Date: Fri, 8 May 2015 09:58:06 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Rik van Riel <riel@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, williams@redhat.com,
        Andrew Lutomirski <luto@kernel.org>, fweisbec@redhat.com,
        Peter Zijlstra <peterz@infradead.org>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable
 & enable from context tracking on syscall entry
Message-ID: <20150508075806.GA5537@gmail.com>
References: <20150501064044.GA18957@gmail.com>
 <554399D1.6010405@redhat.com>
 <1430659432.4233.3.camel@gmail.com>
 <55465B2D.6010300@redhat.com>
 <CALCETrWj35bCgit7Z516+Pk+GrM_rpvFQsJ5t2fF=5722HjXiQ@mail.gmail.com>
 <55466E72.8060602@redhat.com>
 <20150507104845.GB14924@gmail.com>
 <20150507121848.GB32271@lerouge>
 <20150507122913.GA17443@gmail.com>
 <554B891A.9040109@redhat.com>
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* Rik van Riel <riel@redhat.com> wrote:

> On 05/07/2015 08:29 AM, Ingo Molnar wrote:
> > 
> > * Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > 
> >>>> We cannot take the lock_trace(task) from irq context, and we 
> >>>> probably do not need to anyway, since we do not care about a 
> >>>> precise stack trace for the task.
> >>>
> >>> So one worry with this and similar approaches of statistically 
> >>> detecting user mode would be the fact that on the way out to 
> >>> user-space we don't really destroy the previous call trace - we 
> >>> just pop off the stack (non-destructively), restore RIPs and are 
> >>> gone.
> >>>
> >>> We'll need that percpu flag I suspect.
> >>
> >> Note we have the context tracking state which tells where the 
> >> current task is: user/system/guest.
> > 
> > Yes, but that overhead is what I'm suggesting we get rid of, I thought 
> > Rik was trying to find a mechanism that would be independent of that?
> 
> One thing at a time :)
> 
> I am working on the timer sampling stuff, which should be easy to 
> adapt to a different user/system/guest/irq/softirq/... tracking 
> thing, if somebody else comes up with a more efficient way to do 
> that.

So if you make the timer sampling use a percpu variable, and set that 
variable from the existing callbacks, then we could do this gradually: 
first the timer sampling uses the flag, then RCU could use it, and 
finally we could push it out to minimal assembly code.

But it's important to start out with a percpu flag to track this all.

Thanks,

	Ingo
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