Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [RFC PATCH 0/2] A mechanism for efficient support for per-function metrics | Date | Wed, 14 Feb 2024 01:55:23 -0800 |
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Ben Gainey <ben.gainey@arm.com> writes:
> I've been working on an approach to supporting per-function metrics for > aarch64 cores, which requires some changes to the arm_pmuv3 driver, and > I'm wondering if this approach would make sense as a generic feature > that could be used to enable the same on other architectures? > > The basic idea is as follows: > > * Periodically sample one or more counters as needed for the chosen > set of metrics. > * Record a sample count for each symbol so as to identify hot > functions. > * Accumulate counter totals for each of the counters in each of the > metrics *but* only do this where the previous sample's symbol > matches the current sample's symbol.
It sounds very similar to what perf script -F +metric already does (or did if it wasn't broken currently). It would be a straight forward extension here to add this "same as previous" check.
Of course the feature is somewhat dubious in that it will have a very strong systematic bias against short functions and even long functions in some alternating execution patterns. I assume you did some experiments to characterize this. It would be important to emphasize this in any documentation.
> For this to work efficiently, it is useful to provide a means to > decouple the sample window (time over which events are counted) from > the sample period (time between interesting samples). This patcheset > modifies the Arm PMU driver to support alternating between two > sample_period values, providing a simple and inexpensive way for tools > to separate out the sample period and the sample window. It is expected > to be used with the cycle counter event, alternating between a long and > short period and subsequently discarding the counter data for samples > with the long period. The combined long and short period gives the > overall sampling period, and the short sample period gives the sample > window. The symbol taken from the sample at the end of the long period > can be used by tools to ensure correct attribution as described > previously. The cycle counter is recommended as it provides fair > temporal distribution of samples as would be required for the > per-symbol sample count mentioned previously, and because the PMU can > be programmed to overflow after a sufficiently short window; this may > not be possible with software timer (for example). This patch does not > restrict to only the cycle counter, it is possible there could be other > novel uses based on different events.
I don't see anything ARM specific with the technique, so if it's done it should be done generically IMHO
> Cursory testing on a Xeon(R) W-2145 sampling every 300 cycles (without > the patch) suggests this approach would work for some counters. > Calculating branch miss rates for example appears to be correct, > likewise UOPS_EXECUTED.THREAD seems to give something like a sensible > cycles-per-uop value. On the other hand the fixed function instructions > counter does not appear to sample correctly (it seems to report either > very small or very large numbers). No idea whats going on there, so any > insight welcome...
If you use precise samples with 3p there is a restriction on the periods that is enforced by the kernel. Non precise or single/double p should support arbitrary, except that any p is always period + 1.
One drawback of the technique on x86 is that it won't allow multi record pebs (collecting samples without interrupts), so the overhead might be intrinsically higher.
-Andi
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