Re: [PATCH v1] PM: suspend: clean up redundant filesystems_freeze/thaw handling

[Zihuan Zhang <zhangzihuan@kylinos.cn>]

在 2025/7/15 20:48, Rafael J. Wysocki 写道:
> On Tue, Jul 15, 2025 at 8:12 AM Zihuan Zhang <zhangzihuan@kylinos.cn> wrote:
>> Hi Rafael,
>>
>> 在 2025/7/15 01:57, Rafael J. Wysocki 写道:
>>> Hi,
>>>
>>> On Mon, Jul 14, 2025 at 10:44 AM Zihuan Zhang <zhangzihuan@kylinos.cn> wrote:
>>>> Hi Rafael,
>>>>
>>>> Just a gentle ping on this patch.
>>> I've lost track of it for some reason, sorry.
>>>
>>>> I realized I forgot to mention an important motivation in the changelog:
>>>> calling filesystems_freeze() twice (from both suspend_prepare() and
>>>> enter_state()) lead to a black screen and make the system unable to resume..
>>>>
>>>> This patch avoids the duplicate call and resolves that issue.
>>> Now applied as a fix for 6.16-rc7, thank you!
>>
>> Thanks for the reply!
>>
>> Just a quick follow-up question — we noticed that even when the “freeze
>> filesystems” feature is not enabled, the current code still calls
>> filesystems_thaw().
>>
>> Do you think it would make sense to guard this with a static key (or
>> another mechanism) to avoid unnecessary overhead?
> Possibly, if this overhead is significant, but is it?

We've done some testing using ftrace to measure the overhead of 
filesystems_thaw(). When freeze_filesystems is not enabled, the overhead 
is typically around 15–40 microseconds.

However, when freeze is enabled, we observed that filesystems_thaw() can 
take over 3 seconds to complete (e.g., 3,450,644 us in one test case).

freeze_filesystems  not enabled:

# tracer: function_graph
#
# CPU  DURATION                  FUNCTION CALLS
# |     |   |                     |   |   |   |
   4) + 15.740 us   |  filesystems_thaw();
  11) + 16.894 us   |  filesystems_thaw();
  10) + 17.805 us   |  filesystems_thaw();
   8) + 37.762 us   |  filesystems_thaw();
  ------------------------------------------
  11) systemd-54512  => systemd-66433
  ------------------------------------------

  11) + 15.167 us   |  filesystems_thaw();
   6) + 16.760 us   |  filesystems_thaw();
   7) + 14.870 us   |  filesystems_thaw();
   3) + 16.171 us   |  filesystems_thaw();
   1) + 16.461 us   |  filesystems_thaw();
  ------------------------------------------
   3) systemd-71984  => systemd-73036
  ------------------------------------------

   3) + 28.314 us   |  filesystems_thaw();

freeze_filesystems  enabled:

  10)               |  filesystems_thaw() {
   2) $ 3450644 us  |  } /* filesystems_thaw */
  ------------------------------------------
   1) systemd-72561  => systemd-99210
  ------------------------------------------

   1)               |  filesystems_thaw() {
  ------------------------------------------
   7) systemd-71501  => systemd-99210
  ------------------------------------------

   7) $ 3429306 us  |  } /* filesystems_thaw */
  ------------------------------------------
   7) systemd-99210  => systemd-100028
  ------------------------------------------

   7)               |  filesystems_thaw() {
  ------------------------------------------
   4) systemd-53278  => systemd-100028
  ------------------------------------------

   4) $ 3270122 us  |  } /* filesystems_thaw */
  ------------------------------------------
   7) systemd-100028 => systemd-100720
  ------------------------------------------

   7) $ 3446496 us  |  filesystems_thaw();
  ------------------------------------------
   7) systemd-100720 => systemd-112075
  ------------------------------------------

   7)               |  filesystems_thaw() {
  ------------------------------------------
  11) systemd-66433  => systemd-112075
  ------------------------------------------

  11) $ 3454117 us  |  } /* filesystems_thaw */