Re: [XenPPC] Why no do_softirq() after external interrupt ?

[Jimi Xenidis <jimix@xxxxxxxxxxxxxx>]

Yes, I agree, it is strange and could be coded either way.
I doubt there was any real intent here, is is simply how we coded it.
The 970 (our only port) has issues with external interrupt latencies  
already, I doubt this code really effects performance.
At the moment we "share" control if the PIC with Dom0 which we are  
trying to get away from with the newer kernels so once we have  
achieved this "de-coupling" we did plan on revisiting this code path.
I would be happy to take a patch that changed this, especially if you  
could demonstrate that it results in lower-latency for interrupts.
-JX


On May 17, 2007, at 9:03 PM, HYEONSEUNG JANG wrote:

> There are currently three places in xen code where do_softirq() is
> called.
>
>  - return to the guest from the hypervisor decrementer interrupt  
> handler
>  - return to the guest from a hypercall()
>  - in idle_loop()
>
> But it is kind of strange that do_softirq() is not called after  
> external
> interrupt handler,
>
> ex_external_continued:
>     EXCEPTION_SAVE_STATE r1
>     LOADADDR r12, do_external
>     mr r3, r1                           /* pass pointer to  
> cpu_user_regs */
>     subi r1, r1, STACK_FRAME_OVERHEAD   /* make a "caller" stack  
> frame */
>     CALL_CFUNC r12
>
>     addi r1, r1, STACK_FRAME_OVERHEAD   /* restore stack to  
> cpu_user_regs */
>     b fast_resume
>
> This can cause quite large delay in interrupt processing because  
> context
> switching is done by SCHEDULE_SOFTIRQ.
>
>
> For example, Let's assume that domU is now running and there is an
> external interrupt.
>
>   - domU is interrupted and Xen's do_external() is called
>   - an event channel toward dom0 is set to 'pending'
>   - domU is resumed by fast_resume
>   - dom0 is never scheduled even if it has the higher priority until
>     do_softirq() is called by the next hyperviosr decrementer  
> interrupt.
> When I look at the same parts for the x86 arch, it seems that  
> softirq is
> processed in every 'ret_from_int' which, I think, is expected  
> behaviour.
> Is there anyone who can tell me the reason why do_softirq() is not
> called after external interrupt handling? Or is it just a simple
> missing(but important) code in XenPPC port ?
>
>
>
> - HyeonSeung Jang._______________________________________________
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> http://lists.xensource.com/xen-ppc-devel
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