd is not woken here,
> > > but allocations just start coming from zone DMA32 instead.
kswapd is not woken up because we stay in the allocator fastpath once
that much memory hs been freed.
> > > While this
> > > happens, the occasional order=3 allocations coming in via the slab from
> > > __alloc_skb seem to be picking away at the available order=3 chunks.
> > > /proc/buddyinfo shows that there are 10k or so when it starts, so this
> > > succeeds easily.
> > >
> > > After a minute or so, available order-3 start reaching a lower number,
> > > like 20 or so. order-4 then starts dropping as it is split into order-3,
> > > until it reaches 20 or so as well. Then, order-3 hits 0, and kswapd is
> > > woken.
Allocator slowpath.
> > > When this occurs, there are still a few order-5, order-6, etc.,
> > > available.
Watermarks are probably not met though.
> > > I presume the GFP_ATOMIC allocation can still split buddies
> > > here, still making order-3 available without sleeping, because there is
> > > no allocation failure message that I can see.
> > >
Technically it could, but watermark maintenance is important.
> > > Here is a "while true; do sleep 1; grep -v 'DMA ' /proc/buddyinfo; done"
> > > ("DMA" zone is totally untouched, always, so excluded; white space
> > > crushed to avoid wrapping), while it happens:
> > >
> > > Node 0, zone DMA 2 1 1 2 1 1 1 0 1 1 3
> > > Node 0, zone DMA32 25770 29441 14512 10426 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > ...
> > > Node 0, zone DMA32 23343 29405 6062 6478 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 23187 29358 6047 5960 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 23000 29372 6047 5411 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 22714 29391 6076 4225 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 22354 29459 6059 3178 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 22202 29388 6035 2395 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 21971 29411 6036 1032 1901 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 21514 29388 6019 433 1796 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 21334 29387 6019 240 1464 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 21237 29421 6052 216 1336 123 4 0 0 0 0
> > > Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 20968 29378 6020 244 751 123 4 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 20741 29383 6022 134 272 123 4 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 20476 29370 6024 117 48 116 4 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 20343 29369 6020 110 23 10 2 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 21592 30477 4856 22 10 4 2 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 24388 33261 1985 6 10 4 2 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 25358 34080 1068 0 4 4 2 0 0 0 0
> > > Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 75985 68954 5345 87 1 4 2 0 0 0 0
> > > Node 0, zone Normal 18249 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81117 71630 19261 429 3 4 2 0 0 0 0
> > > Node 0, zone Normal 17908 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81226 71299 21038 569 19 4 2 0 0 0 0
> > > Node 0, zone Normal 18559 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81347 71278 21068 640 19 4 2 0 0 0 0
> > > Node 0, zone Normal 17928 21 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81370 71237 21241 1073 29 4 2 0 0 0 0
> > > Node 0, zone Normal 18187 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81401 71237 21314 1139 29 4 2 0 0 0 0
> > > Node 0, zone Normal 16978 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81410 71239 21314 1145 29 4 2 0 0 0 0
> > > Node 0, zone Normal 18156 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81419 71232 21317 1160 30 4 2 0 0 0 0
> > > Node 0, zone Normal 17536 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81347 71144 21443 1160 31 4 2 0 0 0 0
> > > Node 0, zone Normal 18483 7 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81300 71059 21556 1178 38 4 2 0 0 0 0
> > > Node 0, zone Normal 18528 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81315 71042 21577 1180 39 4 2 0 0 0 0
> > > Node 0, zone Normal 18431 2 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81301 71002 21702 1202 39 4 2 0 0 0 0
> > > Node 0, zone Normal 18487 5 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81301 70998 21702 1202 39 4 2 0 0 0 0
> > > Node 0, zone Normal 18311 0 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81296 71025 21711 1208 45 4 2 0 0 0 0
> > > Node 0, zone Normal 17092 5 0 0 0 0 0 0 0 0 0
> > > Node 0, zone DMA32 81299 71023 21716 1226 45 4 2 0 0 0 0
> > > Node 0, zone Normal 18225 12 0 0 0 0 0 0 0 0 0
> > >
> > > Running a perf record on the kswapd wakeup right when it happens shows:
> > > perf record --event vmscan:mm_vmscan_wakeup_kswapd -a --call-graph -c 1 -a sleep 10
> > > perf trace
> > > swapper-0 [002] 1323136.979119: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
> > > swapper-0 [002] 1323136.979131: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
> > > lmtp-20593 [003] 1323136.984066: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
> > > lmtp-20593 [003] 1323136.984079: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
> > > swapper-0 [001] 1323136.985511: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
> > > swapper-0 [001] 1323136.985515: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
> > > lmtp-20593 [003] 1323136.985673: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
> > > lmtp-20593 [003] 1323136.985675: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
> > >
> > > This causes kswapd to throw out a bunch of stuff from Normal and from
> > > DMA32, to try to get zone_watermark_ok() to be happy for order=3.
