Messages in this thread | | | Date | Mon, 01 Dec 2014 18:07:48 +0100 | From | Juergen Gross <> | Subject | Re: [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT |
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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@citrix.com> 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@suse.com> >>>>>>>>> >>>>>>>>> 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?
Juergen
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