Re: ftrace: __start_mcount_loc should be .init.rodata

Steven Rostedt
2009-08-04T18:19:20Z

On Mon, 27 Jul 2009, John Reiser wrote:

> 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.

Thanks for this detailed explanation, I'll pull your patch in an run it
through my tests, and then push it off.

-- Steve