mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-12-26 08:15:03 +07:00
a032d2de0b
This is an attempt to make documentation more user friendly. Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com> Reviewed-by: Doug Smythies <dsmythies@telus.net> Reviewed-by: Chen, Yu C <yu.c.chen@intel.com> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
223 lines
10 KiB
Plaintext
223 lines
10 KiB
Plaintext
Intel P-State driver
|
||
--------------------
|
||
|
||
This driver provides an interface to control the P-State selection for the
|
||
SandyBridge+ Intel processors.
|
||
|
||
The following document explains P-States:
|
||
http://events.linuxfoundation.org/sites/events/files/slides/LinuxConEurope_2015.pdf
|
||
As stated in the document, P-State doesn’t exactly mean a frequency. However, for
|
||
the sake of the relationship with cpufreq, P-State and frequency are used
|
||
interchangeably.
|
||
|
||
Understanding the cpufreq core governors and policies are important before
|
||
discussing more details about the Intel P-State driver. Based on what callbacks
|
||
a cpufreq driver provides to the cpufreq core, it can support two types of
|
||
drivers:
|
||
- with target_index() callback: In this mode, the drivers using cpufreq core
|
||
simply provide the minimum and maximum frequency limits and an additional
|
||
interface target_index() to set the current frequency. The cpufreq subsystem
|
||
has a number of scaling governors ("performance", "powersave", "ondemand",
|
||
etc.). Depending on which governor is in use, cpufreq core will call for
|
||
transitions to a specific frequency using target_index() callback.
|
||
- setpolicy() callback: In this mode, drivers do not provide target_index()
|
||
callback, so cpufreq core can't request a transition to a specific frequency.
|
||
The driver provides minimum and maximum frequency limits and callbacks to set a
|
||
policy. The policy in cpufreq sysfs is referred to as the "scaling governor".
|
||
The cpufreq core can request the driver to operate in any of the two policies:
|
||
"performance: and "powersave". The driver decides which frequency to use based
|
||
on the above policy selection considering minimum and maximum frequency limits.
|
||
|
||
The Intel P-State driver falls under the latter category, which implements the
|
||
setpolicy() callback. This driver decides what P-State to use based on the
|
||
requested policy from the cpufreq core. If the processor is capable of
|
||
selecting its next P-State internally, then the driver will offload this
|
||
responsibility to the processor (aka HWP: Hardware P-States). If not, the
|
||
driver implements algorithms to select the next P-State.
|
||
|
||
Since these policies are implemented in the driver, they are not same as the
|
||
cpufreq scaling governors implementation, even if they have the same name in
|
||
the cpufreq sysfs (scaling_governors). For example the "performance" policy is
|
||
similar to cpufreq’s "performance" governor, but "powersave" is completely
|
||
different than the cpufreq "powersave" governor. The strategy here is similar
|
||
to cpufreq "ondemand", where the requested P-State is related to the system load.
|
||
|
||
Sysfs Interface
|
||
|
||
In addition to the frequency-controlling interfaces provided by the cpufreq
|
||
core, the driver provides its own sysfs files to control the P-State selection.
|
||
These files have been added to /sys/devices/system/cpu/intel_pstate/.
|
||
Any changes made to these files are applicable to all CPUs (even in a
|
||
multi-package system).
|
||
|
||
max_perf_pct: Limits the maximum P-State that will be requested by
|
||
the driver. It states it as a percentage of the available performance. The
|
||
available (P-State) performance may be reduced by the no_turbo
|
||
setting described below.
|
||
|
||
min_perf_pct: Limits the minimum P-State that will be requested by
|
||
the driver. It states it as a percentage of the max (non-turbo)
|
||
performance level.
|
||
|
||
no_turbo: Limits the driver to selecting P-State below the turbo
|
||
frequency range.
|
||
|
||
turbo_pct: Displays the percentage of the total performance that
|
||
is supported by hardware that is in the turbo range. This number
|
||
is independent of whether turbo has been disabled or not.
