On Mon, Dec 01, 2014 at 06:07:48PM +0100, Juergen Gross wrote: > On 12/01/2014 05:19 PM, 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. > > I wonder whether it would be more acceptable to add (or completely > switch to) another preemption model: PREEMPT_SWITCHABLE. This would be > similar to CONFIG_PREEMPT, but the "normal" value of __preempt_count > would be settable via kernel parameter (default 2): > > 0: preempt > 1: preempt_voluntary > 2: preempt_none > > The kernel would run with preemption enabled. cond_sched() would > reschedule if __preempt_count <= 1. And in case of long running kernel > activities (like the hypercall case or other stuff requiring schedule() > calls to avoid hangups) we would just set __preempt_count to 0 during > these periods and restore the old value afterwards. > > This would be a rather intrusive but clean change IMO. > > Any thoughts? I like the idea of dynamically changing at run time the preemption model and personally find this reasonable, however I am not certain if this would introduce a series of issues hard to address. Thoughts by others who linger deep in the cold lonely scheduler caves ? Luis -- 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
