timekeeping: Avoiding false sharing in field access of tk_core

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I47W8L


CVE: NA

---------------------------

We detect a performance deterioration when using Unixbench, we use the dichotomy to
locate the patch 7e66740a ("MPAM / ACPI: Refactoring MPAM init process and set
MPAM ACPI as entrance"), In comparing two commit df5defd9 ("KVM: X86: MMU: Use
the correct inherited permissions to get shadow page") and ac4dbb75 ("ACPI 6.x:
Add definitions for MPAM table") we get following testing result:

CMD: ./Run -c xx context1
RESULT:
+-------------UnixBench context1-----------+
+---------+--------------+-----------------+
+         + ac4dbb75 +  df5defd9   +
+---------+--------------+---------+-------+
+  Cores  +    Score     +      Score      +
+---------+--------------+-----------------+
+    1    +    522.8     +      535.7      +
+---------+--------------+-----------------+
+   24    +   11231.5    +     12111.2     +
+---------+--------------+-----------------+
+   48    +   8535.1     +     8745.1      +
+---------+--------------+-----------------+
+   72    +   10821.9    +     10343.8     +
+---------+--------------+-----------------+
+   96    +   15238.5    +     42947.8     +
+---------+--------------+-----------------+

We found a irrefutable difference in latency sampling when using the perf tool:

HEAD:ac4dbb75                                   HEAD:df5defd9

45.18% [kernel] [k] ktime_get_coarse_real_ts64  ->  1.78% [kernel] [k] ktime_get_coarse_real_ts64
                                                                               ...
                                                                       65.87 │ dmb ishld //smp_rmb()

Through ftrace we get the calltrace and and detected the number of visits of
ktime_get_coarse_real_ts64, which frequently visits tk_core->seq and
tk_core->timekeeper->tkr_mono:

-   48.86%  [kernel]                 [k] ktime_get_coarse_real_ts64
   - 5.76% ktime_get_coarse_real_ts64   #about 111437657 times per 10 seconds
      - 14.70% __audit_syscall_entry
           syscall_trace_enter
           el0_svc_common
           el0_svc_handler
         + el0_svc
      - 2.85% current_time

So this may be performance degradation caused by interference when happened different
fields access, We compare .bss and .data section of this two version:

    HEAD:ac4dbb75
`->
    ffff00000962e680 l     O .bss   0000000000000110 tk_core
    ffff000009355680 l     O .data  0000000000000078 tk_fast_mono
    ffff0000093557a0 l     O .data  0000000000000090 dummy_clock
    ffff000009355700 l     O .data  0000000000000078 tk_fast_raw
    ffff000009355778 l     O .data  0000000000000028 timekeeping_syscore_ops
    ffff00000962e640 l     O .bss   0000000000000008 cycles_at_suspend

    HEAD:df5defd9
`->
    ffff00000957dbc0 l     O .bss   0000000000000110 tk_core
    ffff0000092b4e80 l     O .data  0000000000000078 tk_fast_mono
    ffff0000092b4fa0 l     O .data  0000000000000090 dummy_clock
    ffff0000092b4f00 l     O .data  0000000000000078 tk_fast_raw
    ffff0000092b4f78 l     O .data  0000000000000028 timekeeping_syscore_ops
    ffff00000957db80 l     O .bss   0000000000000008 cycles_at_suspend

By comparing this two version tk_core's address: ffff00000962e680 is 128Byte aligned
but latter df5defd9 is 64Byte aligned, the memory storage layout of tk_core has
undergone subtle changes:

    HEAD:ac4dbb75
`->                     |<--------formmer 64Bytes---------->|<------------latter 64Byte------------->|
    0xffff00000957dbc0_>|<-seq 8Bytes->|<-tkr_mono 56Bytes->|<-thr_raw 56Bytes->|<-xtime_sec 8Bytes->|
    0xffff00000957dc00_>...

    HEAD:df5defd9
`->                     |<------formmer 64Bytes---->|<------------latter 64Byte-------->|
    0xffff00000962e680_>|<-Other variables 64Bytes->|<-seq 8Bytes->|<-tkr_mono 56Bytes->|
    0xffff00000962e6c0_>..

We testified thr_raw,xtime_sec fields interfere strongly with seq,tkr_mono field because of
frequent load/store operation, this will cause as known false sharing.

We add a 64Bytes padding field in tk_core for reservation of any after usefull usage and
keep tk_core 128Byte aligned, this can avoid changes in the way tk_core's layout is stored,
In this solution, layout of tk_core always like this:

crash>  struct -o tk_core_t
struct tk_core_t {
    [0] u64 padding[8];
   [64] seqcount_t seq;
   [72] struct timekeeper timekeeper;
}
SIZE: 336
crash> struct -o timekeeper
struct timekeeper {
    [0] struct tk_read_base tkr_mono;
   [56] struct tk_read_base tkr_raw;
  [112] u64 xtime_sec;
  [120] unsigned long ktime_sec;
  ...
}
SIZE: 264

After appling our own solution:

+---------+--------------+
+         + Our solution +
+---------+--------------+
+  Cores  +    Score     +
+---------+--------------+
+    1    +    548.9     +
+---------+--------------+
+   24    +   11018.3    +
+---------+--------------+
+   48    +   8938.2     +
+---------+--------------+
+   72    +   14610.7    +
+---------+--------------+
+   96    +   40811.7    +
+---------+--------------+

Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>