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

[Juergen Gross <jgross@suse.com>]

On 11/27/2014 07:50 PM, 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@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.

Clever. :-)

>
> 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.

I found

http://lists.xen.org/archives/html/xen-devel/2014-02/msg02540.html

and nothing else.


Juergen