Yep.
> > > However, we have a heavy read load from all of the email stored on SSDs
> > > on this box, and kswapd ends up fighting to try to keep reclaiming the
> > > allocations (mostly order-0). During the whole day, it never wins -- the
> > > allocations are faster. At night, it wins after a minute or two. The
> > > fighting is happening in all of the lines after it awakes above.
> > >
It's probably fighting to keep *all* zones happy even though it's not strictly
necessary. I suspect it's fighting the most for Normal.
> > > slabs_scanned, kswapd_steal, kswapd_inodesteal (slowly),
> > > kswapd_skip_congestion_wait, and pageoutrun go up in vmstat while kswapd
> > > is running. With the box up for 15 days, you can see it struggling on
> > > pgscan_kswapd_normal (from /proc/vmstat):
> > >
> > > pgfree 3329793080
> > > pgactivate 643476431
> > > pgdeactivate 155182710
> > > pgfault 2649106647
> > > pgmajfault 58157157
> > > pgrefill_dma 0
> > > pgrefill_dma32 19688032
> > > pgrefill_normal 7600864
> > > pgrefill_movable 0
> > > pgsteal_dma 0
> > > pgsteal_dma32 465191578
> > > pgsteal_normal 651178518
> > > pgsteal_movable 0
> > > pgscan_kswapd_dma 0
> > > pgscan_kswapd_dma32 768300403
> > > pgscan_kswapd_normal 34614572907
> > > pgscan_kswapd_movable 0
> > > pgscan_direct_dma 0
> > > pgscan_direct_dma32 2853983
> > > pgscan_direct_normal 885799
> > > pgscan_direct_movable 0
> > > pginodesteal 191895
> > > pgrotated 27290463
> > >
> > > So, here are my questions.
> > >
> > > Why do we care about order > 0 watermarks at all in the Normal zone?
> > > Wouldn't it make a lot more sense to just make the DMA32 zone the only
> > > one we care about for larger-order allocations? Or is this required for
> > > the hugepage stuff?
> > >
It's not required. The logic for kswapd is "balance all zones" and
Normal is one of the zones. Even though you know that DMA32 is just
fine, kswapd doesn't.
> > > The fact that so much stuff is evicted just because order-3 hits 0 is
> > > crazy, especially when larger order pages are still free. It seems like
> > > we're trying to keep large orders free here. Why?
Watermarks. The steady stream of order-3 allocations is telling the
allocator and kswapd that these size pages must be available. It doesn't
know that slub can happily fall back to smaller pages because that
information is lost. Even removing __GFP_WAIT won't help because kswapd
still gets woken up for atomic allocation requests.
> > > Maybe things would be
> > > better if kswapd does not reclaim at all unless the requested order is
> > > empty _and_ all orders above are empty. This would require hugepage
> > > users to use CONFIG_COMPACT, and have _compaction_ occur the way the
> > > watermark checks work now, but people without CONFIG_HUGETLB_PAGE could
> > > just actually use the memory. Would this work?
