Re: [RFC PATCH v1 1/6] sched: track time spent in IRQ handler

["Jürgen Groß" <jgross@suse.com>]

On 12.06.20 02:22, Volodymyr Babchuk wrote:
> Add code that saves time spent in IRQ handler, so later we can make
> adjustments to schedule unit run time.
> 
> This and following changes are called upon to provide fair
> scheduling. Problem is that any running vCPU can be interrupted by to
> handle IRQ which is bound to some other vCPU. Thus, current vCPU can
> be charged for a time, it actually didn't used.
> 
> TODO: move vcpu_{begin|end}_irq_handler() calls to entry.S for even
> more fair time tracking.
> 
> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> ---
>   xen/arch/arm/irq.c      |  2 ++
>   xen/arch/x86/irq.c      |  2 ++
>   xen/common/sched/core.c | 29 +++++++++++++++++++++++++++++
>   xen/include/xen/sched.h | 13 +++++++++++++
>   4 files changed, 46 insertions(+)
> 
> diff --git a/xen/arch/arm/irq.c b/xen/arch/arm/irq.c
> index 3877657a52..51b517c0cd 100644
> --- a/xen/arch/arm/irq.c
> +++ b/xen/arch/arm/irq.c
> @@ -201,6 +201,7 @@ void do_IRQ(struct cpu_user_regs *regs, unsigned int irq, int is_fiq)
>       struct irq_desc *desc = irq_to_desc(irq);
>       struct irqaction *action;
>   
> +    vcpu_begin_irq_handler();
>       perfc_incr(irqs);
>   
>       ASSERT(irq >= 16); /* SGIs do not come down this path */
> @@ -267,6 +268,7 @@ out:
>   out_no_end:
>       spin_unlock(&desc->lock);
>       irq_exit();
> +    vcpu_end_irq_handler();
>   }
>   
>   void release_irq(unsigned int irq, const void *dev_id)
> diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
> index a69937c840..3ef4221b64 100644
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -1895,6 +1895,7 @@ void do_IRQ(struct cpu_user_regs *regs)
>       int               irq = this_cpu(vector_irq)[vector];
>       struct cpu_user_regs *old_regs = set_irq_regs(regs);
>   
> +    vcpu_begin_irq_handler();
>       perfc_incr(irqs);
>       this_cpu(irq_count)++;
>       irq_enter();
> @@ -2024,6 +2025,7 @@ void do_IRQ(struct cpu_user_regs *regs)
>    out_no_unlock:
>       irq_exit();
>       set_irq_regs(old_regs);
> +    vcpu_end_irq_handler();
>   }
>   
>   static inline bool is_free_pirq(const struct domain *d,
> diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
> index cb49a8bc02..8f642ada05 100644
> --- a/xen/common/sched/core.c
> +++ b/xen/common/sched/core.c
> @@ -916,6 +916,35 @@ void vcpu_unblock(struct vcpu *v)
>       vcpu_wake(v);
>   }
>   
> +void vcpu_begin_irq_handler(void)
> +{
> +    if (is_idle_vcpu(current))
> +        return;
> +
> +    /* XXX: Looks like ASSERT_INTERRUPTS_DISABLED() is available only for x86 */
> +    if ( current->irq_nesting++ )
> +        return;
> +
> +    current->irq_entry_time = NOW();
> +}
> +
> +void vcpu_end_irq_handler(void)
> +{
> +    int delta;
> +
> +    if (is_idle_vcpu(current))
> +        return;
> +
> +    ASSERT(current->irq_nesting);
> +
> +    if ( --current->irq_nesting )
> +        return;
> +
> +    /* We assume that irq handling time will not overflow int */

This assumption might not hold for long running VMs.


Juergen