Messages in this thread | | | Subject | Re: [Intel-gfx] [PATCH v7 5/8] drm_print: add choice to use dynamic debug in drm-debug | From | Tvrtko Ursulin <> | Date | Mon, 6 Sep 2021 11:20:17 +0100 |
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On 03/09/2021 22:57, jim.cromie@gmail.com wrote: > On Fri, Sep 3, 2021 at 5:15 AM Tvrtko Ursulin > <tvrtko.ursulin@linux.intel.com> wrote: >> >> >> On 31/08/2021 21:21, Jim Cromie wrote: >>> drm's debug system writes 10 distinct categories of messages to syslog >>> using a small API[1]: drm_dbg*(10 names), DRM_DEV_DEBUG*(3 names), >>> DRM_DEBUG*(8 names). There are thousands of these callsites, each >>> categorized in this systematized way. >>> >>> These callsites can be enabled at runtime by their category, each >>> controlled by a bit in drm.debug (/sys/modules/drm/parameter/debug). >>> In the current "basic" implementation, drm_debug_enabled() tests these >>> bits in __drm_debug each time an API[1] call is executed; while cheap >>> individually, the costs accumulate with uptime. >>> >>> This patch uses dynamic-debug with jump-label to patch enabled calls >>> onto their respective NOOP slots, avoiding all runtime bit-checks of >>> __drm_debug by drm_debug_enabled(). >>> >>> Dynamic debug has no concept of category, but we can emulate one by >>> replacing enum categories with a set of prefix-strings; "drm:core:", >>> "drm:kms:" "drm:driver:" etc, and prepend them (at compile time) to >>> the given formats. >>> >>> Then we can use: >>> `echo module drm format "^drm:core: " +p > control` >>> >>> to enable the whole category with one query. >> >> Probably stupid question - enabling stuff at boot time still works as >> described in Documentation/admin-guide/dynamic-debug-howto.rst? >> > > yes. its turned on in earlyinit, and cmdline args are a processed then, > and when modules are added > > >> Second question, which perhaps has been covered in the past so apologies >> if redundant - what is the advantage of allowing this to be >> configurable, versus perhaps always enabling it? Like what would be the >> reasons someone wouldn't just want to have CONFIG_DYNAMIC_DEBUG compiled >> in? Kernel binary size? >> > > Im unaware of anything on this topic, but I can opine :-) > Its been configurable since I saw it and thought "jump-labels are cool!" > > code is small > [jimc@frodo local-i915m]$ size lib/dynamic_debug.o > text data bss dec hex filename > 24016 8041 64 32121 7d79 lib/dynamic_debug.o > > Its data tables are big, particularly the __dyndbg section > builtins: > dyndbg: 108 debug prints in module mptcp > dyndbg: 2 debug prints in module i386 > dyndbg: 2 debug prints in module xen > dyndbg: 2 debug prints in module fixup > dyndbg: 7 debug prints in module irq > dyndbg: 3039 prdebugs in 283 modules, 11 KiB in ddebug tables, 166 kiB > in __dyndbg section > > bash-5.1# > bash-5.1# for m in i915 amdgpu ; do modprobe $m dyndbg=+_ ; done > dyndbg: 384 debug prints in module drm > dyndbg: 211 debug prints in module drm_kms_helper > dyndbg: 2 debug prints in module ttm > dyndbg: 8 debug prints in module video > dyndbg: 1727 debug prints in module i915 > dyndbg: processed 1 queries, with 3852 matches, 0 errs > dyndbg: 3852 debug prints in module amdgpu > [drm] amdgpu kernel modesetting enabled. > amdgpu: CRAT table disabled by module option > amdgpu: Virtual CRAT table created for CPU > amdgpu: Topology: Add CPU node > bash-5.1# > > At 56 bytes / callsite, it adds up. > And teaching DRM to use it enlarges its use dramatically, > not just in drm itself, but in many drivers > > amdgpu has 3852 callsite, (vs 3039 in my kernel), so it has ~240kb. > It has extra (large chunks generated by macros) to trim, > but i915 has ~1700, and drm has ~380 > > I have WIP to reduce the table space, by splitting it into 2 separate ones; > guts and decorations (module, function, file pointers). > The decoration recs are redundant, 45% are copies of previous > (function changes fastest) > It needs much rework, but it should get 20% overall. > decorations are 24/56 of footprint.
I'll try to extract the "executive summary" from this, you tell me if I got it right.
So using or not using dynamic debug for DRM debug ends up being about shifting the cost between kernel binary size (data section grows by each pr_debug call site) and runtime conditionals?
Since the table sizes you mention seem significant enough, I think that justifies existence of DRM_USE_DYNAMIC_DEBUG. It would probably be a good idea to put some commentary on that there. Ideally including some rough estimates both including space cost per call site and space cost for a typical distro kernel build?
Regards,
Tvrtko
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