> > >
> > > There is logic at the end of balance_pgdat() to give up balancing order>0
> > > and just try another loop with order = 0 if sc.nr_reclaimed is <
> > > SWAP_CLUSTER_MAX. However, when this order=0 pass returns, the caller of
> > > balance_pgdat(), kswapd(), gets true from sleeping_prematurely() and just
> > > calls right back to balance_pgdat() again. I think this is why this
> > > logic doesn't seem to work here.
> > >
Ok, this is true. kswapd in balance_pgdat() has given up on the order
but that information is lost when sleeping_prematurely() is called so it
constantly loops. That is a mistake. balance_pgdat() could return the order
so sleeping_prematurely() doesn't do the wrong thing.
> > > Is my assumption about GFP_ATOMIC order=3 working even when order 3 is
> > > empty, but order>3 is not? Regardless, shouldn't kswapd be woken before
> > > order 3 is 0 since it may have nothing above order 3 to split from, thus
> > > actually causing an allocation failure? Does something else do this?
> >
> > even kswapd is woken after order>3 is empty, the issue will occur since
> > the order > 3 pages will be used soon and kswapd still needs to reclaim
> > some pages. So the issue is there is high order page allocation and
> > lumpy reclaim wrongly reclaims some pages. maybe you should use slab
> > instead of slub to avoid high order allocation.
>
> There are actually a few problems here. I think they are worth looking
> at them separately, unless "don't use order 3 allocations" is a valid
> statement, in which case we should fix slub.
>
SLUB can be forced to use smaller orders but I don't think that's the
right fix here.
> The funny thing here is that slub.c's allocate_slab() calls alloc_pages()
> with flags | __GFP_NOWARN | __GFP_NORETRY, and intentionally tries a
> lower order allocation automatically if it fails. This is why there is
> no allocation failure warning when this happens. However, it is too late
> -- kswapd is woken and it ties to bring order 3 up to the watermark.
> If we hacked __alloc_pages_slowpath() to not wake kswapd when
> __GFP_NOWARN is set, we would never see this problem and the slub
> optimization might still mostly work.
Yes, but we'd see more high-order atomic allocation (e.g. jumbo frames)
failures as a result so that fix would cause other regressions.
> Either way, we should "fix" slub
> or "fix" order-3 allocations, so that other people who are using slub
> don't hit the same problem.
>
> kswapd is throwing out many times what is needed for the order 3
> watermark to be met. It seems to be not as bad now, but look at these
> pages being reclaimed (200ms intervals, whitespace-packed buddyinfo
> followed by nr_pages_free calculation and final order-3 watermark test,
> kswapd woken after the second sample):
>
> Zone order:0 1 2 3 4 5 6 7 8 9 A nr_free or3-low-chk
>
> DMA32 20374 35116 975 1 2 5 1 0 0 0 0 94770 257 <= 256
> DMA32 20480 35211 870 1 1 5 1 0 0 0 0 94630 241 <= 256
> (kswapd wakes, gobble gobble)
> DMA32 24387 37009 2910 297 100 5 1 0 0 0 0 114245 4193 <= 256
> DMA32 36169 37787 4676 637 110 5 1 0 0 0 0 137527 7073 <= 256
> DMA32 63443 40620 5716 982 144 5 1 0 0 0 0 177931 10377 <= 256
> DMA32 65866 57006 6462 1180 158 5 1 0 0 0 0 217918 12185 <= 256
