linux-kernel - Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is being made

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Date:   Wed, 24 Nov 2021 18:24:21 +0100
From:   Mike Galbraith <efault@....de>
To:     Mel Gorman <mgorman@...hsingularity.net>,
        "Darrick J. Wong" <djwong@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        NeilBrown <neilb@...e.de>, Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Dave Chinner <david@...morbit.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/8] mm/vmscan: Throttle reclaim when no progress is
 being made

On Wed, 2021-11-24 at 10:32 +0000, Mel Gorman wrote:
> On Tue, Nov 23, 2021 at 05:19:12PM -0800, Darrick J. Wong wrote:
>
> > AFAICT the system is mostly idle, but it's difficult to tell because ps
> > and top also get stuck waiting for this cgroup for whatever reason.
>
> But this is surprising because I expect that ps and top are not running
> within the cgroup. Was /proc/PID/stack readable?

Probably this.

crash> ps | grep UN
   4418   4417   4  ffff8881cae66e40  UN   0.0    7620    980  memcg_test_1 <== the bad guy
   4419   4417   6  ffff8881cae62f40  UN   0.0    7620    980  memcg_test_1
   4420   4417   5  ffff8881cae65e80  UN   0.0    7620    980  memcg_test_1
   4421   4417   7  ffff8881cae63f00  UN   0.0    7620    980  memcg_test_1
   4422   4417   4  ffff8881cae60000  UN   0.0    7620    980  memcg_test_1
   4423   4417   3  ffff888128985e80  UN   0.0    7620    980  memcg_test_1
   4424   4417   7  ffff888117f79f80  UN   0.0    7620    980  memcg_test_1
   4425   4417   2  ffff888117f7af40  UN   0.0    7620    980  memcg_test_1
   4428   2791   6  ffff8881a8253f00  UN   0.0   38868   3568  ps
   4429   2808   4  ffff888100c90000  UN   0.0   38868   3600  ps
crash> bt -sx 4429
PID: 4429   TASK: ffff888100c90000  CPU: 4   COMMAND: "ps"
 #0 [ffff8881af1c3ce0] __schedule+0x285 at ffffffff817ae6c5
 #1 [ffff8881af1c3d68] schedule+0x3a at ffffffff817aed4a
 #2 [ffff8881af1c3d78] rwsem_down_read_slowpath+0x197 at ffffffff817b11a7
 #3 [ffff8881af1c3e08] down_read_killable+0x5c at ffffffff817b142c
 #4 [ffff8881af1c3e18] down_read_killable+0x5c at ffffffff817b142c
 #5 [ffff8881af1c3e28] __access_remote_vm+0x3f at ffffffff8120131f
 #6 [ffff8881af1c3e90] proc_pid_cmdline_read+0x148 at ffffffff812fc9a8
 #7 [ffff8881af1c3ee8] vfs_read+0x92 at ffffffff8126a302
 #8 [ffff8881af1c3f00] ksys_read+0x7d at ffffffff8126a72d
 #9 [ffff8881af1c3f38] do_syscall_64+0x37 at ffffffff817a3f57
#10 [ffff8881af1c3f50] entry_SYSCALL_64_after_hwframe+0x44 at ffffffff8180007c
    RIP: 00007f4b50fe8b5e  RSP: 00007ffdd7f6fe38  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 00007f4b5186a010  RCX: 00007f4b50fe8b5e
    RDX: 0000000000020000  RSI: 00007f4b5186a010  RDI: 0000000000000006
    RBP: 0000000000020000   R8: 0000000000000007   R9: 00000000ffffffff
    R10: 0000000000000000  R11: 0000000000000246  R12: 00007f4b5186a010
    R13: 0000000000000000  R14: 0000000000000006  R15: 0000000000000000
    ORIG_RAX: 0000000000000000  CS: 0033  SS: 002b
crash> mm_struct -x ffff8881021b4800
struct mm_struct {
  {
    mmap = 0xffff8881ccfe6a80,
    mm_rb = {
      rb_node = 0xffff8881ccfe61a0
    },
...
    mmap_lock = {
      count = {
        counter = 0x3
      },
      owner = {
        counter = 0xffff8881cae66e40
...
crash> bt 0xffff8881cae66e40
PID: 4418   TASK: ffff8881cae66e40  CPU: 4   COMMAND: "memcg_test_1"
 #0 [ffff888154097a88] __schedule at ffffffff817ae6c5
 #1 [ffff888154097b10] schedule at ffffffff817aed4a
 #2 [ffff888154097b20] schedule_timeout at ffffffff817b311f
 #3 [ffff888154097b90] reclaim_throttle at ffffffff811d802b
 #4 [ffff888154097bf0] do_try_to_free_pages at ffffffff811da206
 #5 [ffff888154097c40] try_to_free_mem_cgroup_pages at ffffffff811db522
 #6 [ffff888154097cd0] try_charge_memcg at ffffffff81256440
 #7 [ffff888154097d60] obj_cgroup_charge_pages at ffffffff81256c97
 #8 [ffff888154097d88] obj_cgroup_charge at ffffffff8125898c
 #9 [ffff888154097da8] kmem_cache_alloc at ffffffff81242099
#10 [ffff888154097de0] vm_area_alloc at ffffffff8106c87a
#11 [ffff888154097df0] mmap_region at ffffffff812082b2
#12 [ffff888154097e58] do_mmap at ffffffff81208922
#13 [ffff888154097eb0] vm_mmap_pgoff at ffffffff811e259f
#14 [ffff888154097f38] do_syscall_64 at ffffffff817a3f57
#15 [ffff888154097f50] entry_SYSCALL_64_after_hwframe at
ffffffff8180007c
    RIP: 00007f211c36b743  RSP: 00007ffeaac1bd58  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 0000000000000000  RCX: 00007f211c36b743
    RDX: 0000000000000003  RSI: 0000000000001000  RDI: 0000000000000000
    RBP: 0000000000000000   R8: 0000000000000000   R9: 0000000000000000
    R10: 0000000000002022  R11: 0000000000000246  R12: 0000000000000003
    R13: 0000000000001000  R14: 0000000000002022  R15: 0000000000000000
    ORIG_RAX: 0000000000000009  CS: 0033  SS: 002b
crash>