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

[Luis R. Rodriguez]

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?

> >>>
> >>>   	return ret;
> >>>   }
> >>>
> >>
> >> Sorry, I don't think this will solve anything. You're calling schedule()
> >> right after the long running hypercall just nanoseconds before returning
> >> to the user.
> > 
> > Yeah, well that is what [1] tried as well only it tried using
> > preempt_schedule_irq() on the hypercall callback...
> 
> No.  My patch added a schedule point in the middle of a hypercall on the
> return from an interrupt (e.g., the timer interrupt).

OK that provides much better context and given that I do see the above hunk as
pointless. I was completely misrepresenting what the callback was for. Now --
just to address my issues with the use of preempt_schedule_irq(). If the above
is addressed that I think should address most of my concerns, if we can figure
out a way to not deal with it to be arch specific that'd be neat, and if we
could not have to ifdef around stuff even better.

  Luis
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