Re: [Xen-devel] [RFC PATCH v2 1/7] x86/paravirt: Add pv_idle_ops to paravirt ops

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Aug 29, 2017 at 11:46:35AM +0000, Yang Zhang wrote:
> So far, pv_idle_ops.poll is the only ops for pv_idle. .poll is called in
> idle path which will polling for a while before we enter the real idle
> state.
> 
> In virtualization, idle path includes several heavy operations
> includes timer access(LAPIC timer or TSC deadline timer) which will hurt
> performance especially for latency intensive workload like message
> passing task. The cost is mainly come from the vmexit which is a
> hardware context switch between VM and hypervisor. Our solution is to
> poll for a while and do not enter real idle path if we can get the
> schedule event during polling.
> 
> Poll may cause the CPU waste so we adopt a smart polling mechanism to
> reduce the useless poll.
> 
> Signed-off-by: Yang Zhang <yang.zhang.wz@xxxxxxxxx>
> Signed-off-by: Quan Xu <quan.xu0@xxxxxxxxx>
> Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
> Cc: Chris Wright <chrisw@xxxxxxxxxxxx>
> Cc: Alok Kataria <akataria@xxxxxxxxxx>
> Cc: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Adding xen-devel.

Juergen, we really should replace Jeremy's name with xen-devel or
your name.. Wasn't there an patch by you that took some of the 
mainternship over it?

> ---
>  arch/x86/include/asm/paravirt.h       | 5 +++++
>  arch/x86/include/asm/paravirt_types.h | 6 ++++++
>  arch/x86/kernel/paravirt.c            | 6 ++++++
>  3 files changed, 17 insertions(+)
> 
> diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
> index 9ccac19..6d46760 100644
> --- a/arch/x86/include/asm/paravirt.h
> +++ b/arch/x86/include/asm/paravirt.h
> @@ -202,6 +202,11 @@ static inline unsigned long long paravirt_read_pmc(int 
> counter)
>  
>  #define rdpmcl(counter, val) ((val) = paravirt_read_pmc(counter))
>  
> +static inline void paravirt_idle_poll(void)
> +{
> +     PVOP_VCALL0(pv_idle_ops.poll);
> +}
> +
>  static inline void paravirt_alloc_ldt(struct desc_struct *ldt, unsigned 
> entries)
>  {
>       PVOP_VCALL2(pv_cpu_ops.alloc_ldt, ldt, entries);
> diff --git a/arch/x86/include/asm/paravirt_types.h 
> b/arch/x86/include/asm/paravirt_types.h
> index 9ffc36b..cf45726 100644
> --- a/arch/x86/include/asm/paravirt_types.h
> +++ b/arch/x86/include/asm/paravirt_types.h
> @@ -324,6 +324,10 @@ struct pv_lock_ops {
>       struct paravirt_callee_save vcpu_is_preempted;
>  } __no_randomize_layout;
>  
> +struct pv_idle_ops {
> +     void (*poll)(void);
> +} __no_randomize_layout;
> +
>  /* This contains all the paravirt structures: we get a convenient
>   * number for each function using the offset which we use to indicate
>   * what to patch. */
> @@ -334,6 +338,7 @@ struct paravirt_patch_template {
>       struct pv_irq_ops pv_irq_ops;
>       struct pv_mmu_ops pv_mmu_ops;
>       struct pv_lock_ops pv_lock_ops;
> +     struct pv_idle_ops pv_idle_ops;
>  } __no_randomize_layout;
>  
>  extern struct pv_info pv_info;
> @@ -343,6 +348,7 @@ struct paravirt_patch_template {
>  extern struct pv_irq_ops pv_irq_ops;
>  extern struct pv_mmu_ops pv_mmu_ops;
>  extern struct pv_lock_ops pv_lock_ops;
> +extern struct pv_idle_ops pv_idle_ops;
>  
>  #define PARAVIRT_PATCH(x)                                    \
>       (offsetof(struct paravirt_patch_template, x) / sizeof(void *))
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index bc0a849..1b5b247 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -128,6 +128,7 @@ static void *get_call_destination(u8 type)
>  #ifdef CONFIG_PARAVIRT_SPINLOCKS
>               .pv_lock_ops = pv_lock_ops,
>  #endif
> +             .pv_idle_ops = pv_idle_ops,
>       };
>       return *((void **)&tmpl + type);
>  }
> @@ -312,6 +313,10 @@ struct pv_time_ops pv_time_ops = {
>       .steal_clock = native_steal_clock,
>  };
>  
> +struct pv_idle_ops pv_idle_ops = {
> +     .poll = paravirt_nop,
> +};
> +
>  __visible struct pv_irq_ops pv_irq_ops = {
>       .save_fl = __PV_IS_CALLEE_SAVE(native_save_fl),
>       .restore_fl = __PV_IS_CALLEE_SAVE(native_restore_fl),
> @@ -471,3 +476,4 @@ struct pv_mmu_ops pv_mmu_ops __ro_after_init = {
>  EXPORT_SYMBOL    (pv_mmu_ops);
>  EXPORT_SYMBOL_GPL(pv_info);
>  EXPORT_SYMBOL    (pv_irq_ops);
> +EXPORT_SYMBOL    (pv_idle_ops);
> -- 
> 1.8.3.1
> 

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