Re: [PATCH v2] tracing: Do not free snapshot if tracer is on cmdline

From: Ross Zwisler
Date: Wed Oct 05 2022 - 12:08:04 EST

On Wed, Oct 5, 2022 at 9:37 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> The ftrace_boot_snapshot and alloc_snapshot cmdline options allocate the
> snapshot buffer at boot up for use later. The ftrace_boot_snapshot in
> particular requires the snapshot to be allocated because it will take a
> snapshot at the end of boot up allowing to see the traces that happened
> during boot so that it's not lost when user space takes over.
> When a tracer is registered (started) there's a path that checks if it
> requires the snapshot buffer or not, and if it does not and it was
> allocated it will do a synchronization and free the snapshot buffer.
> This is only required if the previous tracer was using it for "max
> latency" snapshots, as it needs to make sure all max snapshots are
> complete before freeing. But this is only needed if the previous tracer
> was using the snapshot buffer for latency (like irqoff tracer and
> friends). But it does not make sense to free it, if the previous tracer
> was not using it, and the snapshot was allocated by the cmdline
> parameters. This basically takes away the point of allocating it in the
> first place!
> Note, the allocated snapshot worked fine for just trace events, but fails
> when a tracer is enabled on the cmdline.
> Further investigation, this goes back even further and it does not require
> a tracer on the cmdline to fail. Simply enable snapshots and then enable a
> tracer, and it will remove the snapshot.
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 45ad21ca5530 ("tracing: Have trace_array keep track if snapshot buffer is allocated")
> Reported-by: Ross Zwisler <zwisler@xxxxxxxxxx>

With this patch I'm able to successfully use the function_graph tracer
with boot snapshots, thanks!

Tested-by: Ross Zwisler <zwisler@xxxxxxxxxx>