Re: [patch 1/2] x86_64 page fault NMI-safe

Mathieu Desnoyers
2010-07-14T23:11:21Z

* Frederic Weisbecker (fweisbec@gmail.com) wrote:
> On Wed, Jul 14, 2010 at 06:31:07PM -0400, Mathieu Desnoyers wrote:
> > * Frederic Weisbecker (fweisbec@gmail.com) wrote:
> > > On Wed, Jul 14, 2010 at 12:54:19PM -0700, Linus Torvalds wrote:
> > > > On Wed, Jul 14, 2010 at 12:36 PM, Frederic Weisbecker
> > > > <fweisbec@gmail.com> wrote:
> > > > >
> > > > > There is also the fact we need to handle the lost NMI, by defering its
> > > > > treatment or so. That adds even more complexity.
> > > >
> > > > I don't think your read my proposal very deeply. It already handles
> > > > them by taking a fault on the iret of the first one (that's why we
> > > > point to the stack frame - so that we can corrupt it and force a
> > > > fault).
> > >
> > >
> > > Ah right, I missed this part.
> >
> > Hrm, Frederic, I hate to ask that but.. what are you doing with those percpu 8k
> > data structures exactly ? :)
> >
> > Mathieu
>
>
>
> So, when an event triggers in perf, we sometimes want to capture the stacktrace
> that led to the event.
>
> We want this stacktrace (here we call that a callchain) to be recorded
> locklessly. So we want this callchain buffer per cpu, with the following
> type:

Ah OK, so you mean that perf now has 2 different ring buffer implementations ?
How about using a single one that is generic enough to handle perf and ftrace
needs instead ?

(/me runs away quickly before the lightning strikes) ;)

Mathieu

>
> #define PERF_MAX_STACK_DEPTH 255
>
> struct perf_callchain_entry {
> __u64 nr;
> __u64 ip[PERF_MAX_STACK_DEPTH];
> };
>
>
> That makes 2048 bytes. But per cpu is not enough for the callchain to be recorded
> locklessly, we also need one buffer per context: task, softirq, hardirq, nmi, as
> an event can trigger in any of these.
> Since we disable preemption, none of these contexts can nest locally. In
> fact hardirqs can nest but we just don't care about this corner case.
>
> So, it makes 2048 * 4 = 8192 bytes. And that per cpu.
>

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.