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

["Luis R. Rodriguez" <mcgrof@xxxxxxxx>]

On Thu, Nov 27, 2014 at 06:50:31PM +0000, Andrew Cooper 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
> >>> multi-call feature whereby Xen lets one hypercall batch out a series
> >>> of other hypercalls on the hypervisor. At times such hypercalls can
> >>> even end up triggering the TASK_UNINTERRUPTIBLE hanger check (default
> >>> 120 seconds), this a non-issue issue on preemptible kernels though as
> >>> the kernel may deschedule such long running tasks. Xen for instance
> >>> supports multicalls to be preempted as well, this is what Xen calls
> >>> continuation (see xen commit 42217cbc5b which introduced this [0]).
> >>> On systems without CONFIG_PREEMPT though -- a kernel with voluntary
> >>> or no preemption -- a long running hypercall will not be descheduled
> >>> until the hypercall is complete and the ioctl returns to user space.
> >>>
> >>> To help with this David had originally implemented support for use
> >>> of preempt_schedule_irq() [1] for non CONFIG_PREEMPT kernels. This
> >>> solution never went upstream though and upon review to help refactor
> >>> this I've concluded that usage of preempt_schedule_irq() would be
> >>> a bit abussive of existing APIs -- for a few reasons:
> >>>
> >>> 0) we want to avoid spreading its use on non CONFIG_PREEMPT kernels
> >>>
> >>> 1) we want try to consider solutions that might work for other
> >>>     hypervisors for this same problem, and identify it its an issue
> >>>     even present on other hypervisors or if this is a self
> >>>     inflicted architectural issue caused by use of multicalls
> >>>
> >>> 2) there is no documentation or profiling of the exact hypercalls
> >>>     that were causing these issues, nor do we have any context
> >>>     to help evaluate this any further
> >>>
> >>> I at least checked with kvm folks and it seems hypercall preemption
> >>> is not needed there. We can survey other hypervisors...
> >>>
> >>> If 'something like preemption' is needed then CONFIG_PREEMPT
> >>> should just be enabled and encouraged, it seems we want to
> >>> encourage CONFIG_PREEMPT on xen, specially when multicalls are
> >>> used. In the meantime this tries to address a solution to help
> >>> xen on non CONFIG_PREEMPT kernels.
> >>>
> >>> One option tested and evaluated was to put private hypercalls in
> >>> process context, however this would introduce complexities such
> >>> originating hypercalls from different contexts. Current xen
> >>> hypercall callback handlers would need to be changed per architecture,
> >>> for instance, we'd also incur the cost of switching states from
> >>> user / kernel (this cost is also present if preempt_schedule_irq()
> >>> is used). There may be other issues which could be introduced with
> >>> this strategy as well. The simplest *shared* alternative is instead
> >>> to just explicitly schedule() at the end of a private hypercall on non
> >>> preempt kernels. This forces our private hypercall call mechanism
> >>> to try to be fair only on non CONFIG_PREEMPT kernels at the cost of
> >>> more context switch but keeps the private hypercall context intact.
> >>>
> >>> [0] 
> >>> http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=42217cbc5b3e84b8c145d8cfb62dd5de0134b9e8;hp=3a0b9c57d5c9e82c55dd967c84dd06cb43c49ee9
> >>> [1] 
> >>> http://ftp.suse.com/pub/people/mcgrof/xen-preempt-hypercalls/0001-x86-xen-allow-privcmd-hypercalls-to-be-preempted.patch
> >>>
> >>> Cc: Davidlohr Bueso <dbueso@xxxxxxx>
> >>> Cc: Joerg Roedel <jroedel@xxxxxxx>
> >>> Cc: Borislav Petkov <bp@xxxxxxx>
> >>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >>> Cc: Jan Beulich <JBeulich@xxxxxxxx>
> >>> Cc: Juergen Gross <JGross@xxxxxxxx>
> >>> Cc: Olaf Hering <ohering@xxxxxxx>
> >>> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> >>> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxx>
> >>> ---
> >>>   drivers/xen/privcmd.c | 3 +++
> >>>   1 file changed, 3 insertions(+)
> >>>
> >>> diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
> >>> index 569a13b..e29edba 100644
> >>> --- 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
> >>>
> >>>           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...
> >
> >> I suppose you were mislead by the "int 0x82" in [0]. This is the
> >> hypercall from the kernel into the hypervisor, e.g. inside of
> >> privcmd_call().
> > Nope, you have to consider what was done in [1], I was trying to
> > do something similar but less complex that didn't involve mucking
> > with the callbacks but also not abusing APIs.
> >
> > I'm afraid we don't have much leg room.
> 
> XenServer uses
> 
> https://github.com/xenserver/linux-3.x.pg/blob/master/master/0001-x86-xen-allow-privcmd-hypercalls-to-be-preempted.patch
> 
> to deal with these issues.  That patch is based on 3.10.
> 
> I can remember whether this has been submitted upstream before (and
> there were outstanding issues), or whether it fell at an inconvenient
> time with our development cycles.
> 
> David: do you recall?

This was precicely the patch I reviewed in [1].

  Luis

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