diff mbox series

[2/2] vmlinux.lds.h: add HEADERED_SECTION_* macros

Message ID 20221117002022.860237-3-jim.cromie@gmail.com
State New
Headers
Series [1/2] vmlinux.lds.h: fix BOUNDED_SECTION_(PRE|POST)_LABEL macros |

Commit Message

Jim Cromie Nov. 17, 2022, 12:20 a.m. UTC 
  These macros elaborate on BOUNDED_SECTION_(PRE|POST)_LABEL macros,
prepending an optional KEEP(.gnu.linkonce##_sec_) reservation, and a
linker-symbol to address it.

This allows a developer to define a header struct (which must fit with
the section's base struct-type), and could contain:

1- fields whose value is common to the entire set of data-records.
   This allows the header & data structs to specialize, complement
   each other, and shrink.

2- an uplink pointer to an organizing struct
   which refs other related/sub data-tables
   header record is addressable via the extern'd header linker-symbol

Once the linker-symbols created by the macro are ref'd extern in code,
that code can compute a record's index (ptr - start) in the "primary"
table, then use it to index into the related/sub tables.  Adding a
primary.map_* field foreach sub-table would then allow deduplication
and remapping of that sub-table.

This is aimed at dyndbg's struct _ddebug __dyndbg[] section, whose 3
columns: function, file, module are 50%, 90%, 100% redundant.  The
module column is fully recoverable after dynamic_debug_init() saves it
to each ddebug_table.module as the builtin __dyndbg[] table is parsed.

Given that those 3 columns use 24/56 of a prdbg record, a dyndbg=y
kernel with ~5k callsites could save substantially.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
---
 include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

Comments

Alexander Lobakin Dec. 16, 2022, 4:25 p.m. UTC | #1 
From: Jim Cromie <jim.cromie@gmail.com>
Date: Wed, 16 Nov 2022 17:20:22 -0700

> These macros elaborate on BOUNDED_SECTION_(PRE|POST)_LABEL macros,
> prepending an optional KEEP(.gnu.linkonce##_sec_) reservation, and a
> linker-symbol to address it.
> 
> This allows a developer to define a header struct (which must fit with
> the section's base struct-type), and could contain:

[...]

> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 85d5d5b203dc..a3b6aa30a525 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -214,6 +214,21 @@
>  
>  #define BOUNDED_SECTION(_sec)	 BOUNDED_SECTION_BY(_sec, _sec)
>  
> +#define HEADERED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> +	_HDR_##_label_	= .;						\
> +	KEEP(*(.gnu.linkonce.##_sec_))					\
> +	BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_)
> +
> +#define HEADERED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> +	_label_##_HDR_ = .;						\
> +	KEEP(*(.gnu.linkonce.##_sec_))					\
> +	BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_)
> +
> +#define HEADERED_SECTION_BY(_sec_, _label_)				\
> +	HEADERED_SECTION_PRE_LABEL(_sec_, _label_, __start, __stop)

Now HEADERED_SECTION_PRE_LABEL() takes 5 arguments, but this line
passes only 4 to it. This went unnoticed probably due to that the
macro is not used anywhere, thus can't trigger a compiler error.
Would you prefer to fix it yourself or me to send the fix?[*]

> +
> +#define HEADERED_SECTION(_sec)	 HEADERED_SECTION_BY(_sec, _sec)
> +
>  #ifdef CONFIG_TRACE_BRANCH_PROFILING
>  #define LIKELY_PROFILE()						\
>  	BOUNDED_SECTION_BY(_ftrace_annotated_branch, _annotated_branch_profile)
> -- 
> 2.38.1

[*] If it needs fixing at all -- some people over the MLs say that
    if there's no trigger, then there's nothing to fix :clownface:

