Re: [RFC PATCH 1/3] Unified trace buffer

[Mathieu Desnoyers]

* Linus Torvalds (torvalds@xxxxxxxxxxxxxxxxxxxx) wrote:
> 
> 
> On Thu, 25 Sep 2008, Mathieu Desnoyers wrote:
> > 
> > We could use a page header instead to contain the "unused_size"
> > information.
> 
> Absolutely. There's no one way to do this.
> 
> > I would prefer to put the extended timestamp within the event header
> > instead of creating a separate entry for this for atomicity concerns
> > (what happens if a long interrupt executes between the TSCExtend marker
> > event and the event expecting to be written right next to it ?).
> 
> The log entries should be reserved with interrupts disabled anyway, and 
> they are per-CPU, so there are no atomicity issues.
> 

I actually do use a lockless algorithm in LTTng and don't have to
disable interrupts around tracing. This is how I get the kind of
performance the Google folks expect. I would recommend to stay with
interrupt disable + per-cpu spinlock (slow and heavy locking) for v1,
but to keep in mind that we might want to go for a more lightweight
locking scheme in v2.

> For NMI's, things get more exciting. I'd really prefer NMI's to go to a 
> separate ring buffer entirely, because otherwise consistency gets really 
> hard. Using lockless algorithms for a variable-sized pool of pages is a 
> disaster waiting to happen.
> 

LTTng does it, no disaster happened in the past 2-3 years. :)

I guess we could manage to deal with NMI tracing specfically using the
in_nmi() helpers.

> I don't think we can currently necessarily reasonably trace NMI's, but 
> it's something to keep in mind as required support eventually.
> 

NMI tracing is a nice-to-have (and lttng does provide it), but the core
thing is really performance; disabling interrupts happens to be fairly
slow on many architectures.

Mathieu

> 		Linus
> 

-- 
Mathieu Desnoyers
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