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? ~Andrew -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
