linux-kernel - Re: [RFC patch 02/15] x86/entry: Remove _TIF_NOHZ from _TIF_WORK_SYSCALL

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Date:   Mon, 23 Sep 2019 22:43:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC patch 02/15] x86/entry: Remove _TIF_NOHZ from
 _TIF_WORK_SYSCALL_ENTRY

On Fri, 20 Sep 2019, Andy Lutomirski wrote:

> On Thu, Sep 19, 2019 at 8:09 AM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > Evaluating _TIF_NOHZ to decide whether to use the slow syscall entry path
> > is not only pointless, it's actually counterproductive:
> >
> >  1) Context tracking code is invoked unconditionally before that flag is
> >     evaluated.
> >
> >  2) If the flag is set the slow path is invoked for nothing due to #1
> 
> Can we also get rid of TIF_NOHZ on x86?

If we make the usage in context_tracking_cpu_set() conditional.

Thanks,

	tglx