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

From: Ingo Molnar
Date: Fri May 08 2015 - 03:58:21 EST

* Rik van Riel <riel@xxxxxxxxxx> wrote:

> On 05/07/2015 08:29 AM, Ingo Molnar wrote:
> >
> > * Frederic Weisbecker <fweisbec@xxxxxxxxx> 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.


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