linux-kernel - Re: [PATCH] x86: enable RCU based table free when PARAVIRT

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Date:   Wed, 23 Aug 2017 16:03:53 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] x86: enable RCU based table free when PARAVIRT

On Wed, Aug 23, 2017 at 3:36 PM, Kirill A. Shutemov
<kirill.shutemov@...ux.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