f861854e1b
Perf doesn't take the left period into account when auto-reload is
enabled with fixed period sampling mode in context switch.
Here is the MSR trace of the perf command as below.
(The MSR trace is simplified from a ftrace log.)
#perf record -e cycles:p -c 2000000 -- ./triad_loop
//The MSR trace of task schedule out
//perf disable all counters, disable PEBS, disable GP counter 0,
//read GP counter 0, and re-enable all counters.
//The counter 0 stops at 0xfffffff82840
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 0
write_msr: MSR_P6_EVNTSEL0(186), value 40003003c
rdpmc: 0, value fffffff82840
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
//The MSR trace of the same task schedule in again
//perf disable all counters, enable and set GP counter 0,
//enable PEBS, and re-enable all counters.
//0xffffffe17b80 (-2000000) is written to GP counter 0.
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
write_msr: MSR_IA32_PMC0(4c1), value ffffffe17b80
write_msr: MSR_P6_EVNTSEL0(186), value 40043003c
write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 1
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
When the same task schedule in again, the counter should starts from
previous left. However, it starts from the fixed period -2000000 again.
A special variant of intel_pmu_save_and_restart() is used for
auto-reload, which doesn't update the hwc->period_left.
When the monitored task schedules in again, perf doesn't know the left
period. The fixed period is used, which is inaccurate.
With auto-reload, the counter always has a negative counter value. So
the left period is -value. Update the period_left in
intel_pmu_save_and_restart_reload().
With the patch:
//The MSR trace of task schedule out
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 0
write_msr: MSR_P6_EVNTSEL0(186), value 40003003c
rdpmc: 0, value ffffffe25cbc
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
//The MSR trace of the same task schedule in again
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value 0
write_msr: MSR_IA32_PMC0(4c1), value ffffffe25cbc
write_msr: MSR_P6_EVNTSEL0(186), value 40043003c
write_msr: MSR_IA32_PEBS_ENABLE(3f1), value 1
write_msr: MSR_CORE_PERF_GLOBAL_CTRL(38f), value f000000ff
Fixes:
|
||
|---|---|---|
| Documentation | ||
| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.