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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
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17
Documentation/admin-guide/pm/intel_idle.rst
View File

@ -170,7 +170,7 @@ and ``idle=nomwait``. If any of them is present in the kernel command line, the
``MWAIT`` instruction is not allowed to be used, so the initialization of
``intel_idle`` will fail.
Apart from that there are four module parameters recognized by ``intel_idle``
Apart from that there are five module parameters recognized by ``intel_idle``
itself that can be set via the kernel command line (they cannot be updated via
sysfs, so that is the only way to change their values).
@ -216,6 +216,21 @@ are ignored).
The idle states disabled this way can be enabled (on a per-CPU basis) from user
space via ``sysfs``.
The ``ibrs_off`` module parameter is a boolean flag (defaults to
false). If set, it is used to control if IBRS (Indirect Branch Restricted
Speculation) should be turned off when the CPU enters an idle state.
This flag does not affect CPUs that use Enhanced IBRS which can remain
on with little performance impact.
For some CPUs, IBRS will be selected as mitigation for Spectre v2 and Retbleed
security vulnerabilities by default. Leaving the IBRS mode on while idling may
have a performance impact on its sibling CPU. The IBRS mode will be turned off
by default when the CPU enters into a deep idle state, but not in some
shallower ones. Setting the ``ibrs_off`` module parameter will force the IBRS
mode to off when the CPU is in any one of the available idle states. This may
help performance of a sibling CPU at the expense of a slightly higher wakeup
latency for the idle CPU.
.. _intel-idle-core-and-package-idle-states:

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");