linux-kernel - Re: [RFC PATCH 1/2] drm: RFC add choice to use dynamic debug in drm-debug

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Date:   Fri, 4 Dec 2020 12:20:21 -0700
From:   jim.cromie@...il.com
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     dri-devel@...ts.freedesktop.org,
        LKML <linux-kernel@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        Jason Baron <jbaron@...mai.com>,
        Thomas Zimmermann <tzimmermann@...e.de>
Subject: Re: [RFC PATCH 1/2] drm: RFC add choice to use dynamic debug in drm-debug

On Fri, Dec 4, 2020 at 8:42 AM Ville Syrjälä
<ville.syrjala@...ux.intel.com> wrote:
>
> On Thu, Dec 03, 2020 at 08:53:17PM -0700, Jim Cromie wrote:
> > drm's debug system uses distinct categories of debug messages, mapped
> > to bits in drm.debug.  Currently, code does a lot of unlikely bit-mask
> > checks on drm.debug (in drm_debug_enabled), we can use dynamic debug
> > instead, and get all that jump_label goodness.
>
> whatis jump_label?

Sorry, I should have at least capitalized that, and spelled it differently

kernel/Makefile
118:obj-$(CONFIG_JUMP_LABEL) += jump_label.o

it is the hot-patching substrate underneath it all.
static-key, static-call, etc?
dynamic-debug uses static-key directly.



>
> One thing that bugs me about the current drm_dbg() stuff is that
> it's a function, and thus we pay the cost of setting up the
> arguments even when debugs are not enabled. I played around a bit
> with making it a macro again with the unlikely bit check moved
> into the macro. That did seem to make some of the asm a bit nicer
> where the debug stuff got shoved out the main codepath, but
> it did result in a slight net increase in code size. What I didn't
> have time to do is check if this has any measurable speed effect
> on eg. TEST_ONLY commits.
>
> And while doing that I started to ponder if we could use something
> like the alternate() instruction stuff to patch the code at runtime
> in order to turn all those debug checks into nops when debugging
> is not enabled. But I couldn't immediately find any generic
> infrastructure for it. So now I wonder if this jump_label is something
> like that?
>

this is the droid youre looking for ;-)

> >

> --
> Ville Syrjälä
> Intel