----- Original Message ----- > > > On 5/6/19 2:35 PM, Jan Stancek wrote: > > > > ----- Original Message ----- > >> > >> On 5/5/19 7:10 AM, Jan Stancek wrote: > >>> Hi, > >>> > >>> I'm seeing userspace program getting stuck on aarch64, on kernels 4.20 > >>> and > >>> newer. > >>> It stalls from seconds to hours. > >>> > >>> I have simplified it to following scenario (reproducer linked below [1]): > >>> while (1): > >>> spawn Thread 1: mmap, write, munmap > >>> spawn Thread 2: <nothing> > >>> > >>> Thread 1 is sporadically getting stuck on write to mapped area. > >>> User-space > >>> is not > >>> moving forward - stdout output stops. Observed CPU usage is however 100%. > >>> > >>> At this time, kernel appears to be busy handling page faults (~700k per > >>> second): > >>> > >>> # perf top -a -g > >>> - 98.97% 8.30% a.out [.] map_write_unmap > >>> - 23.52% map_write_unmap > >>> - 24.29% el0_sync > >>> - 10.42% do_mem_abort > >>> - 17.81% do_translation_fault > >>> - 33.01% do_page_fault > >>> - 56.18% handle_mm_fault > >>> 40.26% __handle_mm_fault > >>> 2.19% __ll_sc___cmpxchg_case_acq_4 > >>> 0.87% mem_cgroup_from_task > >>> - 6.18% find_vma > >>> 5.38% vmacache_find > >>> 1.35% __ll_sc___cmpxchg_case_acq_8 > >>> 1.23% __ll_sc_atomic64_sub_return_release > >>> 0.78% down_read_trylock > >>> 0.93% do_translation_fault > >>> + 8.30% thread_start > >>> > >>> # perf stat -p 8189 -d > >>> ^C > >>> Performance counter stats for process id '8189': > >>> > >>> 984.311350 task-clock (msec) # 1.000 CPUs > >>> utilized > >>> 0 context-switches # 0.000 K/sec > >>> 0 cpu-migrations # 0.000 K/sec > >>> 723,641 page-faults # 0.735 M/sec > >>> 2,559,199,434 cycles # 2.600 GHz > >>> 711,933,112 instructions # 0.28 insn per > >>> cycle > >>> <not supported> branches > >>> 757,658 branch-misses > >>> 205,840,557 L1-dcache-loads # 209.121 M/sec > >>> 40,561,529 L1-dcache-load-misses # 19.71% of all > >>> L1-dcache hits > >>> <not supported> LLC-loads > >>> <not supported> LLC-load-misses > >>> > >>> 0.984454892 seconds time elapsed > >>> > >>> With some extra traces, it appears looping in page fault for same > >>> address, > >>> over and over: > >>> do_page_fault // mm_flags: 0x55 > >>> __do_page_fault > >>> __handle_mm_fault > >>> handle_pte_fault > >>> ptep_set_access_flags > >>> if (pte_same(pte, entry)) // pte: e8000805060f53, entry: > >>> e8000805060f53 > >>> > >>> I had traces in mmap() and munmap() as well, they don't get hit when > >>> reproducer > >>> hits the bad state. > >>> > >>> Notes: > >>> - I'm not able to reproduce this on x86. > >>> - Attaching GDB or strace immediatelly recovers application from stall. > >>> - It also seems to recover faster when system is busy with other tasks. > >>> - MAP_SHARED vs. MAP_PRIVATE makes no difference. > >>> - Turning off THP makes no difference. > >>> - Reproducer [1] usually hits it within ~minute on HW described below. > >>> - Longman mentioned that "When the rwsem becomes reader-owned, it causes > >>> all the spinning writers to go to sleep adding wakeup latency to > >>> the time required to finish the critical sections", but this looks > >>> like busy loop, so I'm not sure if it's related to rwsem issues > >>> identified > >>> in: > >>> https://lore.kernel.org/lkml/20190428212557.13482-2-longman@xxxxxxxxxx/ > >> It sounds possible to me. What the optimization done by the commit ("mm: > >> mmap: zap pages with read mmap_sem in munmap") is to downgrade write > >> rwsem to read when zapping pages and page table in munmap() after the > >> vmas have been detached from the rbtree. > >> > >> So the mmap(), which is writer, in your test may steal the lock and > >> execute with the munmap(), which is the reader after the downgrade, in > >> parallel to break the mutual exclusion. > >> > >> In this case, the parallel mmap() may map to the same area since vmas > >> have been detached by munmap(), then mmap() may create the complete same > >> vmas, and page fault happens on the same vma at the same address. > >> > >> I'm not sure why gdb or strace could recover this, but they use ptrace > >> which may acquire mmap_sem to break the parallel inadvertently. > >> > >> May you please try Waiman's patch to see if it makes any difference? > > I don't see any difference in behaviour after applying: > > [PATCH-tip v7 01/20] locking/rwsem: Prevent decrement of reader count > > before increment > > Issue is still easily reproducible for me. > > > > I'm including output of mem_abort_decode() / show_pte() for sample PTE, > > that > > I see in page fault loop. (I went through all bits, but couldn't find > > anything invalid about it) > > > > mem_abort_decode: Mem abort info: > > mem_abort_decode: ESR = 0x92000047 > > mem_abort_decode: Exception class = DABT (lower EL), IL = 32 bits > > mem_abort_decode: SET = 0, FnV = 0 > > mem_abort_decode: EA = 0, S1PTW = 0 > > mem_abort_decode: Data abort info: > > mem_abort_decode: ISV = 0, ISS = 0x00000047 > > mem_abort_decode: CM = 0, WnR = 1 > > show_pte: user pgtable: 64k pages, 48-bit VAs, pgdp = 0000000067027567 > > show_pte: [0000ffff6dff0000] pgd=000000176bae0003 > > show_pte: , pud=000000176bae0003 > > show_pte: , pmd=000000174ad60003 > > show_pte: , pte=00e80008023a0f53 > > show_pte: , pte_pfn: 8023a > > > > >>> print bin(0x47) > > 0b1000111 > > > > Per D12-2779 (ARM Architecture Reference Manual), > > ISS encoding for an exception from an Instruction Abort: > > IFSC, bits [5:0], Instruction Fault Status Code > > 0b000111 Translation fault, level 3 > > > > --- > > > > My theory is that TLB is getting broken. Theory continued: unmap_region() is batching updates to TLB (for vmas and page tables). And at the same time another thread handles page fault for same mm, which increases "tlb_flush_pending". tlb_finish_mmu() called from unmap_region() will thus set 'force = 1'. And arch_tlb_finish_mmu() will in turn reset TLB range, presumably making it smaller then it would be if force == 0. Change below appears to fix it: diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index f2f03c655807..a4cef21bd62b 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -93,7 +93,7 @@ void arch_tlb_finish_mmu(struct mmu_gather *tlb, struct mmu_gather_batch *batch, *next; if (force) { - __tlb_reset_range(tlb); __tlb_adjust_range(tlb, start, end - start); } > > > > I made a dummy kernel module that exports debugfs file, which on read > > triggers: > > flush_tlb_all(); > > > > Any time reproducer stalls and I read debugfs file, it recovers > > immediately and resumes printing to stdout. > > That commit doesn't change anything about TLB flush, just move zapping > pages under read mmap_sem as what MADV_DONTNEED does. > > I don't have aarch64 board to reproduce and debug it. And, I'm not > familiar with aarch64 architecture either. But, some history told me the > parallel zapping page may run into stale TLB and defer a flush meaning > that this call may