Re: [PATCH] Add need_resched check before go to kvm_resched

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>   
>> Dong, Eddie wrote:
>>     
>>> In this case, IOCTL return to Qemu will trigger scheduling at least.
>>>   
>>>       
>> I think a scheduling change won't happen until the next timer tick. 
>> AFAICT, there's nothing explicit in the ioctl return path that will
>> result in rescheduling.
>>
>> I'm not entirely confident in how the timer interrupt gets handled
>> when it arrives in the guest.  My understanding is that while the
>> timer interrupt is delivered in the host, since preempt is disabled,
>> the rescheduling opportunity is lost even though the current task is
>> marked as needing rescheduling.  I think the next time that bit will
>> get checked is the next timer interrupt but I may be wrong.
>>
>>     
>
> No, any syscall exit triggers a rescheduling check, otherwise a task
> that spends most of its time in a non-preemt kernel would rarely get
> rescheduled.
>   

Ah, okay.

Regards,

Anthony Liguori

> See arch/x86_64/kernel/entry.S, the various checks for TIF_NEED_RESCHED.
>   



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