From: "Gautham R. Shenoy" gautham.shenoy@amd.com
[ Upstream commit cda7ac8ce7de84cf32a3871ba5f318aa3b79381e ]
In the function mperf_start(), mperf_monitor snapshots the time, tsc and finally the aperf,mperf MSRs. However, this order of snapshotting in is reversed in mperf_stop(). As a result, the C0 residency (which is computed as delta_mperf * 100 / delta_tsc) is under-reported on CPUs that is 100% busy.
Fix this by snapshotting time, tsc and then aperf,mperf in mperf_stop() in the same order as in mperf_start().
Link: https://lore.kernel.org/r/20250612122355.19629-2-gautham.shenoy@amd.com Signed-off-by: Gautham R. Shenoy gautham.shenoy@amd.com Signed-off-by: Shuah Khan skhan@linuxfoundation.org Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
**Backport Status: YES**
This commit should be backported to stable kernel trees for the following reasons:
1. **Clear bug fix with user impact**: The commit fixes a calculation bug in the cpupower tool where C0 residency (processor active time) is under-reported on 100% busy CPUs. This directly affects users monitoring CPU performance and power states.
2. **Simple and contained fix**: The change is minimal - it only reorders three function calls in `mperf_stop()` to match the order in `mperf_start()`: - Before: `mperf_measure_stats()` → `mperf_get_tsc()` → `clock_gettime()` - After: `clock_gettime()` → `mperf_get_tsc()` → `mperf_measure_stats()`
3. **Low risk of regression**: The fix simply ensures consistent snapshot ordering between start and stop measurements. This is a logical correction that aligns the measurement methodology without introducing new functionality.
4. **Clear root cause**: The bug occurs because the reversed snapshot order causes timing skew in the C0 residency calculation (`delta_mperf * 100 / delta_tsc`). When snapshots are taken in different orders at start vs stop, the time windows don't align properly, leading to incorrect calculations.
5. **Affects monitoring tool accuracy**: While this is a userspace tool bug rather than a kernel bug, cpupower is an important diagnostic tool shipped with the kernel. Incorrect C0 residency reporting could mislead system administrators and developers analyzing CPU performance.
6. **No architectural changes**: The fix doesn't change any data structures, APIs, or fundamental behavior - it simply corrects the measurement sequence.
The commit meets stable tree criteria as it fixes a real bug affecting users, is minimal in scope, and has very low risk of introducing new issues.
tools/power/cpupower/utils/idle_monitor/mperf_monitor.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c b/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c index 08a399b0be28..6ab9139f16af 100644 --- a/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c +++ b/tools/power/cpupower/utils/idle_monitor/mperf_monitor.c @@ -240,9 +240,9 @@ static int mperf_stop(void) int cpu;
for (cpu = 0; cpu < cpu_count; cpu++) { - mperf_measure_stats(cpu); - mperf_get_tsc(&tsc_at_measure_end[cpu]); clock_gettime(CLOCK_REALTIME, &time_end[cpu]); + mperf_get_tsc(&tsc_at_measure_end[cpu]); + mperf_measure_stats(cpu); }
return 0;