Re: [Xen-devel] [PATCH] x86: enable RCU based table free when PARAVIRT

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:

> On Wed, Aug 23, 2017 at 3:36 PM, Kirill A. Shutemov
> <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
>>
>> Below is test cases that allocates a lot of page tables and measuare
>> fork/exit time. (I'm not entirely sure it's the best way to stress the
>> codepath.)
>
> Looks ok to me. Doing a profile (without the RCU freeing, obviously) gives me
>
>    0.77%  a.out    [kernel.vmlinux]  [k] free_pgd_range
>
>                                           ▒
>
> so it does seem to spend time in the page directory code.
>
>> Unpatched:      average 4.8322s, stddev 0.114s
>> Patched:        average 4.8362s, stddev 0.111s
>
> Ok, I vote for avoiding the complexity of two different behaviors, and
> just making the page table freeing use RCU unconditionally.

Thanks Linus & Kirill,

I actually did a microbenchmark with mmap/munmap too but wasn't able
to see any measurable performace difference.

>
> If actively trying to trigger that code doesn't show a real measurable
> difference, I don't think it matters, and the fewer different code
> paths we have, the better.

I'll send v2 enabling HAVE_RCU_TABLE_FREE on x86 unconditionally, thanks!

-- 
  Vitaly

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