• Andrea Arcangeli's avatar
    mm: thp: relax __GFP_THISNODE for MADV_HUGEPAGE mappings · 818e5846
    Andrea Arcangeli authored
    commit ac5b2c18911ffe95c08d69273917f90212cf5659 upstream.
    
    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:
    
      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)
      kvm cpuset=/ mems_allowed=0-1
      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)
      Hardware name: Supermicro SYS-1029P-WTRT/X11DDW-NT, BIOS 2.0 12/05/2017
      Call Trace:
       dump_stack+0x5c/0x84
       warn_alloc+0xe0/0x180
       __alloc_pages_slowpath+0x820/0xc90
       __alloc_pages_nodemask+0x1cc/0x210
       alloc_pages_vma+0x1e5/0x280
       do_huge_pmd_wp_page+0x83f/0xf00
       __handle_mm_fault+0x93d/0x1060
       handle_mm_fault+0xc6/0x1b0
       __do_page_fault+0x230/0x430
       do_page_fault+0x2a/0x70
       page_fault+0x7b/0x80
       [...]
      Mem-Info:
      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
      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
      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 5265047a
    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 similar, 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] http://lkml.kernel.org/r/20180820032204.9591-1-aarcange@redhat.com
    
    Mel said:
    
    : 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
    : mode.
    :
    : 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;
    :
    : usemem
    :                                   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
    :                             vanillanoreclaim-v1r1
    : 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.
    
    This was a severe regression compared to previous kernels that made
    important workloads unusable and it starts when __GFP_THISNODE was
    added to THP allocations under MADV_HUGEPAGE.  It is not a significant
    risk to go to the previous behavior before __GFP_THISNODE was added, it
    worked like that for years.
    
    This was simply an optimization to some lucky workloads that can fit in
    a single node, but it ended up breaking the VM for others that can't
    possibly fit in a single node, so going back is safe.
    
    [mhocko@suse.com: rewrote the changelog based on the one from Andrea]
    Link: http://lkml.kernel.org/r/20180925120326.24392-2-mhocko@kernel.org
    Fixes: 5265047a ("mm, thp: really limit transparent hugepage allocation to local node")
    Signed-off-by: 's avatarAndrea Arcangeli <aarcange@redhat.com>
    Signed-off-by: 's avatarMichal Hocko <mhocko@suse.com>
    Reported-by: 's avatarStefan Priebe <s.priebe@profihost.ag>
    Debugged-by: 's avatarAndrea Arcangeli <aarcange@redhat.com>
    Reported-by: 's avatarAlex Williamson <alex.williamson@redhat.com>
    Reviewed-by: 's avatarMel Gorman <mgorman@techsingularity.net>
    Tested-by: 's avatarMel Gorman <mgorman@techsingularity.net>
    Cc: Zi Yan <zi.yan@cs.rutgers.edu>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: David Rientjes <rientjes@google.com>
    Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
    Cc: <stable@vger.kernel.org>	[4.1+]
    Signed-off-by: 's avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: 's avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    818e5846