Messages in this thread | | | Date | Fri, 28 Nov 2014 05:49:19 +0100 | From | Juergen Gross <> | Subject | Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT |
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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
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