Messages in this thread | | | Date | Wed, 14 Jul 2010 19:11:17 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [patch 1/2] x86_64 page fault NMI-safe |
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* 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. http://www.efficios.com
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