[lttng-dev] Quick questions about liburcu and RCU in general

[Mathieu Desnoyers]

* Richard Braun (rbraun at sceen.net) wrote:
> On Tue, May 07, 2013 at 07:59:14AM -0400, Mathieu Desnoyers wrote:
> > > 1/ Why use poll instead of sched_yield in e.g. force_mb_all_readers ?
> > > (I guess it's about portability and the effect is expected to be the same,
> > > but is there another reason ?)
> > 
> > poll() allow us to do a millisecond-level wait (timer-based).
> > sched_yield() is pretty much a scheduler hack that just says "be nice to
> > other scheduled processes here". Quoting sched_yield(2):
> > 
> >        If the calling thread is the only thread in the highest  priority list
> >        at that time, it will continue to run after a call to sched_yield().
> > 
> > This is a kind of behavior we don't want.
> 
> Right, I see. But then, poll enforces a minimum delay whatever happens
> (which may be increased up to the scheduler time slice depending on the
> kernel). I guess this isn't something we want either.
> 
> On the other hand, AIUI, poll is called only after all signals were
> already sent, and if a receiving thread didn't update its need_mb yet,
> so I expect it's a rare worst case that doesn't require more care.

Indeed, that would be a rare worst case. Moreover, if we really care
about this code path execution time, we might want to consider using the
wait/wakeup scheme based on sys_futex used elsewhere in the urcu
implementation.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com