Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

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
Date: Fri, 8 May 2015 09:58:06 +0200	[thread overview]
Message-ID: <20150508075806.GA5537@gmail.com> (raw)
In-Reply-To: <554B891A.9040109@redhat.com>

* 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