[PATCH v3 4/5] perf evlist: Respect all_cpus when setting user_requested_cpus
Ian Rogers
irogers at google.com
Fri Apr 29 18:06:07 PDT 2022
On Fri, Apr 29, 2022 at 4:34 AM Adrian Hunter <adrian.hunter at intel.com> wrote:
>
> On 28/04/22 23:49, Ian Rogers wrote:
> > On Thu, Apr 28, 2022 at 1:16 PM Adrian Hunter <adrian.hunter at intel.com <mailto:adrian.hunter at intel.com>> wrote:
> >
> > On 8/04/22 06:56, Ian Rogers wrote:
> > > If all_cpus is calculated it represents the merge/union of all
> > > evsel cpu maps. By default user_requested_cpus is computed to be
> > > the online CPUs. For uncore events, it is often the case currently
> > > that all_cpus is a subset of user_requested_cpus. Metrics printed
> > > without aggregation and with metric-only, in print_no_aggr_metric,
> > > iterate over user_requested_cpus assuming every CPU has a metric to
> > > print. For each CPU the prefix is printed, but then if the
> > > evsel's cpus doesn't contain anything you get an empty line like
> > > the following on a 2 socket 36 core SkylakeX:
> > >
> > > ```
> > > $ perf stat -A -M DRAM_BW_Use -a --metric-only -I 1000
> > > 1.000453137 CPU0 0.00
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137 CPU18 0.00
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 1.000453137
> > > 2.003717143 CPU0 0.00
> > > ...
> > > ```
> > >
> > > While it is possible to be lazier in printing the prefix and
> > > trailing newline, having user_requested_cpus not be a subset of
> > > all_cpus is preferential so that wasted work isn't done elsewhere
> > > user_requested_cpus is used. The change modifies user_requested_cpus
> > > to be the intersection of user specified CPUs, or default all online
> > > CPUs, with the CPUs computed through the merge of all evsel cpu maps.
> > >
> > > New behavior:
> > > ```
> > > $ perf stat -A -M DRAM_BW_Use -a --metric-only -I 1000
> > > 1.001086325 CPU0 0.00
> > > 1.001086325 CPU18 0.00
> > > 2.003671291 CPU0 0.00
> > > 2.003671291 CPU18 0.00
> > > ...
> > > ```
> > >
> > > Signed-off-by: Ian Rogers <irogers at google.com <mailto:irogers at google.com>>
> > > ---
> > > tools/perf/util/evlist.c | 7 +++++++
> > > 1 file changed, 7 insertions(+)
> > >
> > > diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
> > > index 52ea004ba01e..196d57b905a0 100644
> > > --- a/tools/perf/util/evlist.c
> > > +++ b/tools/perf/util/evlist.c
> > > @@ -1036,6 +1036,13 @@ int evlist__create_maps(struct evlist *evlist, struct target *target)
> > > if (!cpus)
> > > goto out_delete_threads;
> > >
> > > + if (evlist->core.all_cpus) {
> > > + struct perf_cpu_map *tmp;
> > > +
> > > + tmp = perf_cpu_map__intersect(cpus, evlist->core.all_cpus);
> >
> > Isn't an uncore PMU represented as being on CPU0 actually
> > collecting data that can be due to any CPU.
> >
> >
> > This is correct but the counter is only opened on CPU0 as the all_cpus cpu_map will only contain CPU0. Trying to dump the counter for say CPU1 will fail as there is no counter there. This is why the metric-only output isn't displaying anything above.
>
> That's not what happens for me:
>
> $ perf stat -A -M DRAM_BW_Use -a --metric-only -I 1000 -- sleep 1
> # time CPU DRAM_BW_Use
> 1.001114691 CPU0 0.00
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.001114691
> 1.002265387 CPU0 0.00
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
> 1.002265387
Thanks! To be clear, getting rid of the unnecessary spew above is what
this patch set is about.