> DMA32 67188 66779 9328 1893 208 5 1 0 0 0 0 256754 18689 <= 256
> DMA32 67909 67356 18307 2268 235 5 1 0 0 0 0 297977 22121 <= 256
> DMA32 68333 67419 20786 4192 298 7 1 0 0 0 0 324907 38585 <= 256
> DMA32 69872 68096 21580 5141 326 7 1 0 0 0 0 339016 46625 <= 256
> DMA32 69959 67970 22339 5657 371 10 1 0 0 0 0 346831 51569 <= 256
> DMA32 70017 67946 22363 6078 417 11 1 0 0 0 0 351073 55705 <= 256
> DMA32 70023 67949 22376 6204 439 12 1 0 0 0 0 352529 57097 <= 256
> DMA32 70045 67937 22380 6262 451 12 1 0 0 0 0 353199 57753 <= 256
> DMA32 70062 67939 22378 6298 456 12 1 0 0 0 0 353580 58121 <= 256
> DMA32 70079 67959 22388 6370 458 12 1 0 0 0 0 354285 58729 <= 256
> DMA32 70079 67959 22388 6387 460 12 1 0 0 0 0 354453 58897 <= 256
> DMA32 70076 67954 22387 6393 460 12 1 0 0 0 0 354484 58945 <= 256
> DMA32 70105 67975 22385 6466 468 12 1 0 0 0 0 355259 59657 <= 256
> DMA32 70110 67972 22387 6466 470 12 1 0 0 0 0 355298 59689 <= 256
> DMA32 70152 67989 22393 6476 470 12 1 0 0 0 0 355478 59769 <= 256
> DMA32 70175 67991 22401 6493 471 12 1 0 0 0 0 355689 59921 <= 256
> DMA32 70175 67991 22401 6493 471 12 1 0 0 0 0 355689 59921 <= 256
> DMA32 70175 67991 22401 6493 471 12 1 0 0 0 0 355689 59921 <= 256
> DMA32 70192 67990 22401 6495 471 12 1 0 0 0 0 355720 59937 <= 256
> DMA32 70192 67988 22401 6496 471 12 1 0 0 0 0 355724 59945 <= 256
> DMA32 70099 68061 22467 6602 477 12 1 0 0 0 0 356985 60889 <= 256
> DMA32 70099 68062 22467 6602 477 12 1 0 0 0 0 356987 60889 <= 256
> DMA32 70099 68062 22467 6602 477 12 1 0 0 0 0 356987 60889 <= 256
> DMA32 70099 68062 22467 6603 477 12 1 0 0 0 0 356995 60897 <= 256
> (kswapd sleeps)
>
> Normal zone at the same time (shown separately for clarity):
>
> Normal 452 1 0 0 0 0 0 0 0 0 0 454 -5 <= 238
> Normal 452 1 0 0 0 0 0 0 0 0 0 454 -5 <= 238
> (kswapd wakes)
> Normal 7618 76 0 0 0 0 0 0 0 0 0 7770 145 <= 238
> Normal 8860 73 1 0 0 0 0 0 0 0 0 9010 143 <= 238
> Normal 8929 25 0 0 0 0 0 0 0 0 0 8979 43 <= 238
> Normal 8917 0 0 0 0 0 0 0 0 0 0 8917 -7 <= 238
> Normal 8978 16 0 0 0 0 0 0 0 0 0 9010 25 <= 238
> Normal 9064 4 0 0 0 0 0 0 0 0 0 9072 1 <= 238
> Normal 9068 2 0 0 0 0 0 0 0 0 0 9072 -3 <= 238
> Normal 8992 9 0 0 0 0 0 0 0 0 0 9010 11 <= 238
> Normal 9060 6 0 0 0 0 0 0 0 0 0 9072 5 <= 238
> Normal 9010 0 0 0 0 0 0 0 0 0 0 9010 -7 <= 238
> Normal 8907 5 0 0 0 0 0 0 0 0 0 8917 3 <= 238
> Normal 8576 0 0 0 0 0 0 0 0 0 0 8576 -7 <= 238
> Normal 8018 0 0 0 0 0 0 0 0 0 0 8018 -7 <= 238
> Normal 6778 0 0 0 0 0 0 0 0 0 0 6778 -7 <= 238
> Normal 6189 0 0 0 0 0 0 0 0 0 0 6189 -7 <= 238
> Normal 6220 0 0 0 0 0 0 0 0 0 0 6220 -7 <= 238
> Normal 6096 0 0 0 0 0 0 0 0 0 0 6096 -7 <= 238
> Normal 6251 0 0 0 0 0 0 0 0 0 0 6251 -7 <= 238
> Normal 6127 0 0 0 0 0 0 0 0 0 0 6127 -7 <= 238
> Normal 6218 1 0 0 0 0 0 0 0 0 0 6220 -5 <= 238
> Normal 6034 0 0 0 0 0 0 0 0 0 0 6034 -7 <= 238
> Normal 6065 0 0 0 0 0 0 0 0 0 0 6065 -7 <= 238
> Normal 6189 0 0 0 0 0 0 0 0 0 0 6189 -7 <= 238
> Normal 6189 0 0 0 0 0 0 0 0 0 0 6189 -7 <= 238
> Normal 6096 0 0 0 0 0 0 0 0 0 0 6096 -7 <= 238
> Normal 6127 0 0 0 0 0 0 0 0 0 0 6127 -7 <= 238
> Normal 6158 0 0 0 0 0 0 0 0 0 0 6158 -7 <= 238
> Normal 6127 0 0 0 0 0 0 0 0 0 0 6127 -7 <= 238
> (kswapd sleeps -- maybe too much turkey)
>
> DMA32 get so much reclaimed that the watermark test succeeded long ago.