Thanks,
Olek
Alexander Lobakin Dec. 16, 2022, 6:11 p.m. UTC | #2 
> From: Jim Cromie <jim.cromie@gmail.com>
> Date: Wed, 16 Nov 2022 17:20:22 -0700
> 
> > These macros elaborate on BOUNDED_SECTION_(PRE|POST)_LABEL macros,
> > prepending an optional KEEP(.gnu.linkonce##_sec_) reservation, and a
> > linker-symbol to address it.
> > 
> > This allows a developer to define a header struct (which must fit with
> > the section's base struct-type), and could contain:
> 
> [...]
> 
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 85d5d5b203dc..a3b6aa30a525 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -214,6 +214,21 @@
> >  
> >  #define BOUNDED_SECTION(_sec)	 BOUNDED_SECTION_BY(_sec, _sec)
> >  

Also, those two pasting ops below:

> > +#define HEADERED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> > +	_HDR_##_label_	= .;						\
> > +	KEEP(*(.gnu.linkonce.##_sec_))					\

                             ^^

> > +	BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_)
> > +
> > +#define HEADERED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
> > +	_label_##_HDR_ = .;						\
> > +	KEEP(*(.gnu.linkonce.##_sec_))					\

                             ^^

will produce incorrect results when @_sec_ starts with a dot (not
a rare thing for section names):

	HEADERED_SECTION_BY(.entry_sites, _entry_sites)

vmlinux.map:

.entry_sites    0xffffffff83c89ca0    0x37118 load address 0x0000000003c89ca0
                0xffffffff83c89ca0                __header_entry_sites = .
 *(.gnu.linkonce. .entry_sites)

here            ^^^

 .entry_sites   0xffffffff83c89ca0    0x37118 vmlinux.o
                0xffffffff83cc0db8                __start_entry_sites = .
 *(.entry_sites)
                0xffffffff83cc0db8                __stop_entry_sites = .

^ as a result, all .entry_sites entries went to the "header" instead
of the "body".

Sorta dangerous stuff as for me, maybe that could be handled
differently?

> > +	BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_)
> > +
> > +#define HEADERED_SECTION_BY(_sec_, _label_)				\
> > +	HEADERED_SECTION_PRE_LABEL(_sec_, _label_, __start, __stop)
> 
> Now HEADERED_SECTION_PRE_LABEL() takes 5 arguments, but this line
> passes only 4 to it. This went unnoticed probably due to that the
> macro is not used anywhere, thus can't trigger a compiler error.
> Would you prefer to fix it yourself or me to send the fix?[*]
> 
> > +
> > +#define HEADERED_SECTION(_sec)	 HEADERED_SECTION_BY(_sec, _sec)
> > +
> >  #ifdef CONFIG_TRACE_BRANCH_PROFILING
> >  #define LIKELY_PROFILE()						\
> >  	BOUNDED_SECTION_BY(_ftrace_annotated_branch, _annotated_branch_profile)
> > -- 
> > 2.38.1
> 
> [*] If it needs fixing at all -- some people over the MLs say that
>     if there's no trigger, then there's nothing to fix :clownface:
> 
> Thanks,
> Olek

Thanks,
Olek
diff mbox series

Patch

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 85d5d5b203dc..a3b6aa30a525 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -214,6 +214,21 @@ 
 
 #define BOUNDED_SECTION(_sec)	 BOUNDED_SECTION_BY(_sec, _sec)
 
+#define HEADERED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
+	_HDR_##_label_	= .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
+	BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_)
+
+#define HEADERED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
+	_label_##_HDR_ = .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
+	BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_)
+
+#define HEADERED_SECTION_BY(_sec_, _label_)				\
+	HEADERED_SECTION_PRE_LABEL(_sec_, _label_, __start, __stop)
+
+#define HEADERED_SECTION(_sec)	 HEADERED_SECTION_BY(_sec, _sec)
+
 #ifdef CONFIG_TRACE_BRANCH_PROFILING
 #define LIKELY_PROFILE()						\
 	BOUNDED_SECTION_BY(_ftrace_annotated_branch, _annotated_branch_profile)