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    SubjectRe: [PATCH][KVM][retry 4] Add support for Pause Filtering to AMD SVM
    Date
    On Thursday 21 May 2009 06:25:17 Mark Langsdorf wrote:
    > This feature creates a new field in the VMCB called Pause
    > Filter Count. If Pause Filter Count is greater than 0 and
    > intercepting PAUSEs is enabled, the processor will increment
    > an internal counter when a PAUSE instruction occurs instead
    > of intercepting. When the internal counter reaches the
    > Pause Filter Count value, a PAUSE intercept will occur.
    >

    (dig it from archives...)

    Any update for the patch(I mean the scheduler part)? I think people agreed on
    the approach?

    --
    regards
    Yang, Sheng

    > This feature can be used to detect contended spinlocks,
    > especially when the lock holding VCPU is not scheduled.
    > Rescheduling another VCPU prevents the VCPU seeking the
    > lock from wasting its quantum by spinning idly. Perform
    > the reschedule by increasing the the credited time on
    > the VCPU.
    >
    > Experimental results show that most spinlocks are held
    > for less than 1000 PAUSE cycles or more than a few
    > thousand. Default the Pause Filter Counter to 3000 to
    > detect the contended spinlocks.
    >
    > Processor support for this feature is indicated by a CPUID
    > bit.
    >
    > On a 24 core system running 4 guests each with 16 VCPUs,
    > this patch improved overall performance of each guest's
    > 32 job kernbench by approximately 1%. Further performance
    > improvement may be possible with a more sophisticated
    > yield algorithm.
    >
    > -Mark Langsdorf
    > Operating System Research Center
    > AMD
    >
    > Signed-off-by: Mark Langsdorf <mark.langsdorf@amd.com>
    > ---
    > arch/x86/include/asm/svm.h | 3 ++-
    > arch/x86/kvm/svm.c | 13 +++++++++++++
    > include/linux/sched.h | 7 +++++++
    > kernel/sched.c | 18 ++++++++++++++++++
    > 4 files changed, 40 insertions(+), 1 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
    > index 85574b7..1fecb7e 100644
    > --- a/arch/x86/include/asm/svm.h
    > +++ b/arch/x86/include/asm/svm.h
    > @@ -57,7 +57,8 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
    > u16 intercept_dr_write;
    > u32 intercept_exceptions;
    > u64 intercept;
    > - u8 reserved_1[44];
    > + u8 reserved_1[42];
    > + u16 pause_filter_count;
    > u64 iopm_base_pa;
    > u64 msrpm_base_pa;
    > u64 tsc_offset;
    > diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
    > index ef43a18..dad6c4b 100644
    > --- a/arch/x86/kvm/svm.c
    > +++ b/arch/x86/kvm/svm.c
    > @@ -45,6 +45,7 @@ MODULE_LICENSE("GPL");
    > #define SVM_FEATURE_NPT (1 << 0)
    > #define SVM_FEATURE_LBRV (1 << 1)
    > #define SVM_FEATURE_SVML (1 << 2)
    > +#define SVM_FEATURE_PAUSE_FILTER (1 << 10)
    >
    > #define DEBUGCTL_RESERVED_BITS (~(0x3fULL))
    >
    > @@ -575,6 +576,11 @@ static void init_vmcb(struct vcpu_svm *svm)
    >
    > svm->nested_vmcb = 0;
    > svm->vcpu.arch.hflags = HF_GIF_MASK;
    > +
    > + if (svm_has(SVM_FEATURE_PAUSE_FILTER)) {
    > + control->pause_filter_count = 3000;
    > + control->intercept |= (1ULL << INTERCEPT_PAUSE);
    > + }
    > }
    >
    > static int svm_vcpu_reset(struct kvm_vcpu *vcpu)
    > @@ -2087,6 +2093,12 @@ static int interrupt_window_interception(struct
    > vcpu_svm *svm, return 1;
    > }
    >
    > +static int pause_interception(struct vcpu_svm *svm, struct kvm_run
    > *kvm_run) +{
    > + sched_delay_yield(1000000);
    > + return 1;
    > +}
    > +
    > static int (*svm_exit_handlers[])(struct vcpu_svm *svm,
    > struct kvm_run *kvm_run) = {
    > [SVM_EXIT_READ_CR0] = emulate_on_interception,
    > @@ -2123,6 +2135,7 @@ static int (*svm_exit_handlers[])(struct vcpu_svm
    > *svm, [SVM_EXIT_CPUID] = cpuid_interception,
    > [SVM_EXIT_IRET] = iret_interception,
    > [SVM_EXIT_INVD] = emulate_on_interception,
    > + [SVM_EXIT_PAUSE] = pause_interception,
    > [SVM_EXIT_HLT] = halt_interception,
    > [SVM_EXIT_INVLPG] = invlpg_interception,
    > [SVM_EXIT_INVLPGA] = invalid_op_interception,
    > diff --git a/include/linux/sched.h b/include/linux/sched.h
    > index b4c38bc..9cde585 100644
    > --- a/include/linux/sched.h
    > +++ b/include/linux/sched.h
    > @@ -2283,6 +2283,9 @@ static inline unsigned int task_cpu(const struct
    > task_struct *p) return task_thread_info(p)->cpu;
    > }
    >
    > +extern void sched_delay_yield(unsigned long ns);
    > +
    > +
    > extern void set_task_cpu(struct task_struct *p, unsigned int cpu);
    >
    > #else
    > @@ -2292,6 +2295,10 @@ static inline unsigned int task_cpu(const struct
    > task_struct *p) return 0;
    > }
    >
    > +void sched_delay_yield(struct task_struct *p, unsigned int delay)
    > +{
    > +}
    > +
    > static inline void set_task_cpu(struct task_struct *p, unsigned int cpu)
    > {
    > }
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index b902e58..3aed2f6 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -1947,6 +1947,24 @@ task_hot(struct task_struct *p, u64 now, struct
    > sched_domain *sd) return delta < (s64)sysctl_sched_migration_cost;
    > }
    >
    > +/*
    > + * Interface for yielding a thread by delaying it for a known
    > + * interval. Use at your own risk and not with real-time.
    > + *
    > + * Like yield, except for SCHED_OTHER/BATCH, where it will
    > + * give us @ns time for the 'good' cause.
    > + */
    > +void sched_delay_yield(unsigned long ns)
    > +{
    > + struct task_struct *curr = current;
    > + if (curr->sched_class == &fair_sched_class) {
    > + struct sched_entity *se = &curr->se;
    > + __update_curr(cfs_rq_of(se), se, ns);
    > + schedule();
    > + } else
    > + yield();
    > +}
    > +EXPORT_SYMBOL_GPL(sched_delay_yield);
    >
    > void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
    > {




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