[PATCH v5] x86: intel_epb: Add earlyparam option to keep bias at performance

Jack Allister
2024-01-03T14:46:23Z

Buggy BIOSes may not set a sane boot-time Energy Performance Bias (EPB).
A result of this may be overheating or excess power usage. The kernel
overrides any boot-time EPB "performance" bias to "normal" to avoid this.

When used in data centers it is preferable keep the EPB at "performance"
when performing a live-update of the host kernel via a kexec to the new
kernel. This is due to boot-time being critical when performing the kexec
as running guest VMs will perceieve this as latency or downtime.

On Intel Xeon Ice Lake platforms it has been observed that a combination of
EPB being set to "normal" alongside HWP (Intel Hardware P-states) being
enabled/configured during or close to the kexec causes an increases the
live-update/kexec downtime by 7 times compared to when the EPB is set to
"performance".

Introduce a command-line parameter, "intel_epb=preserve", to skip the
"performance" -> "normal" override/workaround. This maintains prior
functionality when no parameter is set, but adds in the ability to stay at
performance for a speedy kexec if a user wishes.

Signed-off-by: Jack Allister <jalliste@amazon.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Cc: Paul Durrant <pdurrant@amazon.com>
Cc: Jue Wang <juew@amazon.com>
Cc: Usama Arif <usama.arif@bytedance.com>
---
.../admin-guide/kernel-parameters.txt | 12 +++++++++++
arch/x86/kernel/cpu/intel_epb.c | 21 +++++++++++++++++--
2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 65731b060e3f..5602ee213115 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2148,6 +2148,18 @@
0 disables intel_idle and fall back on acpi_idle.
1 to 9 specify maximum depth of C-state.

+ intel_epb= [X86]
+ auto
+ Same as not passing a parameter to intel_epb. This will
+ ensure that the intel_epb module will restore the energy
+ performance bias to "normal" at boot-time. This workaround
+ is for buggy BIOSes which may not set this value and cause
+ either overheating or excess power usage.
+ preserve
+ At kernel boot-time if the EPB value is read as "performance"
+ keep it at this value. This prevents the "performance" -> "normal"
+ transition which is a workaround mentioned above.
+
intel_pstate= [X86]
disable
Do not enable intel_pstate as the default
diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
index e4c3ba91321c..419e699a43e6 100644
--- a/arch/x86/kernel/cpu/intel_epb.c
+++ b/arch/x86/kernel/cpu/intel_epb.c
@@ -50,7 +50,8 @@
* the OS will do that anyway. That sometimes is problematic, as it may cause
* the system battery to drain too fast, for example, so it is better to adjust
* it on CPU bring-up and if the initial EPB value for a given CPU is 0, the
- * kernel changes it to 6 ('normal').
+ * kernel changes it to 6 ('normal'). However, if it is desirable to retain the
+ * original initial EPB value, intel_epb=preserve can be set to enforce it.
*/

static DEFINE_PER_CPU(u8, saved_epb);
@@ -75,6 +76,8 @@ static u8 energ_perf_values[] = {
[EPB_INDEX_POWERSAVE] = ENERGY_PERF_BIAS_POWERSAVE,
};

+static bool intel_epb_no_override __read_mostly;
+
static int intel_epb_save(void)
{
u64 epb;
@@ -106,7 +109,7 @@ static void intel_epb_restore(void)
* ('normal').
*/
val = epb & EPB_MASK;
- if (val == ENERGY_PERF_BIAS_PERFORMANCE) {
+ if (!intel_epb_no_override && val == ENERGY_PERF_BIAS_PERFORMANCE) {
val = energ_perf_values[EPB_INDEX_NORMAL];
pr_warn_once("ENERGY_PERF_BIAS: Set to 'normal', was 'performance'\n");
}
@@ -213,6 +216,20 @@ static const struct x86_cpu_id intel_epb_normal[] = {
{}
};

+static __init int parse_intel_epb(char *str)
+{
+ if (!str)
+ return 0;
+
+ /* "intel_epb=preserve" prevents PERFORMANCE->NORMAL on restore. */
+ if (!strcmp(str, "preserve"))
+ intel_epb_no_override = true;
+
+ return 0;
+}
+
+early_param("intel_epb", parse_intel_epb);
+
static __init int intel_epb_init(void)
{
const struct x86_cpu_id *id = x86_match_cpu(intel_epb_normal);
--
2.40.1