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

[Steven Rostedt]

On Tue, 5 Sep 2023 20:17:14 +0800
Zheng Yejian <zhengyejian1@xxxxxxxxxx> 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 <zhengyejian1@xxxxxxxxxx>
> ---
>  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