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Date:   Sun, 21 Apr 2019 20:28:42 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, hpa@zytor.com,
        linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] x86_64: uninline TASK_SIZE
Message-ID: <20190421182842.GD35603@gmail.com>
References: <20190421160600.GA31092@avx2>
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* Alexey Dobriyan <adobriyan@gmail.com> wrote:

> TASK_SIZE macro is quite deceptive: it looks like a constant but in fact
> compiles to 50+ bytes.
> 
> Space savings on x86_64 defconfig:
> 
> add/remove: 1/0 grow/shrink: 3/24 up/down: 77/-2247 (-2170)
> Function                                     old     new   delta
> _task_size                                     -      52     +52
> mpol_shared_policy_init                      344     363     +19
> shmem_get_unmapped_area                       92      97      +5
> __rseq_handle_notify_resume.cold              34      35      +1
> copy_from_user_nmi                           123     113     -10
> mmap_address_hint_valid                       92      56     -36
> arch_get_unmapped_area_topdown               471     435     -36
> tlb_gather_mmu                               164     126     -38
> hugetlb_get_unmapped_area                    774     736     -38
> __create_xol_area                            497     458     -39
> arch_tlb_gather_mmu                          160     120     -40
> setup_new_exec                               380     336     -44
> __x64_sys_mlockall                           378     333     -45
> __ia32_sys_mlockall                          378     333     -45
> tlb_flush_mmu                                235     189     -46
> unmap_page_range                            2098    2048     -50
> copy_mount_options                           518     465     -53
> __get_user_pages                            1737    1675     -62
> get_unmapped_area                            270     204     -66
> perf_prepare_sample                         1176    1098     -78
> perf_callchain_user                          549     469     -80
> mremap_to.isra                               545     457     -88
> arch_tlb_finish_mmu                          394     305     -89
> __do_munmap                                 1039     927    -112
> elf_map                                      527     409    -118
> prctl_set_mm                                1509    1335    -174
> __rseq_handle_notify_resume                 1116     906    -210
> load_elf_binary                            11761   11111    -650
> Total: Before=14121337, After=14119167, chg -0.02%
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> ---
> 
>  arch/x86/include/asm/processor.h |    4 ++--
>  arch/x86/kernel/Makefile         |    1 +
>  arch/x86/kernel/task_size_64.c   |    9 +++++++++
>  3 files changed, 12 insertions(+), 2 deletions(-)
> 
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -887,8 +887,8 @@ static inline void spin_lock_prefetch(const void *x)
>  
>  #define TASK_SIZE_LOW		(test_thread_flag(TIF_ADDR32) ? \
>  					IA32_PAGE_OFFSET : DEFAULT_MAP_WINDOW)
> -#define TASK_SIZE		(test_thread_flag(TIF_ADDR32) ? \
> -					IA32_PAGE_OFFSET : TASK_SIZE_MAX)
> +unsigned long _task_size(void);
> +#define TASK_SIZE		_task_size()
>  #define TASK_SIZE_OF(child)	((test_tsk_thread_flag(child, TIF_ADDR32)) ? \
>  					IA32_PAGE_OFFSET : TASK_SIZE_MAX)
>  
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -46,6 +46,7 @@ CFLAGS_irq.o := -I$(src)/../include/asm/trace
>  
>  obj-y			:= process_$(BITS).o signal.o
>  obj-$(CONFIG_COMPAT)	+= signal_compat.o
> +obj-$(CONFIG_X86_64)	+= task_size_64.o
>  obj-y			+= traps.o idt.o irq.o irq_$(BITS).o dumpstack_$(BITS).o
>  obj-y			+= time.o ioport.o dumpstack.o nmi.o
>  obj-$(CONFIG_MODIFY_LDT_SYSCALL)	+= ldt.o
> new file mode 100644
> --- /dev/null
> +++ b/arch/x86/kernel/task_size_64.c
> @@ -0,0 +1,9 @@
> +#include <linux/export.h>
> +#include <linux/sched.h>
> +#include <linux/thread_info.h>
> +
> +unsigned long _task_size(void)
> +{
> +	return test_thread_flag(TIF_ADDR32) ? IA32_PAGE_OFFSET : TASK_SIZE_MAX;
> +}
> +EXPORT_SYMBOL(_task_size);

Good idea - but instead of adding yet another compilation unit, why not 
stick _task_size() into arch/x86/kernel/process_64.c, which is the 
canonical place for process management related arch functions?

Thanks,

	Ingo