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

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Message-ID: <20170824084746.brbfcth76yew6lr2@node.shutemov.name>
Date:   Thu, 24 Aug 2017 11:47:46 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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 04:03:53PM -0700, Linus Torvalds wrote:
> 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.

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