|
||
|
||
num_pstates: Displays the number of P-States that are supported
|
||
by hardware. This number is independent of whether turbo has
|
||
been disabled or not.
|
||
|
||
For example, if a system has these parameters:
|
||
Max 1 core turbo ratio: 0x21 (Max 1 core ratio is the maximum P-State)
|
||
Max non turbo ratio: 0x17
|
||
Minimum ratio : 0x08 (Here the ratio is called max efficiency ratio)
|
||
|
||
Sysfs will show :
|
||
max_perf_pct:100, which corresponds to 1 core ratio
|
||
min_perf_pct:24, max_efficiency_ratio / max 1 Core ratio
|
||
no_turbo:0, turbo is not disabled
|
||
num_pstates:26 = (max 1 Core ratio - Max Efficiency Ratio + 1)
|
||
turbo_pct:39 = (max 1 core ratio - max non turbo ratio) / num_pstates
|
||
|
||
Refer to "Intel® 64 and IA-32 Architectures Software Developer’s Manual
|
||
Volume 3: System Programming Guide" to understand ratios.
|
||
|
||
cpufreq sysfs for Intel P-State
|
||
|
||
Since this driver registers with cpufreq, cpufreq sysfs is also presented.
|
||
There are some important differences, which need to be considered.
|
||
|
||
scaling_cur_freq: This displays the real frequency which was used during
|
||
the last sample period instead of what is requested. Some other cpufreq driver,
|
||
like acpi-cpufreq, displays what is requested (Some changes are on the
|
||
way to fix this for acpi-cpufreq driver). The same is true for frequencies
|
||
displayed at /proc/cpuinfo.
|
||
|
||
scaling_governor: This displays current active policy. Since each CPU has a
|
||
cpufreq sysfs, it is possible to set a scaling governor to each CPU. But this
|
||
is not possible with Intel P-States, as there is one common policy for all
|
||
CPUs. Here, the last requested policy will be applicable to all CPUs. It is
|
||
suggested that one use the cpupower utility to change policy to all CPUs at the
|
||
same time.
|
||
|
||
scaling_setspeed: This attribute can never be used with Intel P-State.
|
||
|
||
scaling_max_freq/scaling_min_freq: This interface can be used similarly to
|
||
the max_perf_pct/min_perf_pct of Intel P-State sysfs. However since frequencies
|
||
are converted to nearest possible P-State, this is prone to rounding errors.
|
||
This method is not preferred to limit performance.
|
||
|
||
affected_cpus: Not used
|
||
related_cpus: Not used
|
||
|
||
For contemporary Intel processors, the frequency is controlled by the
|
||
processor itself and the P-State exposed to software is related to
|
||
performance levels. The idea that frequency can be set to a single
|
||
frequency is fictional for Intel Core processors. Even if the scaling
|
||
driver selects a single P-State, the actual frequency the processor
|
||
will run at is selected by the processor itself.
|
||
|
||
Tuning Intel P-State driver
|
||
|
||
When HWP mode is not used, debugfs files have also been added to allow the
|
||
tuning of the internal governor algorithm. These files are located at
|
||
/sys/kernel/debug/pstate_snb/. The algorithm uses a PID (Proportional
|
||
Integral Derivative) controller. The PID tunable parameters are:
|
||
|
||
deadband
|
||
d_gain_pct
|
||
i_gain_pct
|
||
p_gain_pct
|
||
sample_rate_ms
|
||
setpoint
|
||
|
||
To adjust these parameters, some understanding of driver implementation is
|
||
necessary. There are some tweeks described here, but be very careful. Adjusting
|
||
them requires expert level understanding of power and performance relationship.
|
||
These limits are only useful when the "powersave" policy is active.
|
||
|
||
-To make the system more responsive to load changes, sample_rate_ms can
|
||
be adjusted (current default is 10ms).
|
||
-To make the system use higher performance, even if the load is lower, setpoint
|
||
can be adjusted to a lower number. This will also lead to faster ramp up time
|
||
to reach the maximum P-State.
