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Message-ID: <499A1A43.6000009@redhat.com>
Date: Mon, 16 Feb 2009 21:00:35 -0500
From: Masami Hiramatsu <mhiramat@...hat.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
Nick Piggin <npiggin@...e.de>
CC: Peter Zijlstra <peterz@...radead.org>,
akpm <akpm@...ux-foundation.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>,
Ananth N Mavinakayanahalli <ananth@...ibm.com>,
Jim Keniston <jkenisto@...ibm.com>
Subject: Re: irq-disabled vs vmap vs text_poke
Mathieu Desnoyers wrote:
> * Nick Piggin (npiggin@...e.de) wrote:
>> On Mon, Feb 16, 2009 at 10:04:43AM -0500, Masami Hiramatsu wrote:
>>>>>>>> BTW, what about using map_vm_area() in text_poke() instead of vmap()?
>>>>>>>> Since text_poke() just maps text pages to alias pages temporarily,
>>>>>>>> I think we don't need to use delayed vunmap().
>> [...]
>>
>>> Here is the patch which replace v(un)map with (un)map_vm_area.
>> I don't quite understand the point of this... delayed vunmap() is
>> just an implementation detail of vmap subsystem. Callers should not
>> have to care.
>>
>
> AFAIK, map_vm_area/unmap_vm_area is faster than vmap/vunmap. This is
> the point of this patch. Masami, could you provide a quick benchmark of
> text_poke()/seconds before and after this optimization is applied to
> confirm this ?
Sure, here is the result of calling text_poke() 2^14 times.
<Without this patch>
Total: 3634133356(cycles), 221809(cycles/text_poke)
Total: 3699532690(cycles), 225801(cycles/text_poke)
Total: 3249855588(cycles), 198355(cycles/text_poke)
<With this patch>
Total: 483467579(cycles), 29508(cycles/text_poke)
Total: 497441301(cycles), 30361(cycles/text_poke)
Total: 497604548(cycles), 30371(cycles/text_poke)
BTW, this is not only for performance, but also simplicity and its need.
Vmap may allocate new vm_area. However, since text_poke() just needs to
map pages temporarily (yeah, very short time), we don't want to call
kmalloc or any other memory allocators.
And since text_poke() makes WRITABLE aliases of READ-ONLY pages, we
want to purge these pages ASAP.
So, I think just reserving a small vm_area for text_poke() and
reusing it is enough.
Thanks,
>
> Thanks,
>
> Mathieu
>
>>
>>> Signed-off-by: Masami Hiramatsu <mhiramat@...hat.com>
>>> ---
>>> arch/x86/include/asm/alternative.h | 1 +
>>> arch/x86/kernel/alternative.c | 25 ++++++++++++++++++++-----
>>> init/main.c | 3 +++
>>> 3 files changed, 24 insertions(+), 5 deletions(-)
>>>
>>> Index: linux-2.6/arch/x86/include/asm/alternative.h
>>> ===================================================================
>>> --- linux-2.6.orig/arch/x86/include/asm/alternative.h
>>> +++ linux-2.6/arch/x86/include/asm/alternative.h
>>> @@ -177,6 +177,7 @@ extern void add_nops(void *insns, unsign
>>> * The _early version expects the memory to already be RW.
>>> */
>>>
>>> +extern void text_poke_init(void);
>>> extern void *text_poke(void *addr, const void *opcode, size_t len);
>>> extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>>>
>>> Index: linux-2.6/arch/x86/kernel/alternative.c
>>> ===================================================================
>>> --- linux-2.6.orig/arch/x86/kernel/alternative.c
>>> +++ linux-2.6/arch/x86/kernel/alternative.c
>>> @@ -485,6 +485,16 @@ void *text_poke_early(void *addr, const
>>> return addr;
>>> }
>>>
>>> +static struct vm_struct *text_poke_area[2];
>>> +static DEFINE_SPINLOCK(text_poke_lock);
>>> +
>>> +void __init text_poke_init(void)
>>> +{
>>> + text_poke_area[0] = get_vm_area(PAGE_SIZE, VM_ALLOC);
>>> + text_poke_area[1] = get_vm_area(2 * PAGE_SIZE, VM_ALLOC);
>>> + BUG_ON(!text_poke_area[0] || !text_poke_area[1]);
>>> +}
>>> +
>>> /**
>>> * text_poke - Update instructions on a live kernel
>>> * @addr: address to modify
>>> @@ -501,8 +511,9 @@ void *__kprobes text_poke(void *addr, co
>>> unsigned long flags;
>>> char *vaddr;
>>> int nr_pages = 2;
>>> - struct page *pages[2];
>>> - int i;
>>> + struct page *pages[2], **pgp = pages;
>>> + int i, ret;
>>> + struct vm_struct *vma;
>>>
>>> if (!core_kernel_text((unsigned long)addr)) {
>>> pages[0] = vmalloc_to_page(addr);
>>> @@ -515,12 +526,16 @@ void *__kprobes text_poke(void *addr, co
>>> BUG_ON(!pages[0]);
>>> if (!pages[1])
>>> nr_pages = 1;
>>> - vaddr = vmap(pages, nr_pages, VM_MAP, PAGE_KERNEL);
>>> - BUG_ON(!vaddr);
>>> + spin_lock(&text_poke_lock);
>>> + vma = text_poke_area[nr_pages-1];
>>> + ret = map_vm_area(vma, PAGE_KERNEL, &pgp);
>>> + BUG_ON(ret);
>>> + vaddr = vma->addr;
>>> local_irq_save(flags);
>>> memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
>>> local_irq_restore(flags);
>>> - vunmap(vaddr);
>>> + unmap_kernel_range((unsigned long)vma->addr, (unsigned
>>> long)vma->size);
>>> + spin_unlock(&text_poke_lock);
>>> sync_core();
>>> /* Could also do a CLFLUSH here to speed up CPU recovery; but
>>> that causes hangs on some VIA CPUs. */
>>> Index: linux-2.6/init/main.c
>>> ===================================================================
>>> --- linux-2.6.orig/init/main.c
>>> +++ linux-2.6/init/main.c
>>> @@ -676,6 +676,9 @@ asmlinkage void __init start_kernel(void
>>> taskstats_init_early();
>>> delayacct_init();
>>>
>>> +#ifdef CONFIG_X86
>>> + text_poke_init();
>>> +#endif
>>> check_bugs();
>>>
>>> acpi_early_init(); /* before LAPIC and SMP init */
>>>
>>> --
>>> Masami Hiramatsu
>>>
>>> Software Engineer
>>> Hitachi Computer Products (America) Inc.
>>> Software Solutions Division
>>>
>>> e-mail: mhiramat@...hat.com
>
--
Masami Hiramatsu
Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division
e-mail: mhiramat@...hat.com
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