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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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
>>> 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@suse.de>
>>> Cc: Joerg Roedel <jroedel@suse.de>
>>> Cc: Borislav Petkov <bp@suse.de>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> Cc: Jan Beulich <JBeulich@suse.com>
>>> Cc: Juergen Gross <JGross@suse.com>
>>> Cc: Olaf Hering <ohering@suse.de>
>>> Cc: David Vrabel <david.vrabel@citrix.com>
>>> Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
>>> ---
>>>   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?

~Andrew