Re: [PATCH] 2.4.0-prerelease: preemptive kernel.

[george anzinger]

Roger Larsson wrote:
> 
> On Thursday 04 January 2001 09:43, ludovic fernandez wrote:
> > Daniel Phillips wrote:
> > > The key idea here is to disable preemption on spin lock and reenable on
> > > spin unlock.  That's a practical idea, highly compatible with the
> > > current way of doing things.  Its a fairly heavy hit on spinlock
> > > performance, but maybe the overall performance hit is small.  Benchmarks
> > > are needed.
> >
> > I'm not sure the hit on spinlock is this heavy (one increment for lock
> > and one dec + test on unlock), but I completely agree (and volonteer)
> > for benchmarking.
> 
> And the conditional jump is usually predicted correctly :-)
> +static inline void enable_preempt(void)
> 
> +{
> +        if (atomic_read(&current->preemptable) <= 0) {
> +                BUG();
> +        }
> +        if (atomic_read(&current->preemptable) == 1) {
> 
> This part can probably be put in a proper non inline function.
> Cache issues...
> +                /*
> +                * At that point a scheduling is healthy iff:
> +                * - a scheduling request is pending.
> +                * - the task is in running state.
> +                * - this is not an interrupt context.
> +                * - local interrupts are enabled.
> +                */
> +                if (current->need_resched == 1     &&
> +                   current->state == TASK_RUNNING &&
> +                   !in_interrupt()                &&
> +                   local_irq_are_enabled())
> +                {
> +                        schedule();
> +                }
>
Actually the MontaVista Patch cleverly removes the tests for
in_interrupt() and local_irq_are_enabled() AND the state ==
TASK_RUNNING.  In actual fact these states can be considered way points
on the system status vector.  For example the interrupts off state
implies all the rest, the in_interrupt() implies not preemptable and
finally, not preemptable is one station away from fully preemptable.  

TASK_RUNNING is easily solved by makeing schedule() aware that it is
being called for preemption.  See the MontaVista patch for details.

ftp://ftp.mvista.com/pub/Area51/preemptible_kernel/

 
> +        }
> +        atomic_dec(&current->preemptable);
> 
> What if something happens during the schedule() that would require
> another thread...?
> 
> +}
> 
> I have been discussing different layout with George on Montavista
> also doing this kind of work... (different var and value range)
> 
> static incline void enable_preempt(void) {
>     if (--current->preempt_count) {
>         smp_mb(); /* not shure if needed... */
>         preempt_schedule();
>     }
> }
> 
> in sched.c (some smp_mb might be needed here too...)
> void preempt_schedule() {
>     while (current->need_resched) {
>         current->preempt->count++; /* prevent competition with IRQ code */
>         if (current->need_resched)
>             schedule();
>         current->preempt_count--;
>     }
> }
> 
> > I'm not convinced a full preemptive kernel is something
> > interesting mainly due to the context switch cost (actually mmu contex
> > switch).
> 
> It will NOT be fully, it will be mostly.
> You will only context switch when a higher prio thread gets runnable, two
> ways:
> 1) external intterupt waking higher prio process, same context swithes as
> when running in user code. We won't get more interrupts.
> 2) wake up due to something we do. Not that many places, mostly due to
> releasing syncronization objects (spinlocks does not count).
> 
> If this still is a problem, we can select to only preemt to processes running
> RT stuff. SCHED_FIFO and SCHED_RR by letting them set need_resched to 2...

The preemption ususally just switches earlier.  The switch would happen
soon anyway.  That is what need_resched =1; means.
> 
> > Benchmarking is a good way to get a global overview on this.
> 
> Remember to benchmark with stuff that will make the positive aspects visible
> too. Playing audio (with smaller buffers), more reliably burning CD ROMs,
> less hichups while playing video [if run with higher prio...]
> Plain throuput tests won't tell the whole story!
> 
> see
>  http://www.gardena.net/benno/linux/audio
>  http://www.linuxdj.com/latency-graph/
> 
> > What about only preemptable kernel threads ?
> 
> No, it won't help enough.
> 
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