observe pte_none and fails to flush the TLB. But, > this has been solved by commit 56236a59556c ("mm: refactor TLB gathering > API") and 99baac21e458 ("mm: fix MADV_[FREE|DONTNEED] TLB flush miss > problem"). > > For more detail, please refer to commit 4647706ebeee ("mm: always flush > VMA ranges affected by zap_page_range"). Copied Mel and Rik in this > thread. Also added Will Deacon and Catalin Marinas, who are aarch64 > maintainers, in this loop Thanks > > But, your test (triggering TLB flush) does demonstrate TLB flush is > *not* done properly at some point as expected for aarch64. Could you > please give the below patch a try? Your patch also fixes my reproducer. > > diff --git a/mm/memory.c b/mm/memory.c > index ab650c2..ef41ad5 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -1336,8 +1336,10 @@ void unmap_vmas(struct mmu_gather *tlb, > > mmu_notifier_range_init(&range, vma->vm_mm, start_addr, end_addr); > mmu_notifier_invalidate_range_start(&range); > - for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) > + for ( ; vma && vma->vm_start < end_addr; vma = vma->vm_next) { > unmap_single_vma(tlb, vma, start_addr, end_addr, NULL); > + flush_tlb_range(vma, start_addr, end_addr); > + } > mmu_notifier_invalidate_range_end(&range); > } > > > > >>> - I tried 2 different aarch64 systems so far: APM X-Gene CPU Potenza A3 > >>> and > >>> Qualcomm 65-LA-115-151. > >>> I can reproduce it on both with v5.1-rc7. It's easier to reproduce > >>> on latter one (for longer periods of time), which has 46 CPUs. > >>> - Sample output of reproducer on otherwise idle system: > >>> # ./a.out > >>> [00000314] map_write_unmap took: 26305 ms > >>> [00000867] map_write_unmap took: 13642 ms > >>> [00002200] map_write_unmap took: 44237 ms > >>> [00002851] map_write_unmap took: 992 ms > >>> [00004725] map_write_unmap took: 542 ms > >>> [00006443] map_write_unmap took: 5333 ms > >>> [00006593] map_write_unmap took: 21162 ms > >>> [00007435] map_write_unmap took: 16982 ms > >>> [00007488] map_write unmap took: 13 ms^C > >>> > >>> I ran a bisect, which identified following commit as first bad one: > >>> dd2283f2605e ("mm: mmap: zap pages with read mmap_sem in munmap") > >>> > >>> I can also make the issue go away with following change: > >>> diff --git a/mm/mmap.c b/mm/mmap.c > >>> index 330f12c17fa1..13ce465740e2 100644 > >>> --- a/mm/mmap.c > >>> +++ b/mm/mmap.c > >>> @@ -2844,7 +2844,7 @@ EXPORT_SYMBOL(vm_munmap); > >>> SYSCALL_DEFINE2(munmap, unsigned long, addr, size_t, len) > >>> { > >>> profile_munmap(addr); > >>> - return __vm_munmap(addr, len, true); > >>> + return __vm_munmap(addr, len, false); > >>> } > >>> > >>> # cat /proc/cpuinfo | head > >>> processor : 0 > >>> BogoMIPS : 40.00 > >>> Features : fp asimd evtstrm aes pmull sha1 sha2 crc32 cpuid > >>> asimdrdm > >>> CPU implementer : 0x51 > >>> CPU architecture: 8 > >>> CPU variant : 0x0 > >>> CPU part : 0xc00 > >>> CPU revision : 1 > >>> > >>> # numactl -H > >>> available: 1 nodes (0) > >>> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 > >>> 23 > >>> 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 > >>> node 0 size: 97938 MB > >>> node 0 free: 95732 MB > >>> node distances: > >>> node 0 > >>> 0: 10 > >>> > >>> Regards, > >>> Jan > >>> > >>> [1] > >>> https://github.com/jstancek/reproducers/blob/master/kernel/page_fault_stall/mmap5.c > >>> [2] > >>> https://github.com/jstancek/reproducers/blob/master/kernel/page_fault_stall/config > >> > >
