On 01/12/14 16:19, Luis R. Rodriguez wrote: > On Mon, Dec 01, 2014 at 03:54:24PM +0000, David Vrabel wrote: >> On 01/12/14 15:44, Luis R. Rodriguez wrote: >>> On Mon, Dec 1, 2014 at 10:18 AM, David Vrabel <david.vrabel@xxxxxxxxxx> wrote: >>>> On 01/12/14 15:05, Luis R. Rodriguez wrote: >>>>> On Mon, Dec 01, 2014 at 11:11:43AM +0000, David Vrabel wrote: >>>>>> On 27/11/14 18:36, Luis R. Rodriguez wrote: >>>>>>> On Thu, Nov 27, 2014 at 07:36:31AM +0100, Juergen Gross wrote: >>>>>>>> On 11/26/2014 11:26 PM, Luis R. Rodriguez wrote: >>>>>>>>> From: "Luis R. Rodriguez" <mcgrof@xxxxxxxx> >>>>>>>>> >>>>>>>>> Some folks had reported that some xen hypercalls take a long time >>>>>>>>> to complete when issued from the userspace private ioctl mechanism, >>>>>>>>> this can happen for instance with some hypercalls that have many >>>>>>>>> sub-operations, this can happen for instance on hypercalls that use >>>>>> [...] >>>>>>>>> --- a/drivers/xen/privcmd.c >>>>>>>>> +++ b/drivers/xen/privcmd.c >>>>>>>>> @@ -60,6 +60,9 @@ static long privcmd_ioctl_hypercall(void __user *udata) >>>>>>>>> hypercall.arg[0], hypercall.arg[1], >>>>>>>>> hypercall.arg[2], hypercall.arg[3], >>>>>>>>> hypercall.arg[4]); >>>>>>>>> +#ifndef CONFIG_PREEMPT >>>>>>>>> + schedule(); >>>>>>>>> +#endif >>>>>> >>>>>> As Juergen points out, this does nothing. You need to schedule while in >>>>>> the middle of the hypercall. >>>>>> >>>>>> Remember that Xen's hypercall preemption only preempts the hypercall to >>>>>> run interrupts in the guest. >>>>> >>>>> How is it ensured that when the kernel preempts on this code path on >>>>> CONFIG_PREEMPT=n kernel that only interrupts in the guest are run? >>>> >>>> Sorry, I really didn't describe this very well. >>>> >>>> If a hypercall needs a continuation, Xen returns to the guest with the >>>> IP set to the hypercall instruction, and on the way back to the guest >>>> Xen may schedule a different VCPU or it will do any upcalls (as per normal). >>>> >>>> The guest is free to return from the upcall to the original task >>>> (continuing the hypercall) or to a different one. >>> >>> OK so that addresses what Xen will do when using continuation and >>> hypercall preemption, my concern here was that using >>> preempt_schedule_irq() on CONFIG_PREEMPT=n kernels in the middle of a >>> hypercall on the return from an interrupt (e.g., the timer interrupt) >>> would still let the kernel preempt to tasks other than those related >>> to Xen. >> >> Um. Why would that be a problem? We do want to switch to any task the >> Linux scheduler thinks is best. > > Its safe but -- it technically is doing kernel preemption, unless we want > to adjust the definition of CONFIG_PREEMPT=n to exclude hypercalls. This > was my original concern with the use of preempt_schedule_irq() to do this. > I am afraid of setting precedents without being clear or wider review and > acceptance. It's voluntary preemption at a well defined point. It's no different to a cond_resched() call. Note that we're not trying to fix this for the non-voluntary-preempt kernels. David -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
