Re: [PATCH 1/2] mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings

From: Mel Gorman
Date: Tue Sep 25 2018 - 08:20:17 EST

On Tue, Sep 25, 2018 at 02:03:25PM +0200, Michal Hocko wrote:
> From: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> THP allocation might be really disruptive when allocated on NUMA system
> with the local node full or hard to reclaim. Stefan has posted an
> allocation stall report on 4.12 based SLES kernel which suggests the
> same issue:
> [245513.362669] kvm: page allocation stalls for 194572ms, order:9, mode:0x4740ca(__GFP_HIGHMEM|__GFP_IO|__GFP_FS|__GFP_COMP|__GFP_NOMEMALLOC|__GFP_HARDWALL|__GFP_THISNODE|__GFP_MOVABLE|__GFP_DIRECT_RECLAIM), nodemask=(null)
> [245513.363983] kvm cpuset=/ mems_allowed=0-1
> [245513.364604] CPU: 10 PID: 84752 Comm: kvm Tainted: G W 4.12.0+98-ph <a href="/view.php?id=1" title="[geschlossen] Integration Ramdisk" class="resolved">0000001</a> SLE15 (unreleased)
> [245513.365258] Hardware name: Supermicro SYS-1029P-WTRT/X11DDW-NT, BIOS 2.0 12/05/2017
> [245513.365905] Call Trace:
> [245513.366535] dump_stack+0x5c/0x84
> [245513.367148] warn_alloc+0xe0/0x180
> [245513.367769] __alloc_pages_slowpath+0x820/0xc90
> [245513.368406] ? __slab_free+0xa9/0x2f0
> [245513.369048] ? __slab_free+0xa9/0x2f0
> [245513.369671] __alloc_pages_nodemask+0x1cc/0x210
> [245513.370300] alloc_pages_vma+0x1e5/0x280
> [245513.370921] do_huge_pmd_wp_page+0x83f/0xf00
> [245513.371554] ? set_huge_zero_page.isra.52.part.53+0x9b/0xb0
> [245513.372184] ? do_huge_pmd_anonymous_page+0x631/0x6d0
> [245513.372812] __handle_mm_fault+0x93d/0x1060
> [245513.373439] handle_mm_fault+0xc6/0x1b0
> [245513.374042] __do_page_fault+0x230/0x430
> [245513.374679] ? get_vtime_delta+0x13/0xb0
> [245513.375411] do_page_fault+0x2a/0x70
> [245513.376145] ? page_fault+0x65/0x80
> [245513.376882] page_fault+0x7b/0x80
> [...]
> [245513.382056] Mem-Info:
> [245513.382634] active_anon:126315487 inactive_anon:1612476 isolated_anon:5
> active_file:60183 inactive_file:245285 isolated_file:0
> unevictable:15657 dirty:286 writeback:1 unstable:0
> slab_reclaimable:75543 slab_unreclaimable:2509111
> mapped:81814 shmem:31764 pagetables:370616 bounce:0
> free:32294031 free_pcp:6233 free_cma:0
> [245513.386615] Node 0 active_anon:254680388kB inactive_anon:1112760kB active_file:240648kB inactive_file:981168kB unevictable:13368kB isolated(anon):0kB isolated(file):0kB mapped:280240kB dirty:1144kB writeback:0kB shmem:95832kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 81225728kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
> [245513.388650] Node 1 active_anon:250583072kB inactive_anon:5337144kB active_file:84kB inactive_file:0kB unevictable:49260kB isolated(anon):20kB isolated(file):0kB mapped:47016kB dirty:0kB writeback:4kB shmem:31224kB shmem_thp: 0kB shmem_pmdmapped: 0kB anon_thp: 31897600kB writeback_tmp:0kB unstable:0kB all_unreclaimable? no
> The defrag mode is "madvise" and from the above report it is clear that
> the THP has been allocated for MADV_HUGEPAGA vma.
> Andrea has identified that the main source of the problem is
> __GFP_THISNODE usage:
> : The problem is that direct compaction combined with the NUMA
> : __GFP_THISNODE logic in mempolicy.c is telling reclaim to swap very
> : hard the local node, instead of failing the allocation if there's no
> : THP available in the local node.
> :
> : Such logic was ok until __GFP_THISNODE was added to the THP allocation
> : path even with MPOL_DEFAULT.
> :
> : The idea behind the __GFP_THISNODE addition, is that it is better to
> : provide local memory in PAGE_SIZE units than to use remote NUMA THP
> : backed memory. That largely depends on the remote latency though, on
> : threadrippers for example the overhead is relatively low in my
> : experience.
> :
> : The combination of __GFP_THISNODE and __GFP_DIRECT_RECLAIM results in
> : extremely slow qemu startup with vfio, if the VM is larger than the
> : size of one host NUMA node. This is because it will try very hard to
> : unsuccessfully swapout get_user_pages pinned pages as result of the
> : __GFP_THISNODE being set, instead of falling back to PAGE_SIZE
> : allocations and instead of trying to allocate THP on other nodes (it
> : would be even worse without vfio type1 GUP pins of course, except it'd
> : be swapping heavily instead).
> Fix this by removing __GFP_THISNODE for THP requests which are
> requesting the direct reclaim. This effectivelly reverts 5265047ac301 on
> the grounds that the zone/node reclaim was known to be disruptive due
> to premature reclaim when there was memory free. While it made sense at
> the time for HPC workloads without NUMA awareness on rare machines, it
> was ultimately harmful in the majority of cases. The existing behaviour
> is similiar, if not as widespare as it applies to a corner case but
> crucially, it cannot be tuned around like zone_reclaim_mode can. The
> default behaviour should always be to cause the least harm for the
> common case.
> If there are specialised use cases out there that want zone_reclaim_mode
> in specific cases, then it can be built on top. Longterm we should
> consider a memory policy which allows for the node reclaim like behavior
> for the specific memory ranges which would allow a
> [1]
> [mhocko@xxxxxxxx: rewrote the changelog based on the one from Andrea]
> Fixes: 5265047ac301 ("mm, thp: really limit transparent hugepage allocation to local node")
> Cc: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> Cc: stable # 4.1+
> Reported-by: Stefan Priebe <s.priebe@xxxxxxxxxxxx>
> Debugged-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Reviewed-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

