Re: [RFC] sched_tick with interrupts enabled

[Nick Piggin]

Christoph Lameter wrote:
> scheduler_tick() has the potential of running for some time if f.e.
> sched_domains for a system with 1024 processors have to be balanced.
> We currently do all of that with interrupts disabled. So we may be unable
> to service interrupts for some time.
>
> I wonder if it would be possible to put the sched_tick() into a tasklet and
> allow interrupts to be enabled? Preemption is still disabled and so we
> are stuck on a cpu.

I don't think so because it also does accounting which probably wants to
be precisely on a tick.

Also the timeslice accounting takes the rq lock without disabling interrupt,
and task wakeups can easily happen from interrupt / softirq.

Taking rebalance_tick out of scheduler_tick, and not calling rebalance_tick
from sched_fork is probably a good idea.

After that, it might be acceptable to call rebalance_tick from a tasklet,
although it would be uneeded overhead on small systems. It might be better
to have a special case for your large systems which does the full balance
and runs less frequently in a tasklet (and make your regular rebalance_tick
skip the top level balancing).

--
SUSE Labs, Novell Inc.
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