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

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Date:   Wed, 23 Aug 2017 11:26:46 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     "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>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.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 6:45 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Solve the issue by enabling RCU-based table free mechanism when PARAVIRT
> is selected in config. Testing with kernbench doesn't show any notable
> performance impact:

I wonder if we should just make it unconditional if it doesn't really
show any performance difference. One less config complexity to worry
about (and in this case I'm not so much worried about Kconfig itself,
as just "oh, you have totally different paths in the core VM depending
on PARAVIRT".

That said, the thing to test for these kinds of things is often
heavily scripted loads that just run thousands and thousands of really
small processes, and build up and tear down page tables all the time
because of fork/exit.

The load I've used occasionally is just "make test" in the git source
tree. Tons and tons of trivial fork/exec/exit things for all those
small tests and shell scripts.

I think 'kernbench' just does kernel compiles. Which is not very
kernel or VM intensive at all. It's mostly just user mode compilers in
parallel.

               Linus