Both patches look correct to me but I'm responding to this one because
it's the fix. The change makes sense and moves further away from the
severe stalling behaviour we used to see with both THP and zone reclaim

I put together a basic experiment with usemem configured to reference a
buffer multiple times that is 80% the size of main memory on a 2-socket box
with symmetric node sizes and defrag set to "always". The defrag setting
is not the default but it would be functionally similar to accessing a
buffer with madvise(MADV_HUGEPAGE). Usemem is configured to reference
the buffer multiple times and while it's not an interesting workload,
it would be expected to complete reasonably quickly as it fits within
memory. The results were;

vanilla noreclaim-v1
Amean Elapsd-1 42.78 ( 0.00%) 26.87 ( 37.18%)
Amean Elapsd-3 27.55 ( 0.00%) 7.44 ( 73.00%)
Amean Elapsd-4 5.72 ( 0.00%) 5.69 ( 0.45%)

This shows the elapsed time in seconds for 1 thread, 3 threads and 4 threads
referencing buffers 80% the size of memory. With the patches applied, it's
37.18% faster for the single thread and 73% faster with two threads. Note
that 4 threads showing little difference does not indicate the problem is
related to thread counts. It's simply the case that 4 threads gets spread
so their workload mostly fits in one node.

The overall view from /proc/vmstats is more startling

4.19.0-rc1 4.19.0-rc1
Minor Faults 35593425 708164
Major Faults 484088 36
Swap Ins 3772837 0
Swap Outs 3932295 0

Massive amounts of swap in/out without the patch

Direct pages scanned 6013214 0
Kswapd pages scanned 0 0
Kswapd pages reclaimed 0 0
Direct pages reclaimed 4033009 0

Lots of reclaim activity without the patch

Kswapd efficiency 100% 100%
Kswapd velocity 0.000 0.000
Direct efficiency 67% 100%
Direct velocity 11191.956 0.000

Mostly from direct reclaim context as you'd expect without the patch.

Page writes by reclaim 3932314.000 0.000
Page writes file 19 0
Page writes anon 3932295 0
Page reclaim immediate 42336 0

Writes from reclaim context is never good but the patch eliminates it.

We should never have default behaviour to thrash the system for such a
basic workload. If zone reclaim mode behaviour is ever desired but on a
single task instead of a global basis then the sensible option is to build
a mempolicy that enforces that behaviour.

Mel Gorman