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   1 From foo@baz Sat Apr  3 10:50:40 AM CEST 2021
   2 From: Frank van der Linden <fllinden@amazon.com>
   3 Date: Tue, 30 Mar 2021 18:19:08 +0000
   4 Subject: mem_cgroup: make sure moving_account, move_lock_task and stat_cpu in the same cacheline
   5 To: <stable@vger.kernel.org>
   6 Cc: <fllinden@amazon.com>
   7 Message-ID: <20210330181910.15378-4-fllinden@amazon.com>
   8
   9 From: Frank van der Linden <fllinden@amazon.com>
  10
  11 From: Aaron Lu <aaron.lu@intel.com>
  12
  13 commit e81bf9793b1861d74953ef041b4f6c7faecc2dbd upstream.
  14
  15 The LKP robot found a 27% will-it-scale/page_fault3 performance
  16 regression regarding commit e27be240df53("mm: memcg: make sure
  17 memory.events is uptodate when waking pollers").
  18
  19 What the test does is:
  20  1 mkstemp() a 128M file on a tmpfs;
  21  2 start $nr_cpu processes, each to loop the following:
  22    2.1 mmap() this file in shared write mode;
  23    2.2 write 0 to this file in a PAGE_SIZE step till the end of the file;
  24    2.3 unmap() this file and repeat this process.
  25  3 After 5 minutes, check how many loops they managed to complete, the
  26    higher the better.
  27
  28 The commit itself looks innocent enough as it merely changed some event
  29 counting mechanism and this test didn't trigger those events at all.
  30 Perf shows increased cycles spent on accessing root_mem_cgroup->stat_cpu
  31 in count_memcg_event_mm()(called by handle_mm_fault()) and in
  32 __mod_memcg_state() called by page_add_file_rmap().  So it's likely due
  33 to the changed layout of 'struct mem_cgroup' that either make stat_cpu
  34 falling into a constantly modifying cacheline or some hot fields stop
  35 being in the same cacheline.
  36
  37 I verified this by moving memory_events[] back to where it was:
  38
  39 : --- a/include/linux/memcontrol.h
  40 : +++ b/include/linux/memcontrol.h
  41 : @@ -205,7 +205,6 @@ struct mem_cgroup {
  42 :       int             oom_kill_disable;
  43 :
  44 :       /* memory.events */
  45 : -     atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
  46 :       struct cgroup_file events_file;
  47 :
  48 :       /* protect arrays of thresholds */
  49 : @@ -238,6 +237,7 @@ struct mem_cgroup {
  50 :       struct mem_cgroup_stat_cpu __percpu *stat_cpu;
  51 :       atomic_long_t           stat[MEMCG_NR_STAT];
  52 :       atomic_long_t           events[NR_VM_EVENT_ITEMS];
  53 : +     atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
  54 :
  55 :       unsigned long           socket_pressure;
  56
  57 And performance restored.
  58
  59 Later investigation found that as long as the following 3 fields
  60 moving_account, move_lock_task and stat_cpu are in the same cacheline,
  61 performance will be good.  To avoid future performance surprise by other
  62 commits changing the layout of 'struct mem_cgroup', this patch makes
  63 sure the 3 fields stay in the same cacheline.
  64
  65 One concern of this approach is, moving_account and move_lock_task could
  66 be modified when a process changes memory cgroup while stat_cpu is a
  67 always read field, it might hurt to place them in the same cacheline.  I
  68 assume it is rare for a process to change memory cgroup so this should
  69 be OK.
  70
  71 Link: https://lkml.kernel.org/r/20180528114019.GF9904@yexl-desktop
  72 Link: http://lkml.kernel.org/r/20180601071115.GA27302@intel.com
  73 Signed-off-by: Aaron Lu <aaron.lu@intel.com>
  74 Reported-by: kernel test robot <xiaolong.ye@intel.com>
  75 Cc: Johannes Weiner <hannes@cmpxchg.org>
  76 Cc: Michal Hocko <mhocko@kernel.org>
  77 Cc: Tejun Heo <tj@kernel.org>
  78 Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
  79 Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
  80 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  81 ---
  82  include/linux/memcontrol.h |   23 +++++++++++++++++++----
  83  1 file changed, 19 insertions(+), 4 deletions(-)
  84
  85 --- a/include/linux/memcontrol.h
  86 +++ b/include/linux/memcontrol.h
  87 @@ -155,6 +155,15 @@ enum memcg_kmem_state {
  88         KMEM_ONLINE,
  89  };
  90
  91 +#if defined(CONFIG_SMP)
  92 +struct memcg_padding {
  93 +       char x[0];
  94 +} ____cacheline_internodealigned_in_smp;
  95 +#define MEMCG_PADDING(name)      struct memcg_padding name;
  96 +#else
  97 +#define MEMCG_PADDING(name)
  98 +#endif
  99 +
 100  /*
 101   * The memory controller data structure. The memory controller controls both
 102   * page cache and RSS per cgroup. We would eventually like to provide
 103 @@ -202,7 +211,6 @@ struct mem_cgroup {
 104         int             oom_kill_disable;
 105
 106         /* memory.events */
 107 -       atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
 108         struct cgroup_file events_file;
 109
 110         /* protect arrays of thresholds */
 111 @@ -222,19 +230,26 @@ struct mem_cgroup {
 112          * mem_cgroup ? And what type of charges should we move ?
 113          */
 114         unsigned long move_charge_at_immigrate;
 115 +       /* taken only while moving_account > 0 */
 116 +       spinlock_t              move_lock;
 117 +       unsigned long           move_lock_flags;
 118 +
 119 +       MEMCG_PADDING(_pad1_);
 120 +
 121         /*
 122          * set > 0 if pages under this cgroup are moving to other cgroup.
 123          */
 124         atomic_t                moving_account;
 125 -       /* taken only while moving_account > 0 */
 126 -       spinlock_t              move_lock;
 127         struct task_struct      *move_lock_task;
 128 -       unsigned long           move_lock_flags;
 129
 130         /* memory.stat */
 131         struct mem_cgroup_stat_cpu __percpu *stat_cpu;
 132 +
 133 +       MEMCG_PADDING(_pad2_);
 134 +
 135         atomic_long_t           stat[MEMCG_NR_STAT];
 136         atomic_long_t           events[NR_VM_EVENT_ITEMS];
 137 +       atomic_long_t memory_events[MEMCG_NR_MEMORY_EVENTS];
 138
 139         unsigned long           socket_pressure;
 140