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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Wed, 23 Aug 2017 16:03:53 -0700
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Subject: Re: [PATCH] x86: enable RCU based table free when PARAVIRT
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
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On Wed, Aug 23, 2017 at 3:36 PM, Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> 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.

              Linus