Xen project Mailing List

On Tue, Apr 2, 2013 at 5:57 AM, George Dunlap <dunlapg@xxxxxxxxx> wrote:

On Fri, Mar 29, 2013 at 9:43 PM, Sisu Xi <xisisu@xxxxxxxxx> wrote:
> Hi, all:
>
> Does anyone know how does the yield function works in the Xen scheduler?
> Right now only credit scheduler uses it to put it behind one lower-priority
> runnable vcpus. But what is this used for?
>
> I checked [1], the author also mentioned the yield function to prevent lhp.
> Is this the same function?
>
> On Xen part, the yield function is called in do_yield(), which is called in
> do_sched_op(), by passing in SCHEDOP_YIELD.
>
> However, if you grep the whole Linux Kernel Code, (3.8.0 in my case), the
> place where SCHEDOP_YIELD appears are:
> 1) drivers\tty\hvc\hvc_xen.c, domU_write_console()
> 2) arch\x86\xen\smp.c, xen_cpu_up()
> 3) arch\x86\xen\smp.c, xen_smp_send_call_function_ipi()
> 4) arch\ia64\xen\xcom_hcall.c, xencomm_hypercall_sched_op()
>
> None of this seems related to the spinlock.

YIELD is used by the Citrix Windows PV drivers to patch Windows
spinlocks. ÂThe pvops kernels have a different solution to the
lock-holder problem that's a bit more complicated. Â But the
lock-holder problem isn't limited only to actual locks; it's a
potential problem any time any cpu waits on another cpu. ÂI haven't
looked at the code, but in the case of #3, the caller spins waiting
for the other cpu to execute the function; the basic problem is the
same as the spinlock problem. ÂI suspect if you look at the other 3,
you'll also find someone spinning while waiting for another cpu.

Â-George

--
Sisu Xi, PhD Candidate

http://www.cse.wustl.edu/~xis/
Department of Computer Science and Engineering
Campus Box 1045
Washington University in St. Louis
One Brookings Drive
St. Louis, MO 63130

Re: [Xen-devel] yield hypercall to prevent lock holder problem ?