linux-kernel - Re: ftrace: __start_mcount

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Date:	Mon, 27 Jul 2009 17:11:14 -0700
From:	John Reiser <jreiser@...wagon.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Steven Rostedt <srostedt@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Sam Ravnborg <sam@...nborg.org>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: ftrace: __start_mcount_loc should be .init.rodata

Steven Rostedt wrote:
> 
> 
> On Mon, 27 Jul 2009, John Reiser wrote:
> 
>> __start_mcount_loc[] is unused after init, yet occupies RAM forever
>> as part of .rodata.  152kiB is typical on a 64-bit architecture.  Instead,
>> __start_mcount_loc should be in the interval [__init_begin, __init_end)
>> so that the space is reclaimed after init.
> 
> Are you sure about that?

Data follows from a direct A<->B comparison.

My .config is same as Fedora Project, except for CONFIG_DEBUG_PAGEALLOC=y.
----- /var/log/messages
ftrace: allocating 18889 entries ...
	    # 18889 * 8 = 151112 bytes (147kiB) for __start_mcount_loc[]
		on 64-bit architecture
-----

With MCOUNT_REC() in original RO_DATA in include/asm-generic/vmlinux.lds.h:
===================================
----- /var/log/messages
kernel: Freeing initrd memory: 3134k freed
kernel: debug: unmapping init memory ffffffff8163d000..ffffffff81783000
	## The only message about unmapping;  length is 0x146000 or 1335296 bytes
kernel: Write protecting the kernel read-only data: 1844k   ## 148kiB more than later
-----

----- System.map
ffffffff8155c850 R __start_mcount_loc  # below unmapped range
ffffffff81581698 R __stop_mcount_loc
-----

----- "readelf --sections vmlinux" has these "\.init" strings:
  [12] __init_rodata     PROGBITS         ffffffff8155c850  0075c850  # _below_ unmapped range
  [15] .init.rodata      PROGBITS         ffffffff815f01c0  007f01c0  # _below_ unmapped range
  [25] .data.init_task   PROGBITS         ffffffff81612000  00a12000
  [28] .init.text        PROGBITS         ffffffff8163d000  00a3d000  # low end of unmapped range
  [29] .init.data        PROGBITS         ffffffff81675620  00a75620
  [30] .init.setup       PROGBITS         ffffffff8175d980  00b5d980
  [31] .initcall.init    PROGBITS         ffffffff8175eca0  00b5eca0
  [33] .x86_cpu_dev.init PROGBITS         ffffffff8175fa00  00b5fa00
  [39] .init.ramfs       PROGBITS         ffffffff81776000  00b76000  # still in unmapped range
   01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
## Note that .init.rodata is in segment 1, with .data
   03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
## Note that .init.text and .init.data are in segment 3
-----


With MCOUNT_REC() moved to INIT_DATA as patched to include/asm-generic/vmlinux.lds.h:
==================================
----- /var/log/messages:
kernel: Freeing initrd memory: 3134k freed
kernel: debug: unmapping init memory ffffffff81619000..ffffffff81784000
	## The only message about unmapping;  length is 0x16b000 or 1486848 bytes
kernel: Write protecting the kernel read-only data: 1696k   ## 148kiB less than before
-----

----- System.map
ffffffff816a28d0 T __start_mcount_loc  # within unmapped range
ffffffff816c7718 T __stop_mcount_loc
-----

----- "readelf --sections vmlinux" has these "\.init" strings:
  [14] .init.rodata      PROGBITS         ffffffff815cb1c0  007cb1c0  # _below_ unmapped range
  [24] .data.init_task   PROGBITS         ffffffff815ee000  009ee000
  [27] .init.text        PROGBITS         ffffffff81619000  00a19000  # low end of unmapped range
  [28] .init.data        PROGBITS         ffffffff81651620  00a51620
  [29] .init.setup       PROGBITS         ffffffff8175e7c0  00b5e7c0
  [30] .initcall.init    PROGBITS         ffffffff8175fae0  00b5fae0
  [32] .x86_cpu_dev.init PROGBITS         ffffffff81760840  00b60840
  [38] .init.ramfs       PROGBITS         ffffffff81777000  00b77000  # still in unmapped range
   01     .data .init.rodata .data.cacheline_aligned .data.read_mostly
   03     .data.init_task .data.page_aligned .smp_locks .init.text .init.data .init.setup .initcall.init .con_initcall.init .x86_cpu_dev.init .secu
rity_initcall.init .parainstructions .altinstructions .altinstr_replacement .exit.text .init.ramfs .data.percpu .data_nosave .bss
-----

Perusing the source:
arch/x86/mm/init_64.c: free_init_pages() called from
   ./arch/x86/kernel/alternative.c:		free_init_pages("SMP alternatives",
   ./arch/x86/mm/init_64.c:	free_init_pages("unused kernel memory",
   ./arch/x86/mm/init_64.c:	free_init_pages("initrd memory", start, end);
All other calls also are architecture-specific and in other architectures,
and also are similar to these.


>> @@ -331,7 +332,6 @@
>>  	/* __*init sections */						\
>>  	__init_rodata : AT(ADDR(__init_rodata) - LOAD_OFFSET) {		\
> 
> Isn't this in __init_rodata? Doesn't this get removed too?

In my vmlinux it is _not_ removed; it is with .rodata, not .init.rodata
(and not .init.data), and is outside (below) the removed range.

-- 
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