> Meanwhile, Normal is being reclaimed as well, but because it's fighting
> with allocations, it tries for a while and eventually succeeds (I think),
> but the 200ms samples didn't catch it.
>
So, the key here is kswapd didn't need to balance all zones, any one of
them would have been fine.
> KOSAKI Motohiro, I'm interested in your commit 73ce02e9. This seems
> to be similar to this problem, but your change is not working here.
It's not because sleeping_prematurely() interferes with it.
> We're seeing kswapd run without sleeping, KSWAPD_SKIP_CONGESTION_WAIT
> is increasing (so has_under_min_watermark_zone is true), and pageoutrun
> increasing all the time. This means that balance_pgdat() keeps being
> called, but sleeping_prematurely() is returning true, so kswapd() just
> keeps re-calling balance_pgdat(). If your approach is correct to stop
> kswapd here, the problem seems to be that balance_pgdat's copy of order
> and sc.order is being set to 0, but not pgdat->kswapd_max_order, so
> kswapd never really sleeps. How is this supposed to work?
>
It doesn't.
> Our allocation load here is mostly file pages, some anon pages, and
> relatively little slab and anything else.
>
I think there are at least two fixes required here.
1. sleeping_prematurely() must be aware that balance_pgdat() has dropped
the order.
2. kswapd is trying to balance all zones for higher orders even though
it doesn't really have to.
This patch has potential fixes for both of these problems. I have a split-out
series but I'm posting it as a single patch so see if it allows kswapd to
go to sleep as expected for you and whether it stops hammering the Normal
zone unnecessarily. I tested it locally here (albeit with compaction
enabled) and it did reduce the amount of time kswapd spent awake.
==== CUT HERE ====
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 39c24eb..25fe08d 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -645,6 +645,7 @@ typedef struct pglist_data {
wait_queue_head_t kswapd_wait;
struct task_struct *kswapd;
int kswapd_max_order;
+ enum zone_type high_zoneidx;
} pg_data_t;
#define node_present_pages(nid) (NODE_DATA(nid)->node_present_pages)
@@ -660,7 +661,7 @@ typedef struct pglist_data {
extern struct mutex zonelists_mutex;
void build_all_zonelists(void *data);
-void wakeup_kswapd(struct zone *zone, int order);
+void wakeup_kswapd(struct zone *zone, int order, enum zone_type high_zoneidx);
int zone_watermark_ok(struct zone *z, int order, unsigned long mark,
int classzone_idx, int alloc_flags);
enum memmap_context {
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 07a6544..344b597 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1921,7 +1921,7 @@ void wake_all_kswapd(unsigned int order, struct zonelist *zonelist,
struct zone *zone;
for_each_zone_zonelist(zone, z, zonelist, high_zoneidx)
- wakeup_kswapd(zone, order);
+ wakeup_kswapd(zone, order, high_zoneidx);
}
static inline int
diff --git a/mm/vmscan.c b/mm/vmscan.c
index d31d7ce..00529a0 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2118,15 +2118,17 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *mem_cont,
#endif
/* is kswapd sleeping prematurely? */
-static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
+static bool sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
{
int i;
