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Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT

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],
+ schedule();

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

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?


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