----- 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. 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. > > > - 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 > >
