| Message ID | 20230710194857.2898284-5-longman@redhat.com |
|---|---|
| State | Superseded |
| Headers | show |
| Series | x86/speculation: Disable IBRS when idle | expand |
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 <longman@redhat.com> > Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > --- > 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");
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");
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 <longman@redhat.com> Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> --- Documentation/admin-guide/pm/intel_idle.rst | 17 ++++++++++++++++- drivers/idle/intel_idle.c | 11 ++++++++++- 2 files changed, 26 insertions(+), 2 deletions(-)