RE: [Xen-devel] Scheduler guts question

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

Perfect!  Thanks!  Because Linux/ia64 uses the architectural
timer (ar.itc) for all timer events, I missed the critical
dependency on the APIC timer. 

> -----Original Message-----
> From: Andrew Biggadike [mailto:biggadike@xxxxxxx] 
> Sent: Friday, March 04, 2005 7:21 PM
> To: Magenheimer, Dan (HP Labs Fort Collins)
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Scheduler guts question
> 
> Magenheimer, Dan (HP Labs Fort Collins) 
> <dan.magenheimer@xxxxxx> wrote:
> > Looking at the code, I am missing some key connection
> > between the interrupt routines and the scheduler routines.
> > Perhaps there is some hidden call in a macro or in
> > a function pointer indirection that I am missing.
> >
> > Could someone walk me through at a low level the sequence
> > of Xen/x86 calls that occur when a timer interrupt results in
> > some timer expiration that results in the currently running
> > domain getting "paused" and another domain launched?
> 
> There are certainly better people to address this, since I 
> just started
> looking at this code last week, but here's what I've figured 
> out so far.
> 
> start_of_day() calls scheduler_init(), in which an ac_timer named
> s_timer is initialized for each CPU's schedule_data.  Each time one of
> these goes off the s_timer_fn() function is invoked which raises the
> SCHEDULER_SOFTIRQ.  Later, setup() calls schedulers_start(), which
> invokes s_timer_fn() for the first time to raise the first scheduler
> softirq.
> 
> __enter_scheduler(), which is attached to the scheduler softirq by the
> call to open_softirq() in scheduler_init(), removes the ac_timer when
> starting, then adds a new one based on the time slice provided by the
> scheduler's do_schedule() routine.  See the "reprogram the timer"
> comment in __enter_scheduler().  When that timer goes off the 
> scheduler
> softirq will be raised by s_timer_fn() again, causing
> __enter_scheduler() to be invoked again.
> 
> The softirqs are handled in entry.S.  ret_from_intr checks 
> whether there
> are events pending and, if so, jumps to test_all_events.  (It appears
> that several places jump to test_all_events and it naturally follows a
> hypercall invocation.)  test_all_events checks for pending 
> softirqs, and
> jumps to process_softirqs to handle them.  This calls do_softirq() (in
> softirq.c), which indirectly invokes __enter_scheduler() through the
> softirq_handlers method table.
> 
> The ac_timer functions (s_timer_fn for the scheduler timer) 
> are invoked
> by ac_timer_softirq_action() which is invoked (also by do_softirq())
> when AC_TIMER_SOFTIRQ is raised.  This ac_timer softirq is raised by
> smp_apic_timer_interrupt() (and timer_interrupt() if there is 
> no apic).
> Note that smp_apic_timer_interrupt() is invoked by the
> apic_timer_interrupt() function created by the
> BUILD_SMP_TIMER_INTERRUPT() macro in irq.h (the use of the macro is in
> i8259.c) and registered in the timer's interrupt gate in init_IRQ()
> (which is also called by start_of_day()).
> 
> 
> So, a timer interrupt gives:
> o timer interrupt ->
>    apic_timer_interrupt() ->
>     smp_apic_timer_interrupt() -> raises AC_TIMER_SOFTIRQ
> 
> Then, next time leaving Xen (through entry.S):
> o test_all_events: ->
>    process_softirqs: ->
>     do_softirq() ->
>      softirq_handlers[AC_TIMER_SOFTIRQ] == 
> ac_timer_softirq_action() ->
>       s_timer_fn() -> raises SCHEDULER_SOFTIRQ 
> 
> Then, since AC_TIMER_SOFTIRQ == SCHEDULER_SOFTIRQ - 1, the next
> iteration of do_softirq()'s loop invokes the scheduler:
> o do_softirq() ->
>    softirq_handlers[SCHEDULER_SOFTIRQ] == __enter_scheduler()
> 
> 
> Note that I'm looking at 2.0.4.  Also, there's a good chance that some
> of this is not quite right or complete, in which case I'd appreciate
> being corrected by someone who has a better understanding of the code
> base.
> 
> Andrew
> 


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