Re: [Xen-devel] [PATCH 2/2] xen: sched: we never get into context_switch() with prev==next

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Dario,

On 20.04.19 18:24, Dario Faggioli wrote:
> In schedule(), if we pick, as the next vcpu to run (next) the same one
> that is running already (prev), we never get to call context_switch().
And what about `if ( prev != current )` in arm/domain.c:schedule_tail() ?

> We can, therefore, get rid of all the `if`-s testing prev and next being
> different, trading them with an ASSERT() (on ARM, the ASSERT() was even
> already there!)> 
> Signed-off-by: Dario Faggioli <dfaggioli@xxxxxxxx>
> Suggested-by: Juergen Gross <jgross@xxxxxxxx>
For all below:

Reviewed-by: Andrii Anisov <andrii_anisov@xxxxxxxx>


> ---
> Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> Cc: Julien Grall <julien.grall@xxxxxxx>
> Cc: Jan Beulich <jbeulich@xxxxxxxx>
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> Cc: "Roger Pau Monné" <roger.pau@xxxxxxxxxx>
> Cc: George Dunlap <george.dunlap@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> ---
>   xen/arch/arm/domain.c |    3 +--
>   xen/arch/x86/domain.c |   22 ++++++++--------------
>   2 files changed, 9 insertions(+), 16 deletions(-)
>
> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> index 6dc633ed50..915ae0b4c6 100644
> --- a/xen/arch/arm/domain.c
> +++ b/xen/arch/arm/domain.c
> @@ -343,8 +343,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
>       ASSERT(prev != next);
>       ASSERT(!vcpu_cpu_dirty(next));
>   
> -    if ( prev != next )
> -        update_runstate_area(prev);
> +    update_runstate_area(prev);
>   
>       local_irq_disable();
>   
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 9eaa978ce5..d2d9f2fc3c 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -1721,6 +1721,7 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
>       const struct domain *prevd = prev->domain, *nextd = next->domain;
>       unsigned int dirty_cpu = next->dirty_cpu;
>   
> +    ASSERT(prev != next);
>       ASSERT(local_irq_is_enabled());
>   
>       get_cpu_info()->use_pv_cr3 = false;
> @@ -1732,12 +1733,9 @@ void context_switch(struct vcpu *prev, struct vcpu *next)
>           flush_mask(cpumask_of(dirty_cpu), FLUSH_VCPU_STATE);
>       }
>   
> -    if ( prev != next )
> -    {
> -        _update_runstate_area(prev);
> -        vpmu_switch_from(prev);
> -        np2m_schedule(NP2M_SCHEDLE_OUT);
> -    }
> +    _update_runstate_area(prev);
> +    vpmu_switch_from(prev);
> +    np2m_schedule(NP2M_SCHEDLE_OUT);
>   
>       if ( is_hvm_domain(prevd) && !list_empty(&prev->arch.hvm.tm_list) )
>           pt_save_timer(prev);
> @@ -1794,14 +1792,10 @@ void context_switch(struct vcpu *prev, struct vcpu 
> *next)
>   
>       context_saved(prev);
>   
> -    if ( prev != next )
> -    {
> -        _update_runstate_area(next);
> -
> -        /* Must be done with interrupts enabled */
> -        vpmu_switch_to(next);
> -        np2m_schedule(NP2M_SCHEDLE_IN);
> -    }
> +    _update_runstate_area(next);
> +    /* Must be done with interrupts enabled */
> +    vpmu_switch_to(next);
> +    np2m_schedule(NP2M_SCHEDLE_IN);
>   
>       /* Ensure that the vcpu has an up-to-date time base. */
>       update_vcpu_system_time(next);
>
>
> _______________________________________________
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--
Sincerely,
Andrii Anisov.

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