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When ignore_nice_load is toggled from 0 to 1 via sysfs, dbs_update() may
run concurrently and observe the new tunable value while prev_cpu_nice
still holds a stale baseline, producing a spurious massive idle_time that
results in an incorrect CPU load value.
The race can be illustrated with two concurrent paths:
Path A (sysfs write, holds attr_set->update_lock):
governor_store()
mutex_lock(&attr_set->update_lock)
ignore_nice_load_store()
dbs_data->ignore_nice_load = 1 /* (A1) */
gov_update_cpu_data(dbs_data)
mutex_lock(&policy_dbs->update_mutex) /* (A2) */
j_cdbs->prev_cpu_nice = kcpustat_field(...)
mutex_unlock(&policy_dbs->update_mutex)
mutex_unlock(&attr_set->update_lock)
Path B (work queue, wins the race between A1 and A2):
dbs_work_handler()
mutex_lock(&policy_dbs->update_mutex) /* acquired before A2 */
dbs_update()
ignore_nice = dbs_data->ignore_nice_load /* sees new value: 1 */
cur_nice = kcpustat_field(...)
idle_time += div_u64(cur_nice - j_cdbs->prev_cpu_nice, ..) /* stale */
j_cdbs->prev_cpu_nice = cur_nice
mutex_unlock(&policy_dbs->update_mutex)
Fix this by unconditionally sampling cur_nice and advancing prev_cpu_nice
in dbs_update() on every call, regardless of ignore_nice. With
prev_cpu_nice always reflecting the most recent sample, enabling
ignore_nice_load can never produce a stale-baseline spike: the delta will
always be the nice time accumulated in the last sampling interval, not
since boot. The additional kcpustat_field() call per CPU per sample is
negligible given that the sampling path already reads idle and load
accounting.
To keep prev_cpu_nice handling consistent with the always-tracking
semantics introduced above:
- gov_update_cpu_data() unconditionally resets prev_cpu_nice alongside
prev_cpu_idle, so both baselines share the same timestamp when
io_is_busy changes. This prevents an interval mismatch between
idle_time and nice_delta on the next dbs_update() when
ignore_nice_load is enabled.
- cpufreq_dbs_governor_start() unconditionally initializes prev_cpu_nice
so the baseline is always valid from the first dbs_update() call;
remove the ignore_nice guard and the now-unused ignore_nice variable.
Fixes:
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|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .pylintrc | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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