Re: [PATCH] tracing: Fix unexpected ring buffer expand by instance

On Tue, 5 Sep 2023 20:17:14 +0800
Zheng Yejian [off-list ref] wrote:

The ring buffer of global_trace is set to the minimum size in
order to save memory on boot up and then it will be expand when
some trace feature enabled.

However currently operations under an instance can also cause
global_trace ring buffer being expanded, and the expanded memory
would be wasted if global_trace then not being used.

See following case, we enable 'sched_switch' event in instance 'A', then
ring buffer of global_trace is unexpectedly expanded to be 1410KB, also
the '(expanded: 1408)' from 'buffer_size_kb' of instance is confusing.

  # cd /sys/kernel/tracing
  # mkdir instances/A
  # cat buffer_size_kb
  7 (expanded: 1408)
  # cat instances/A/buffer_size_kb
  1410 (expanded: 1408)
  # echo sched:sched_switch > instances/A/set_event
  # cat buffer_size_kb
  1410
  # cat instances/A/buffer_size_kb
  1410

To fix it, we can:
  - Make 'ring_buffer_expanded' as a member of 'struct trace_array';
  - Make 'ring_buffer_expanded' of instance is defaultly true,
    global_trace is defaultly false;
  - In order not to expose 'global_trace' outside of file
    'kernel/trace/trace.c', introduce trace_set_ring_buffer_expanded()
    to set 'ring_buffer_expanded' as 'true';
  - Pass the expected trace_array to tracing_update_buffers().

Signed-off-by: Zheng Yejian <redacted>
---
 kernel/trace/trace.c        | 46 ++++++++++++++++++++-----------------
 kernel/trace/trace.h        |  9 ++++++--
 kernel/trace/trace_events.c | 22 ++++++++++--------
 3 files changed, 44 insertions(+), 33 deletions(-)

Looks good, but as the merge window is still open, it will need to wait
till it is closed to be reviewed.

Thanks,

-- Steve