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From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: Stephen Boyd <sboyd@kernel.org>,
	John Stultz <john.stultz@linaro.org>,
	michael-git@rodin.online, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] arch/x86/entry/vsyscall/vsyscall_gtod.c: remove __read_mostly from vclocks_used
Date: Sat, 23 Jun 2018 11:04:25 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1806231057040.8650@nanos.tec.linutronix.de> (raw)
In-Reply-To: <CALCETrVDuqie6WS--gyb9ohEKXmreA+iN5P-raJbnfS7Tp8nkQ@mail.gmail.com>

On Fri, 22 Jun 2018, Andy Lutomirski wrote:
> On Fri, Jun 22, 2018 at 3:47 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Mon, 4 Jun 2018, Michael Rodin wrote:
> >
> > > The variable "vclocks_used" doesn't appear to be "read mostly".
> > > Measurements of the access frequency with perf stat [1] and
> > > perf report show, that approximately half of the accesses to
> > > this variable are write accesses and happen in update_vsyscall()
> > > in the file arch/x86/entry/vsyscall/vsyscall_gtod.c.
> > > The measurements were done with the kernel 4.13.0-43-generic used by
> > > ubuntu as well as with the stable kernel 4.16.7 with a custom config.
> > > I've used "perf bench sched" and iperf3 as workloads.
> > >
> > > This was discovered during my master thesis in the CADOS project [2].
> >
> > Nice find, but ...
> >
> > The point is that the content of that variable changes once in a blue moon,
> > so the intent of marking it read_mostly is almost correct.
> 
> I would propose a rather different fix.  Add a an
> arch_change_clocksource() function.  Do:
> 
> static inline void arch_change_clocksource(struct clocksource
> *new_clocksource) { ... }
> #define arch_change_clocksource arch_change_clocksource
> 
> and
> 
> #ifndef arch_change_clocksource
> static inline void arch_change_clocksource(struct clocksource
> *new_clocksource) {}
> #endif
>
> in the generic header.  In change_clocksource(), add a call to
> arch_change_clocksource() right after tk_setup_internals().  In x86's
> arch_change_clocksource, update vclocks_used.
> 
> Now it's genuinely read_mostly, and we don't need to touch that
> cacheline at all in the normal clock tick code.  Everyone wins.
> (vclocks_used is actually rather rarely read.  It's only used to
> prevent user code from accessing a never-used clocksource through the
> vvar area, which is a hardening measure.  It's only referenced from
> the vvar fault handler code.)

Agreed.

Thanks,

	tglx

      reply	other threads:[~2018-06-23  9:04 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-06-04 17:27 Michael Rodin
2018-06-22 22:47 ` Thomas Gleixner
2018-06-22 23:25   ` Andy Lutomirski
2018-06-23  9:04     ` Thomas Gleixner [this message]

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