On Wed, Apr 21, 2010 at 2:11 PM, Peter Zijlstra <peterz@infradead.org> wrote:
I wonder why SAMPLE_RAW went in in the first place, then. What was the
justification for it, traces?
Okay, so you're suggesting everything is exposed via PERF_SAMPLE_REGS.
PEBS does capture machine state which is easily mapped onto PERF_SAMPLE_REGS.
Well, that's until you look at PEB-LL on Nehalem where is captures
latencies and data
addresses.
IBS does not capture machine state in the sense of general purpose registers.
IBS captures micro-architectural info about an instruction and stores
this into a
handful of IBS registers. Those could be funneled through PERF_SAMPLE_REGS
as well, I believe. But that means, PERF_SAMPLE_REGS would need some
configuration
bitmask to name the registers of interest, e.g. EAX, EDX, IBSOP_DATA,
IBSOP_PHYSAD,
and so on.
As you pointed out a long time ago, IBS returns to much information to
be abstracted
easily unless you're willing to drop part of the information it
returns, e.g., concentrate
on cache miss info only. But this goes back to a point I made early
on: there are
many usage models, different metrics need different data. You don't
want to prevent
any usage model.
By construction it is given that it tracks an instruction. Thus, the
IP is always
that of the monitored instruction: no skid. The difference, though, it
that is not
associated with an actual counter+event. IBS keeps its own counter. You can
easily create a pseudo event (which is what Robert does in his patch).
I think we can avoid model_spec.
Concept is simple: track an instruction (uop) as it traverses the pipeline and
gather all sort of micro-architectural info. When the instruction retires,
interrupt the CPU and deliver the info via registers. The current implementation
is not without problems, but you can already gather useful info such as instrs
latencies, data cache misses.
Speaking of data cache misses, I believe there may be a way to abstract
sampling on cache misses, i.e, capture where they occur, that would work
with both IBS and PEBS-LL. That may be a good way to start exposing
some of the IBS features.
That is fixed now.
--