+ bool all_zones_ok = true;
+ bool any_zone_ok = false;
/* If a direct reclaimer woke kswapd within HZ/10, it's premature */
if (remaining)
return 1;
- /* If after HZ/10, a zone is below the high mark, it's premature */
+ /* Check the watermark levels */
for (i = 0; i < pgdat->nr_zones; i++) {
struct zone *zone = pgdat->node_zones + i;
@@ -2138,10 +2140,20 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
if (!zone_watermark_ok(zone, order, high_wmark_pages(zone),
0, 0))
- return 1;
+ all_zones_ok = false;
+ else
+ any_zone_ok = true;
}
- return 0;
+ /*
+ * For high-order requests, any zone meeting the watermark is enough
+ * to allow kswapd go back to sleep
+ * For order-0, all zones must be balanced
+ */
+ if (order)
+ return !any_zone_ok;
+ else
+ return !all_zones_ok;
}
/*
@@ -2168,6 +2180,7 @@ static int sleeping_prematurely(pg_data_t *pgdat, int order, long remaining)
static unsigned long balance_pgdat(pg_data_t *pgdat, int order)
{
int all_zones_ok;
+ int any_zone_ok;
int priority;
int i;
unsigned long total_scanned;
@@ -2201,6 +2214,7 @@ loop_again:
disable_swap_token();
all_zones_ok = 1;
+ any_zone_ok = 0;
/*
* Scan in the highmem->dma direction for the highest
@@ -2310,10 +2324,12 @@ loop_again:
* spectulatively avoid congestion waits
*/
zone_clear_flag(zone, ZONE_CONGESTED);
+ if (i <= pgdat->high_zoneidx)
+ any_zone_ok = 1;
}
}
- if (all_zones_ok)
+ if (all_zones_ok || (order && any_zone_ok))
break; /* kswapd: all done */
/*
* OK, kswapd is getting into trouble. Take a nap, then take
@@ -2336,7 +2352,7 @@ loop_again:
break;
}
out:
- if (!all_zones_ok) {
+ if (!(all_zones_ok || (order && any_zone_ok))) {
cond_resched();
try_to_freeze();
@@ -2361,7 +2377,13 @@ out:
goto loop_again;
}
- return sc.nr_reclaimed;
+ /*
+ * Return the order we were reclaiming at so sleeping_prematurely()
+ * makes a decision on the order we were last reclaiming at. However,
+ * if another caller entered the allocator slow path while kswapd
+ * was awake, order will remain at the higher level
+ */
+ return order;
}
/*
@@ -2417,6 +2439,7 @@ static int kswapd(void *p)
prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE);
new_order = pgdat->kswapd_max_order;
pgdat->kswapd_max_order = 0;
+ pgdat->high_zoneidx = MAX_ORDER;
if (order < new_order) {
/*
* Don't sleep if someone wants a larger 'order'
@@ -2464,7 +2487,7 @@ static int kswapd(void *p)
*/
if (!ret) {
trace_mm_vmscan_kswapd_wake(pgdat->node_id, order);
- balance_pgdat(pgdat, order);
+ order = balance_pgdat(pgdat, order);
}
}
return 0;
@@ -2473,7 +2496,7 @@ static int kswapd(void *p)
/*
* A zone is low on free memory, so wake its kswapd task to service it.
*/
-void wakeup_kswapd(struct zone *zone, int order)
+void wakeup_kswapd(struct zone *zone, int order, enum zone_type high_zoneidx)
{
pg_data_t *pgdat;
@@ -2483,8 +2506,10 @@ void wakeup_kswapd(struct zone *zone, int order)
pgdat = zone->zone_pgdat;
if (zone_watermark_ok(zone, order, low_wmark_pages(zone), 0, 0))
return;
- if (pgdat->kswapd_max_order < order)
+ if (pgdat->kswapd_max_order < order) {
pgdat->kswapd_max_order = order;
+ pgdat->high_zoneidx = min(pgdat->high_zoneidx, high_zoneidx);
+ }
trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);
if (!cpuset_zone_allowed_hardwall(zone, GFP_KERNEL))
return;
--
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