> perf stat -A -M DRAM_BW_Use -a --metric-only -I 1000 -C 1 -- sleep 1
> # time CPU DRAM_BW_Use
> 1.001100827 CPU1 0.00
> 1.002128527 CPU1 0.00
So this case I'd not been thinking about. What does it mean? We have a
metric that is computed using uncore_imc that has a cpumask of 0 on
your machine. You are requesting the data for CPU1. This feels like
user error. After this change the behavior is:
$ perf stat -A -M DRAM_BW_Use -a --metric-only -C 1 -- sleep 1
Error:
The sys_perf_event_open() syscall returned with 22 (Invalid argument)
for event (uncore_imc/cas_count_write/).
/bin/dmesg | grep -i perf may provide additional information.
That kind of goes along with this. What has actually happened is the
intersect has resulted in an empty cpu_map and so we try to program
the events for CPU -1 and that fails.
The existing behavior is to open the event for CPU1 and more than that
it succeeds in the perf_event_open for CPU 1. I find that weird. With
the CPU being 1 we have the user_requested_cpus being {1} and all_cpus
being {0} (I'm using the curlies just to say it is really a set). That
means, for example, the affinity iterator evlist__for_each_cpu [1]
will not iterate over any of the evsels as there is no CPU where the
evsel's cpu_map agrees with the all_cpu cpu_map.
Now, I can imagine it being said the existing behavior is value add.
We can move events away from the crowded CPU0 to some arbitrary CPU
and I'm sympathetic to that. I think the bug this is highlighting is
that in this case all_cpus should be {1} and not {0} as the code
currently computes (hence the empty intersect and death). This would
also fix the evlist__for_each_cpu loop.
[1] https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/util/evlist.h?h=perf/core#n353
> >
> >
> > Or for an uncore PMU represented as being on CPU0-CPU4 on a
> > 4 core 8 hyperthread processor, actually 1 PMU per core.
> >
> >
> > In this case I believe the CPU map will be CPU0, CPU2, CPU4, CPU6. To get the core counter for hyperthreads on CPU0 and CPU1 you read on CPU0, there is no counter on CPU1 and trying to read it will fail as the counters are indexed by a cpu map index into the all_cpus . Not long ago I cleaned up the cpu_map code as there was quite a bit of confusion over cpus and indexes which were both of type int.
> >
> >
> > So I am not sure intersection makes sense.
> >
> > Also it is not obvious what happens with hybrid CPUs or
> > per thread recording.
> >
> >
> > The majority of code is using all_cpus, and so is unchanged by this change.
>
> I am not sure what you mean. Every tool uses this code. It affects everything when using PMUs with their own cpus.
My point was that because we use iterators on all_cpus then the common
case is the all_cpus case, this code only affects user_requested_cpus.
Looking at raw references there are more to user_requested_cpus than
all_cpus, but I believe the main iterators in the stat code are
largely based on all_cpus. In any case this doesn't matter given what
I found below.
> > Code that is affected, when it say needs to use counters, needs to check that the user CPU was valid in all_cpus, and use the all_cpus index. The metric-only output could be fixed in the same way, ie don't display lines when the user_requested_cpu isn't in all_cpus. I prefered to solve the problem this way as it is inefficient to be processing cpus where there can be no corresponding counters, etc. We may be setting something like affinity unnecessarily - although that doesn't currently happen as that code iterates over all_cpus. I also think it is confusing from its name when the variable all_cpus is for a cpu_map that contains fewer cpus than user_requested_cpus - albeit that was worse when user_requested_cpus was called just cpus.
> >
> > It could be hybrid or intel-pt have different assumptions on these cpu_maps. I don't have access to a hybrid test system. For intel-pt it'd be great if there were a perf test. Given that most code is using all_cpus and was cleaned up as part of the cpu_map work, I believe the change to be correct.
>
> Mainly what happens if you try to intersect all_cpus with dummy cpus?
The intersect of two dummy cpu_maps is a dummy cpu_map, as with merge.
