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

["Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx>]

On Wed, Aug 23, 2017 at 04:03:53PM -0700, Linus Torvalds wrote:
> 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.
> 
> 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.

Numbers from bigger 2-socket machine:

Unpatched:      average 5.0542s, stddev 0.058s
Patched:        average 5.0440s, stddev 0.072s

Still fine.

I don't see a reason not to go this path.

-- 
 Kirill A. Shutemov

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