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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
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Subject: Re: [PATCH] x86: enable RCU based table free when PARAVIRT
References: <20170823134521.5068-1-vkuznets@redhat.com>
        <CA+55aFzFnbnFnY0ht15aOY7jq4Kyy-uzP9DAyswH6UaU6UhF4g@mail.gmail.com>
        <20170823195955.wnyg2dcv4c23kdoj@node.shutemov.name>
        <CA+55aFxDwWgMQa2HGfgWKOxqfepiBu5XVpGj3VJ=f53a=w0kpA@mail.gmail.com>
        <20170823223637.bjke4w3wpolrn7md@black.fi.intel.com>
        <CA+55aFy5M-0Ru-SMoyymewffawyMs8cS2TzjodyjbAtXBp24xw@mail.gmail.com>
Date: Thu, 24 Aug 2017 10:47:41 +0200
In-Reply-To: <CA+55aFy5M-0Ru-SMoyymewffawyMs8cS2TzjodyjbAtXBp24xw@mail.gmail.com>
        (Linus Torvalds's message of "Wed, 23 Aug 2017 16:03:53 -0700")
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Linus Torvalds <torvalds@linux-foundation.org> writes:

> 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.

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