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intel_idle: Add ibrs_off module parameter to force-disable IBRS

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Commit bf5835bcdb ("intel_idle: Disable IBRS during long idle")
disables IBRS when the cstate is 6 or lower. However, there are
some use cases where a customer may want to use max_cstate=1 to
lower latency. Such use cases will suffer from the performance
degradation caused by the enabling of IBRS in the sibling idle thread.
Add a "ibrs_off" module parameter to force disable IBRS and the
CPUIDLE_FLAG_IRQ_ENABLE flag if set.

In the case of a Skylake server with max_cstate=1, this new ibrs_off
option will likely increase the IRQ response latency as IRQ will now
be disabled.

When running SPECjbb2015 with cstates set to C1 on a Skylake system.

First test when the kernel is booted with: "intel_idle.ibrs_off":

  max-jOPS = 117828, critical-jOPS = 66047

Then retest when the kernel is booted without the "intel_idle.ibrs_off"
added:

  max-jOPS = 116408, critical-jOPS = 58958

That means booting with "intel_idle.ibrs_off" improves performance by:

  max-jOPS:      +1.2%, which could be considered noise range.
  critical-jOPS: +12%,  which is definitely a solid improvement.

The admin-guide/pm/intel_idle.rst file is updated to add a description
about the new "ibrs_off" module parameter.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20230727184600.26768-5-longman@redhat.com
This commit is contained in:
Waiman Long
2023-07-27 14:46:00 -04:00
committed by Ingo Molnar
parent 7506203089
commit aa1567a7e6
2 changed files with 26 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files

11
drivers/idle/intel_idle.c
View File

@@ -68,6 +68,7 @@ static int max_cstate = CPUIDLE_STATE_MAX - 1;
static unsigned int disabled_states_mask __read_mostly;
static unsigned int preferred_states_mask __read_mostly;
static bool force_irq_on __read_mostly;
static bool ibrs_off __read_mostly;
static struct cpuidle_device __percpu *intel_idle_cpuidle_devices;
@@ -1852,11 +1853,13 @@ static void state_update_enter_method(struct cpuidle_state *state, int cstate)
}
if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS) &&
state->flags & CPUIDLE_FLAG_IBRS) {
((state->flags & CPUIDLE_FLAG_IBRS) || ibrs_off)) {
/*
* IBRS mitigation requires that C-states are entered
* with interrupts disabled.
*/
if (ibrs_off && (state->flags & CPUIDLE_FLAG_IRQ_ENABLE))
state->flags &= ~CPUIDLE_FLAG_IRQ_ENABLE;
WARN_ON_ONCE(state->flags & CPUIDLE_FLAG_IRQ_ENABLE);
state->enter = intel_idle_ibrs;
return;
@@ -2175,3 +2178,9 @@ MODULE_PARM_DESC(preferred_cstates, "Mask of preferred idle states");
* 'CPUIDLE_FLAG_INIT_XSTATE' and 'CPUIDLE_FLAG_IBRS' flags.
*/
module_param(force_irq_on, bool, 0444);
/*
* Force the disabling of IBRS when X86_FEATURE_KERNEL_IBRS is on and
* CPUIDLE_FLAG_IRQ_ENABLE isn't set.
*/
module_param(ibrs_off, bool, 0444);
MODULE_PARM_DESC(ibrs_off, "Disable IBRS when idle");