Am 10.09.2018 um 22:08 schrieb David Rientjes: > On Fri, 7 Sep 2018, Michal Hocko wrote: > >> From: Michal Hocko <mhocko@xxxxxxxx> >> >> Andrea has noticed [1] that a 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 > > Since we don't have __GFP_REPEAT, this suggests that > __alloc_pages_direct_compact() took >100s to complete. The memory > capacity of the system isn't shown, but I assume it's around 768GB? This > should be with COMPACT_PRIO_ASYNC, and MIGRATE_ASYNC compaction certainly > should abort much earlier. Yes it's 768GB. Greets, Stefan >> [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 handling from alloc_pages_vma where >> it doesn't belong and move it to alloc_hugepage_direct_gfpmask where we >> juggle gfp flags for different allocation modes. The rationale is that >> __GFP_THISNODE is helpful in relaxed defrag modes because falling back >> to a different node might be more harmful than the benefit of a large page. >> If the user really requires THP (e.g. by MADV_HUGEPAGE) then the THP has >> a higher priority than local NUMA placement. >> > > That's not entirely true, the remote access latency for remote thp on all > of our platforms is greater than local small pages, this is especially > true for remote thp that is allocated intersocket and must be accessed > through the interconnect. > > Our users of MADV_HUGEPAGE are ok with assuming the burden of increased > allocation latency, but certainly not remote access latency. There are > users who remap their text segment onto transparent hugepages are fine > with startup delay if they are access all of their text from local thp. > Remote thp would be a significant performance degradation. > > When Andrea brought this up, I suggested that the full solution would be a > MPOL_F_HUGEPAGE flag that could define thp allocation policy -- the added > benefit is that we could replace the thp "defrag" mode default by setting > this as part of default_policy. Right now, MADV_HUGEPAGE users are > concerned about (1) getting thp when system-wide it is not default and (2) > additional fault latency when direct compaction is not default. They are > not anticipating the degradation of remote access latency, so overloading > the meaning of the mode is probably not a good idea. >
