linux-kernel - Re: [PATCH v6 28/57] drm_print: refine drm_debug

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Message-ID: <Yxg86v7UsB8jtyYi@phenom.ffwll.local>
Date:   Wed, 7 Sep 2022 08:40:42 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Jim Cromie <jim.cromie@...il.com>
Cc:     jbaron@...mai.com, gregkh@...uxfoundation.org,
        dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        daniel.vetter@...ll.ch, seanpaul@...omium.org, robdclark@...il.com,
        linux@...musvillemoes.dk, joe@...ches.com
Subject: Re: [PATCH v6 28/57] drm_print: refine drm_debug_enabled for
 jump-label

On Sun, Sep 04, 2022 at 03:41:05PM -0600, Jim Cromie wrote:
> In order to use dynamic-debug's jump-label optimization in drm-debug,
> its clarifying to refine drm_debug_enabled into 3 uses:
> 
> 1.   drm_debug_enabled - legacy, public
> 2. __drm_debug_enabled - optimized for dyndbg jump-label enablement.
> 3.  _drm_debug_enabled - pr_debug instrumented, observable
> 
> 1. The legacy version always checks the bits.
> 
> 2. is privileged, for use by __drm_dbg(), __drm_dev_dbg(), which do an
> early return unless the category is enabled.  For dyndbg builds, debug
> callsites are selectively "pre-enabled", so __drm_debug_enabled()
> short-circuits to true there.  Remaining callers of 1 may be able to
> use 2, case by case.
> 
> 3. is 1st wrapped in a macro, with a pr_debug, which reports each
> usage in /proc/dynamic_debug/control, making it observable in the
> logs.  The macro lets the pr_debug see the real caller, not an inline
> function.
> 
> When plugged into 1, 3 identified ~10 remaining callers of the
> function, leading to the follow-on cleanup patch, and would allow
> activating the pr_debugs, estimating the callrate, and the potential
> savings by using the wrapper macro.  It is unused ATM, but it fills
> out the picture.
> 
> Signed-off-by: Jim Cromie <jim.cromie@...il.com>

So instead of having 3 here as a "you need to hack it in to see what
should be converted" I have a bit a different idea: Could we make the
public version also a dyndbg callsite (like the printing wrappers), but
instead of a dynamic call we'd have a dynamically fixed value we get out?
I think that would take care of everything you have here as an open.

Otherwise I'd just drop 3 for the series we're going to merge.
-Daniel

> ---
>  drivers/gpu/drm/drm_print.c |  4 ++--
>  include/drm/drm_print.h     | 28 ++++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
> index 29a29949ad0b..cb203d63b286 100644
> --- a/drivers/gpu/drm/drm_print.c
> +++ b/drivers/gpu/drm/drm_print.c
> @@ -285,7 +285,7 @@ void __drm_dev_dbg(const struct device *dev, enum drm_debug_category category,
>  	struct va_format vaf;
>  	va_list args;
>  
> -	if (!drm_debug_enabled(category))
> +	if (!__drm_debug_enabled(category))
>  		return;
>  
>  	va_start(args, format);
> @@ -308,7 +308,7 @@ void ___drm_dbg(enum drm_debug_category category, const char *format, ...)
>  	struct va_format vaf;
>  	va_list args;
>  
> -	if (!drm_debug_enabled(category))
> +	if (!__drm_debug_enabled(category))
>  		return;
>  
>  	va_start(args, format);
> diff --git a/include/drm/drm_print.h b/include/drm/drm_print.h
> index dfdd81c3287c..7631b5fb669e 100644
> --- a/include/drm/drm_print.h
> +++ b/include/drm/drm_print.h
> @@ -321,11 +321,39 @@ enum drm_debug_category {
>  	DRM_UT_DRMRES
>  };
>  
> +/*
> + * 3 name flavors of drm_debug_enabled:
> + *   drm_debug_enabled - public/legacy, always checks bits
> + *  _drm_debug_enabled - instrumented to observe call-rates, est overheads.
> + * __drm_debug_enabled - privileged - knows jump-label state, can short-circuit
> + */
>  static inline bool drm_debug_enabled(enum drm_debug_category category)
>  {
>  	return unlikely(__drm_debug & BIT(category));
>  }
>  
> +/*
> + * Wrap fn in macro, so that the pr_debug sees the actual caller, not
> + * the inline fn.  Using this name creates a callsite entry / control
> + * point in /proc/dynamic_debug/control.
> + */
> +#define _drm_debug_enabled(category)				\
> +	({							\
> +		pr_debug("todo: maybe avoid via dyndbg\n");     \
> +		drm_debug_enabled(category);			\
> +	})
> +
> +#if defined(CONFIG_DRM_USE_DYNAMIC_DEBUG)
> +/*
> + * dyndbg is wrapping the drm.debug API, so as to avoid the runtime
> + * bit-test overheads of drm_debug_enabled() in those api calls.
> + * In this case, executed callsites are known enabled, so true.
> + */
> +#define __drm_debug_enabled(category)	true
> +#else
> +#define __drm_debug_enabled(category)	drm_debug_enabled(category)
> +#endif
> +
>  /*
>   * struct device based logging
>   *
> -- 
> 2.37.2
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch