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From:   Jack Allister <jalliste@amazon.com>
To:     <rafael@kernel.org>
CC:     Jack Allister <jalliste@amazon.com>,
        Paul Durrant <pdurrant@amazon.com>, Jue Wang <juew@amazon.com>,
        Usama Arif <usama.arif@bytedance.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "Borislav Petkov" <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Hans de Goede <hdegoede@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        <linux-kernel@vger.kernel.org>
Subject: [PATCH v4] x86: intel_epb: Add earlyparam option to keep bias at performance
Date:   Tue, 5 Dec 2023 15:14:02 +0000
Message-ID: <20231205151403.76015-1-jalliste@amazon.com>
In-Reply-To: <CAJZ5v0jYqQc4UztGzMDP5m5xKejrEQkMyyt12nsHdQ=qiULpTQ@mail.gmail.com>
References: <CAJZ5v0jYqQc4UztGzMDP5m5xKejrEQkMyyt12nsHdQ=qiULpTQ@mail.gmail.com>
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There are certain scenarios where it may be intentional that the EPB was
set at to 0/ENERGY_PERF_BIAS_PERFORMANCE on kernel boot. For example, in
data centers a kexec/live-update of the kernel may be performed regularly.

Usually this live-update is time critical and defaulting of the bias back
to ENERGY_PERF_BIAS_NORMAL may actually be detrimental to the overall
update time if processors' time to ramp up/boost are affected.

This patch introduces a kernel command line "intel_epb_no_override"
which will leave the EPB at performance if during the restoration code path
it is detected as such.

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>
---
 arch/x86/kernel/cpu/intel_epb.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/intel_epb.c b/arch/x86/kernel/cpu/intel_epb.c
index e4c3ba91321c..4a3523225572 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_no_override 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 +216,12 @@ static const struct x86_cpu_id intel_epb_normal[] = {
 	{}
 };
 
+static __init int intel_epb_no_override_setup(char *str)
+{
+	return kstrtobool(str, &intel_epb_no_override);
+}
+early_param("intel_epb_no_override", intel_epb_no_override_setup);
+
 static __init int intel_epb_init(void)
 {
 	const struct x86_cpu_id *id = x86_match_cpu(intel_epb_normal);
-- 
2.40.1