On 13/02/2025 13:36, Liang, Kan wrote:
On 2025-02-12 11:13 p.m., Lucas De Marchi wrote:
On Tue, Jan 21, 2025 at 12:19:15PM -0500, Liang, Kan wrote:
On 2025-01-21 11:59 a.m., Lucas De Marchi wrote:
On Tue, Jan 21, 2025 at 10:53:31AM -0500, Liang, Kan wrote:
On 2025-01-21 9:29 a.m., Lucas De Marchi wrote:
On Mon, Jan 20, 2025 at 08:42:41PM -0500, Liang, Kan wrote:
-static int i915_pmu_cpu_offline(unsigned int cpu, struct
hlist_node
*node)
-{
- struct i915_pmu *pmu = hlist_entry_safe(node, typeof(*pmu),
cpuhp.node);
- unsigned int target = i915_pmu_target_cpu;
-
- /*
- * Unregistering an instance generates a CPU offline event
which
we must
- * ignore to avoid incorrectly modifying the shared
i915_pmu_cpumask.
- */
- if (!pmu->registered)
- return 0;
-
- if (cpumask_test_and_clear_cpu(cpu, &i915_pmu_cpumask)) {
- target = cpumask_any_but(topology_sibling_cpumask(cpu),
cpu);
-
I'm not familar with the i915 PMU, but it seems suggest a core
scope
PMU, not a system-wide scope.
counter is in a complete separate device - it doesn't depend on
core or
die or pkg - not sure why it cared about topology_sibling_cpumask
here.
OK. But it's still a behavior change. Please make it clear in the
description that the patch also changes/fixes the scope from core
scope
to system-wide.
sure... do you have a suggestion how to test the hotplug? For testing
purposes, can I force the perf cpu assigned to be something other than
the cpu0?
Yes, it's a bit tricky to verify the hotplug if the assigned CPU is
CPU0. I don't know a way to force another CPU without changing the
code.
You may have to instrument the code for the test.
Another test you may want to do is the perf system-wide test, e.g.,
perf
stat -a -e i915/actual-frequency/ sleep 1.
The existing code assumes the counter is core scope. So the result
should be huge, since perf will read the counter on each core and add
them up.
that is not allowed and it simply fails to init the counter:
static int i915_pmu_event_init(struct perf_event *event)
...
if (event->cpu < 0)
return -EINVAL;
if (!cpumask_test_cpu(event->cpu, &i915_pmu_cpumask))
return -EINVAL;
...
}
event only succeeds the initialization in the assigned cpu. I see no
differences in results (using i915/interrupts/ since freq is harder to
compare):
$ sudo perf stat -e i915/interrupts/ sleep 1
Performance counter stats for 'system wide':
253 i915/
interrupts/
1.002215175 seconds time elapsed
$ sudo perf stat -a -e i915/interrupts/ sleep 1
Performance counter stats for 'system wide':
251 i915/
interrupts/
1.000900818 seconds time elapsed
Note that our cpumask attr already returns just the assigned cpu and
perf-stat only tries to open on that cpu:
$ strace --follow -s 1024 -e perf_event_open -- perf stat -a -e i915/
interrupts/ sleep 1
[pid 55777] perf_event_open({type=0x24 /* PERF_TYPE_??? */, size=0x88 /*
PERF_ATTR_SIZE_??? */, config=0x100002, sample_period=0,
sample_type=PERF_SAMPLE_IDENTIFIER,
read_format=PERF_FORMAT_TOTAL_TIME_ENABLED|
PERF_FORMAT_TOTAL_TIME_RUNNING, disabled=1, inherit=1, precise_ip=0 /*
arbitrary skid */, exclude_guest=1, ...}, -1, 0, -1,
PERF_FLAG_FD_CLOEXEC) = 3
I see. The behavior is not changed with the patch. It should be just the
humn... the behavior doesn't change when using perf because perf will
read the cpumask and use it accordingly. However apparently now it's not
working anymore to reject calls to perf_event_open() that have a cpu
that doesn't match the cpumask.
Just like before I have this output:
$ sudo cat /sys/devices/i915/cpumask 0
However if perf_event_open() is called with cpu == 1, it succeeds.
Example:
attr_init(&attr);
perf_event_open(&attr, -1, 1, -1, 0);
I was expecting it to fail and set errno to ENODEV, but that is not the
case. For this particular system I'm seeing these values in
perf_try_init_event():
event->cpu == 1
cpumask=0-19
pmu_cpumask=0
Re-reading this: it will accept any (online) cpu of the system. Same
behavior occurs with other scopes: any cpu of that scope is accepted and
event->cpu will still keep what the user passed in (rather than the
calculated by perf_try_init_event(). Is that expected?
Yes, for a system-wide event, it can be read from any CPU. The CPU mask
in the sysfs only tells the perf tool that only 1 CPU is required to get
system-wide information. It doesn't have to be the advised CPU. It can
be any CPU in the scope.
https://lore.kernel.org/all/20240802151643.1691631-3-kan.li...@linux.intel.com/
I was asking about this during review - will it also allow for group
reads to mix cpus and if yes are there any downsides etc?
Regards,
Tvrtko
