Commit bf5835bcdb96 ("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 <[email protected]>
Acked-by: Rafael J. Wysocki <[email protected]>
---
Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
drivers/idle/intel_idle.c | 11 ++++++++++-
2 files changed, 26 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
index b799a43da62e..39bd6ecce7de 100644
--- a/Documentation/admin-guide/pm/intel_idle.rst
+++ b/Documentation/admin-guide/pm/intel_idle.rst
@@ -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:
diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
index c9479f089037..e1b826344682 100644
--- a/drivers/idle/intel_idle.c
+++ b/drivers/idle/intel_idle.c
@@ -69,6 +69,7 @@ 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;
@@ -1919,11 +1920,13 @@ 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;
@@ -2346,3 +2349,9 @@ 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");
--
2.31.1
On 7/10/23 15:48, Waiman Long wrote:
> Commit bf5835bcdb96 ("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 <[email protected]>
> Acked-by: Rafael J. Wysocki <[email protected]>
> ---
> Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
> drivers/idle/intel_idle.c | 11 ++++++++++-
> 2 files changed, 26 insertions(+), 2 deletions(-)
Ping! Is there further suggested changes for this patch series or is it
good enough to be merged?
Thanks,
Longman
>
> diff --git a/Documentation/admin-guide/pm/intel_idle.rst b/Documentation/admin-guide/pm/intel_idle.rst
> index b799a43da62e..39bd6ecce7de 100644
> --- a/Documentation/admin-guide/pm/intel_idle.rst
> +++ b/Documentation/admin-guide/pm/intel_idle.rst
> @@ -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:
>
> diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> index c9479f089037..e1b826344682 100644
> --- a/drivers/idle/intel_idle.c
> +++ b/drivers/idle/intel_idle.c
> @@ -69,6 +69,7 @@ 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;
>
> @@ -1919,11 +1920,13 @@ 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;
> @@ -2346,3 +2349,9 @@ 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");
Waiman Long, Jul 10, 2023 at 21:48:
> Commit bf5835bcdb96 ("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 <[email protected]>
> Acked-by: Rafael J. Wysocki <[email protected]>
Hi Longman,
Adding back my tag that I had set in v1. Thanks!
Tested-by: Robin Jarry <[email protected]>
On Tue, Jul 25, 2023 at 04:45:49PM -0400, Waiman Long wrote:
> On 7/10/23 15:48, Waiman Long wrote:
> > Commit bf5835bcdb96 ("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 <[email protected]>
> > Acked-by: Rafael J. Wysocki <[email protected]>
> > ---
> > Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++-
> > drivers/idle/intel_idle.c | 11 ++++++++++-
> > 2 files changed, 26 insertions(+), 2 deletions(-)
>
> Ping! Is there further suggested changes for this patch series or is it good
> enough to be merged?
I suppose I'll go stick it in x86/core unless someone objects.