What happens on timeslice overrun?

Udo A. Steinberg us15 at os.inf.tu-dresden.de
Mon Aug 15 17:10:35 CEST 2005

On Mon, 15 Aug 2005 15:17:20 +0200 Rene Wittmann (RW) wrote:

RW> > When a thread is preempted (that means it gets involuntarily 
RW> > descheduled), the remaining time quantum is saved and later 
RW> > restored when the thread is scheduled again. If the thread 
RW> > yields its scheduling context (that means it voluntarily 
RW> > gives it away), the time quantum is no longer available to 
RW> > the thread. A yield can happen in two ways depending on the 
RW> > target thread id specified:
RW> > 
RW> > 1) if the target ID is the NIL_ID, then the thread yields its active
RW> >    scheduling context to noone - the time is effectively gone. This is
RW> >    what next_reservation does. Note that the kernel checks 
RW> > the user-specified
RW> >    ID, to guard against the case where a thread wants to 
RW> > yield its active
RW> >    scheduling context and the time quantum on that scheduling 
RW> > context expires
RW> >    simultaneously.
RW> > 
RW> > 2) if the target ID is a valid thread ID in the system, then 
RW> > the action
RW> >    depends on whether the specified thread is ready to run:
RW> >    a) if it is, then the current thread donates the current scheduling
RW> >       context to the specified thread and that thread then becomes the
RW> >       current thread. This is similar to what happens during 
RW> > a donating IPC.
RW> > 
RW> >    b) if the specified thread is not ready, then the current 
RW> > scheduling
RW> >       context is gone, similar to 1)
RW> > 
RW> How can I specify the target ID? When creating the thread? But how?

The target thread ID for l4_thread_switch, i.e. the thread to yield to,
is a parameter to that system call. The L4 interface (see L4.V2 manual)
describes the ABI for x86: The following registers are defined for Fiasco:

EAX scheduling context ID
ESI low part of the target thread ID

After the system call returns, the following registers are defined:

EAX return value (0 = success, -1 = failure)
ECX low part of remaining time on yielded sched. context  (if EAX is 0)
EDX high part of remaining time on yielded sched. context (if EAX is 0)

You probably need not concern yourself with these low-level details of
the L4 ABI - there should be bindings in l4/pkg/l4sys that provide
high-level interfaces to l4_thread_switch.

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