When all_cpus is computed during add:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evlist.c?h=perf/core#n72
dummy maps are treated as empty and evsel's cpumap is replaced with
the user_requested_cpus which is empty:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evlist.c?h=perf/core#n47
It will then merge this empty cpu_map into all_cpus:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/lib/perf/evlist.c?h=perf/core#n55
Rather than rely on the copy of an empty user_requested_cpus, I think
the merge and intersect functions should explicitly handle dummy as an
empty cpu_map.
This code then sets user_requested_cpus and it will intersect it with
all_cpus. This is then propagated to the empty cpu_maps in the evsels.
This shows another bug in this change:
Current behavior:
```
$ perf stat -A -M DRAM_BW_Use -e faults -a sleep 1
Performance counter stats for 'system wide':
CPU0 280.09 MiB uncore_imc/cas_count_read/ #
0.00 DRAM_BW_Use
CPU18 184.07 MiB uncore_imc/cas_count_read/ #
0.00 DRAM_BW_Use
CPU0 167.29 MiB uncore_imc/cas_count_write/
CPU18 149.95 MiB uncore_imc/cas_count_write/
CPU0 1,002,927,474 ns duration_time
CPU0 42 faults
CPU1 2 faults
CPU2 19 faults
CPU3 0 faults
CPU4 2 faults
CPU5 88 faults
CPU6 0 faults
CPU7 6 faults
CPU8 2 faults
CPU9 2 faults
CPU10 2 faults
CPU11 0 faults
CPU12 0 faults
CPU13 12 faults
CPU14 158 faults
CPU15 31 faults
CPU16 0 faults
CPU17 18 faults
CPU18 108 faults
CPU19 27 faults
CPU20 23 faults
CPU21 9 faults
CPU22 10 faults
CPU23 0 faults
CPU24 0 faults
CPU25 1 faults
CPU26 38 faults
CPU27 1 faults
CPU28 16 faults
CPU29 0 faults
CPU30 3 faults
CPU31 5 faults
CPU32 2 faults
CPU33 1 faults
CPU34 3 faults
CPU35 41 faults
1.002927474 seconds time elapsed
```
New behavior:
```
$ /tmp/perf/perf stat -A -M DRAM_BW_Use -e faults -a sleep 1
Performance counter stats for 'system wide':
CPU0 389.17 MiB uncore_imc/cas_count_write/ #
0.00 DRAM_BW_Use
CPU18 158.69 MiB uncore_imc/cas_count_write/ #
0.00 DRAM_BW_Use
CPU0 465.99 MiB uncore_imc/cas_count_read/
CPU18 242.37 MiB uncore_imc/cas_count_read/
CPU0 1,003,287,405 ns duration_time
CPU0 176 faults
CPU18 110 faults
1.003287405 seconds time elapsed
```
So I think I've come around to the idea that user_requested_cpus needs
to be the original non-intersected value for the setting of
empty/dummy evsel's cpu_map. We could add a valid_user_requested_cpus
variable, which would be the intersect of the user_requested_cpus with
all_cpus, but there seems little point.
So I still need to follow this up to fix the bug and the bugs
discovered in this thread. The main follow-up actions are:
1) modify merge to explicitly handle dummy maps - and intersect if we
still need intersect after these changes.
2) document user_requested_cpus, detail why it can have more cpus than
"all_cpus" and the behavior for dummy cpu maps
3) rename all_cpus, to perhaps merged_evsel_cpus - I want to get away
from the implication all_cpus is a super set of cpu maps like
user_requested_cpus.
4) fixup evsel cpu maps when the event will be opened on a cpu that
isn't in the cpu_map.
5) update the stat-display print_no_aggr_metric of user_requested_cpus
so that CPUs not in all_cpus/merged_evsel_cpus don't get new lines
printed.
Thanks,
Ian
> >
> > Thanks,
> > Ian
> >
> >
> > > + perf_cpu_map__put(cpus);
> > > + cpus = tmp;
> > > + }
> > > evlist->core.has_user_cpus = !!target->cpu_list && !target->hybrid;
> > >
> > > perf_evlist__set_maps(&evlist->core, cpus, threads);
> >
>
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