Re: [PATCH v10 08/10] dyndbg: add print-to-tracefs, selftest with it - RFC

[Vincent Whitchurch <vincent.whitchurch@axis.com>]

On Thu, Nov 11, 2021 at 03:02:04PM -0700, Jim Cromie wrote:
> Sean Paul proposed, in:
> https://patchwork.freedesktop.org/series/78133/
> drm/trace: Mirror DRM debug logs to tracefs
> 
> His patchset's objective is to be able to independently steer some of
> the drm.debug stream to an alternate tracing destination, by splitting
> drm_debug_enabled() into syslog & trace flavors, and enabling them
> separately.  2 advantages were identified:
> 
> 1- syslog is heavyweight, tracefs is much lighter
> 2- separate selection of enabled categories means less traffic
> 
> Dynamic-Debug can do 2nd exceedingly well:
> 
> A- all work is behind jump-label's NOOP, zero off cost.
> B- exact site selectivity, precisely the useful traffic.
>    can tailor enabled set interactively, at shell.
> 
> Since the tracefs interface is effective for drm (the threads suggest
> so), adding that interface to dynamic-debug has real potential for
> everyone including drm.
> 
> if CONFIG_TRACING:
> 
> Grab Sean's trace_init/cleanup code, use it to provide tracefs
> available by default to all pr_debugs.  This will likely need some
> further per-module treatment; perhaps something reflecting hierarchy
> of module,file,function,line, maybe with a tuned flattening.
> 
> endif CONFIG_TRACING
> 
> Add a new +T flag to enable tracing, independent of +p, and add and
> use 3 macros: dyndbg_site_is_enabled/logging/tracing(), to encapsulate
> the flag checks.  Existing code treats T like other flags.

I posted a patchset a while ago to do something very similar, but that
got stalled for some reason and I unfortunately didn't follow it up:

 https://lore.kernel.org/lkml/20200825153338.17061-1-vincent.whitchurch@axis.com/

A key difference between that patchset and this patch (besides that
small fact that I used +x instead of +T) was that my patchset allowed
the dyndbg trace to be emitted to the main buffer and did not force them
to be in an instance-specific buffer.

That feature is quite important at least for my use case since I often
use dyndbg combined with function tracing, and the latter doesn't work
on non-main instances according to Documentation/trace/ftrace.rst.

For example, here's a random example of a bootargs from one of my recent
debugging sessions:

 trace_event=printk:* ftrace_filter=_mmc*,mmc*,sd*,dw_mci*,mci*
 ftrace=function trace_buf_size=20M dyndbg="file drivers/mmc/* +x"