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Re: [libvirt] [RFC 0/2] util: introduce timeout mode in virCommandRun/Async and virKModLoad



On Mon, Aug 13, 2018 at 07:37:33PM +0800, shilei massclouds gmx com wrote:
> On Monday, August 13, 2018 at 10:02 AM, Daniel P. Berrangé wrote:
> >On Mon, Aug 13, 2018 at 02:32:17PM +0800, Shi Lei wrote:
> >> Hi, everyone!
> >> It's possible that the running-time of a command is long than its caller
> >> expected. Perhaps, it's useful to provide timeout mode for virCommandRun or
> >> virCommandRunAsync + virCommandWait. And The caller can restrict the
> >> running time of the command if necessary. I introduce a new function
> >> virCommandSetTimeout for that.
> >>
> >> And then, virKModLoad will get a default timeout of 3 seconds. I hope it
> >> could solve the problem of "strange delay" as John mentioned.
> >
> >I'm not really see what this is hoping to solve. If something in libvirt
> >is running virKModLoad, I cannot see how it will work better by not waiting
> >for it to complete. 
> 
> Thanks for your quick reply! I hope I could explain my idea more clear:)
> 
> The virKModLoad is used for loading external kernel modules which are not under
> the control of Libvirt. We cannot make sure they're always solid and strong
> (especially for various pci drivers). If the INIT process of a module is blocked
> for some reasons, the caller of the virKModLoad will be blocked itself and
> cannot known what happened internally and have no chance to handle it.
> Most modules need only dozens of milliseconds to complete loading.
> On my slow PC, module 'nbd' needs about 2 milliseconds to load and '8021q' needs
> about 30 milliseconds. And I will try to test other pci drivers.
> If loading time is longer than a few seconds, we can presume there are some exceptions
> or errors in the loading process of the module. With timeout mode, virKModLoad can
> report it to the caller and the caller could decide how to handle it.
> Timeout mode for virKModLoad is just an insurance which could avoid blocking indefinitely.

virKModLoad is only used in two places, one to load the nbd module, and one
place to load either pci-stub or pciback or vfio-pci. It is not used to load
arbitrary external device specific kernel modules. So this still doesn't
explain why a timeout is required here.


Regards,
Daniel
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