Re: Add top down metrics to perf stat v2

Stephane Eranian
2015-12-17T10:28:05Z

Andi,

On Tue, Dec 15, 2015 at 4:54 PM, Andi Kleen <andi@firstfloor.org> wrote:
> Note to reviewers: includes both tools and kernel patches.
> The kernel patches are at the end.
>
> This patchkit adds support for TopDown measurements to perf stat
> It applies on top of my earlier metrics patchkit, posted
> separately, and the --metric-only patchkit (also
> posted separately)
>
> TopDown is intended to replace the frontend cycles idle/
> backend cycles idle metrics in standard perf stat output.
> These metrics are not reliable in many workloads,
> due to out of order effects.
>
> This implements a new --topdown mode in perf stat
> (similar to --transaction) that measures the pipe line
> bottlenecks using standardized formulas. The measurement
> can be all done with 5 counters (one fixed counter)
>
> The result are four metrics:
> FrontendBound, BackendBound, BadSpeculation, Retiring
>
> that describe the CPU pipeline behavior on a high level.
>
> FrontendBound and BackendBound
> BadSpeculation is a higher
>
> The full top down methology has many hierarchical metrics.
> This implementation only supports level 1 which can be
> collected without multiplexing. A full implementation
> of top down on top of perf is available in pmu-tools toplev.
> (http://github.com/andikleen/pmu-tools)
>
> The current version works on Intel Core CPUs starting
> with Sandy Bridge, and Atom CPUs starting with Silvermont.
> In principle the generic metrics should be also implementable
> on other out of order CPUs.
>
> TopDown level 1 uses a set of abstracted metrics which
> are generic to out of order CPU cores (although some
> CPUs may not implement all of them):
>
> topdown-total-slots Available slots in the pipeline
> topdown-slots-issued Slots issued into the pipeline
> topdown-slots-retired Slots successfully retired
> topdown-fetch-bubbles Pipeline gaps in the frontend
> topdown-recovery-bubbles Pipeline gaps during recovery
> from misspeculation
>
> These metrics then allow to compute four useful metrics:
> FrontendBound, BackendBound, Retiring, BadSpeculation.
>
> The formulas to compute the metrics are generic, they
> only change based on the availability on the abstracted
> input values.
>
> The kernel declares the events supported by the current
> CPU and perf stat then computes the formulas based on the
> available metrics.
>
>
> Example output:
>
> $ ./perf stat --topdown -a ./BC1s
>
> Performance counter stats for 'system wide':
>
> S0-C0 2 19650790 topdown-total-slots (100.00%)
> S0-C0 2 4445680.00 topdown-fetch-bubbles # 22.62% frontend bound (100.00%)
> S0-C0 2 1743552.00 topdown-slots-retired (100.00%)
> S0-C0 2 622954 topdown-recovery-bubbles (100.00%)
> S0-C0 2 2025498.00 topdown-slots-issued # 63.90% backend bound
> S0-C1 2 16685216540 topdown-total-slots (100.00%)
> S0-C1 2 962557931.00 topdown-fetch-bubbles (100.00%)
> S0-C1 2 4175583320.00 topdown-slots-retired (100.00%)
> S0-C1 2 1743329246 topdown-recovery-bubbles # 22.22% bad speculation (100.00%)
> S0-C1 2 6138901193.50 topdown-slots-issued # 46.99% backend bound
>
I don't see how this output could be very useful. What matters is the
percentage in the comments
and not so much the raw counts because what is the unit? Same remark
holds for the percentage.
I think you need to explain or show that this is % of issue slots and
not cycles.

> 1.535832673 seconds time elapsed
>
> $ perf stat --topdown --topdown --metric-only -I 100 ./BC1s

When I tried from your git tree the --metric-only option was not recognized.

> 0.100576098 frontend bound retiring bad speculation backend bound
> 0.100576098 8.83% 48.93% 35.24% 7.00%
> 0.200800845 8.84% 48.49% 35.53% 7.13%
> 0.300905983 8.73% 48.64% 35.58% 7.05%
> ...
>
This kind of output is more meaningful and clearer for end-users based
on my experience
and you'd like it per-core possibly.

>
>
> On Hyper Threaded CPUs Top Down computes metrics per core instead of per logical CPU.
> In this case perf stat automatically enables --per-core mode and also requires
> global mode (-a) and avoiding other filters (no cgroup mode)
>
> One side effect is that this may require root rights or a
> kernel.perf_event_paranoid=-1 setting.
>
> On systems without Hyper Threading it can be used per process.
>
> Full tree available in
> git://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc perf/top-down-11

That is in the top-down-2 branch instead, I think.

>
> No changelog against previous version. There were lots of changes.