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Date: Wed, 23 Aug 2017 22:59:55 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        xen-devel <xen-devel@lists.xenproject.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Jork Loeser <Jork.Loeser@microsoft.com>,
        KY Srinivasan <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        Steven Rostedt <rostedt@goodmis.org>, Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH] x86: enable RCU based table free when PARAVIRT
Message-ID: <20170823195955.wnyg2dcv4c23kdoj@node.shutemov.name>
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On Wed, Aug 23, 2017 at 11:26:46AM -0700, Linus Torvalds wrote:
> On Wed, Aug 23, 2017 at 6:45 AM, Vitaly Kuznetsov <vkuznets@redhat.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".

In this case we need performance numbers for !PARAVIRT kernel.

> 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.

Numbers for tight loop of "mmap(MAP_POPULATE); munmap()" might be
interesting too for worst case scenario.

-- 
 Kirill A. Shutemov