Re: [Xen-devel] Proposed new "memory capacity claim" hypercall/feature

[Keir Fraser <keir@xxxxxxx>]

On 08/11/2012 09:47, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>> Ah, then I am out of date on how Linux services softirqs and preemption? Can
>> softirqs/preemption occur any time, even in kernel mode, so long as no locks
>> are held?
>> 
>> I thought softirq-type work only happened during event servicing, only if
>> the event servicing had interrupted user context (ie, would not happen if
>> started from within kernel mode). So the restart of the hypercall trap
>> instruction would be an opportunity to service hardirqs, but not softirqs or
>> scheduler...
> 
> No, irq_exit() can invoke softirqs, provided this isn't a nested IRQ
> (soft as well as hard) or softirqs weren't disabled in the interrupted
> context.

Ah, okay. In fact maybe that's always been the case and I have misremembered
this detail, since condition for softirq entry in Xen has always been more
strict than this.

> The only thing that indeed is - on non-preemptible kernels - done
> only on exit to user mode is the eventual entering of the scheduler.

That alone may still be an argument for restricting the batch size from the
toolstack?

 -- Keir



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