|
||
If there are no derivative and integral coefficients, The next P-State will be
|
||
equal to:
|
||
current P-State - ((setpoint - current cpu load) * p_gain_pct)
|
||
|
||
For example, if the current PID parameters are (Which are defaults for the core
|
||
processors like SandyBridge):
|
||
deadband = 0
|
||
d_gain_pct = 0
|
||
i_gain_pct = 0
|
||
p_gain_pct = 20
|
||
sample_rate_ms = 10
|
||
setpoint = 97
|
||
|
||
If the current P-State = 0x08 and current load = 100, this will result in the
|
||
next P-State = 0x08 - ((97 - 100) * 0.2) = 8.6 (rounded to 9). Here the P-State
|
||
goes up by only 1. If during next sample interval the current load doesn't
|
||
change and still 100, then P-State goes up by one again. This process will
|
||
continue as long as the load is more than the setpoint until the maximum P-State
|
||
is reached.
|
||
|
||
For the same load at setpoint = 60, this will result in the next P-State
|
||
= 0x08 - ((60 - 100) * 0.2) = 16
|
||
So by changing the setpoint from 97 to 60, there is an increase of the
|
||
next P-State from 9 to 16. So this will make processor execute at higher
|
||
P-State for the same CPU load. If the load continues to be more than the
|
||
setpoint during next sample intervals, then P-State will go up again till the
|
||
maximum P-State is reached. But the ramp up time to reach the maximum P-State
|
||
will be much faster when the setpoint is 60 compared to 97.
|
||
|
||
Debugging Intel P-State driver
|
||
|
||
Event tracing
|
||
To debug P-State transition, the Linux event tracing interface can be used.
|
||
There are two specific events, which can be enabled (Provided the kernel
|
||
configs related to event tracing are enabled).
|
||
|
||
# cd /sys/kernel/debug/tracing/
|
||
# echo 1 > events/power/pstate_sample/enable
|
||
# echo 1 > events/power/cpu_frequency/enable
|
||
# cat trace
|
||
gnome-terminal--4510 [001] ..s. 1177.680733: pstate_sample: core_busy=107
|
||
scaled=94 from=26 to=26 mperf=1143818 aperf=1230607 tsc=29838618
|
||
freq=2474476
|
||
cat-5235 [002] ..s. 1177.681723: cpu_frequency: state=2900000 cpu_id=2
|
||
|
||
|
||
Using ftrace
|
||
|
||
If function level tracing is required, the Linux ftrace interface can be used.
|
||
For example if we want to check how often a function to set a P-State is
|
||
called, we can set ftrace filter to intel_pstate_set_pstate.
|
||
|
||
# cd /sys/kernel/debug/tracing/
|
||
# cat available_filter_functions | grep -i pstate
|
||
intel_pstate_set_pstate
|
||
intel_pstate_cpu_init
|
||
...
|
||
|
||
# echo intel_pstate_set_pstate > set_ftrace_filter
|
||
# echo function > current_tracer
|
||
# cat trace | head -15
|
||
# tracer: function
|
||
#
|
||
# entries-in-buffer/entries-written: 80/80 #P:4
|
||
#
|
||
# _-----=> irqs-off
|
||
# / _----=> need-resched
|
||
# | / _---=> hardirq/softirq
|
||
# || / _--=> preempt-depth
|
||
# ||| / delay
|
||
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
|
||
# | | | |||| | |
|
||
Xorg-3129 [000] ..s. 2537.644844: intel_pstate_set_pstate <-intel_pstate_timer_func
|
||
gnome-terminal--4510 [002] ..s. 2537.649844: intel_pstate_set_pstate <-intel_pstate_timer_func
|
||
gnome-shell-3409 [001] ..s. 2537.650850: intel_pstate_set_pstate <-intel_pstate_timer_func
|
||
<idle>-0 [000] ..s. 2537.654843: intel_pstate_set_pstate <-intel_pstate_timer_func
|