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SubjectRe: irq-disabled vs vmap vs text_poke
Nick Piggin wrote:
> On Mon, Feb 16, 2009 at 09:00:35PM -0500, Masami Hiramatsu wrote:
>> Mathieu Desnoyers wrote:
>>> * Nick Piggin ( 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)
> Hmm, on bigger SMP systems, I think the global TLB flush required
> for unmap_kernel_range will reverse these numbers.

Sure, that's possible. unfortunately, I don't have that bigger machine...
It's just the result on 4-core smp machine.

>> 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.
> It is not a bad idea, but I don't think it quite goes far enough.
> IMO we should reserve 2 pages of virtual memory for each CPU, and
> then do the mapping/unmapping without locking, and with another
> variant of unmap_kernel_range that does not do the global TLB
> flush.
> Unless performance doesn't really matter much, in which case, I
> guess your patch is nice because it avoids doing the allocations.

Thanks, I think text_poke() doesn't need high performance currently,
because it's not called so frequently, nor from the normal operation.

However, Would dynamic ftrace need performance?

Thank you,

Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division


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