Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Jul 20, 2009 at 01:17:02PM +0200, Peter Zijlstra wrote:
> On Sat, 2009-07-18 at 15:30 -0700, Arjan van de Ven wrote:
> > On Sat, 18 Jul 2009 15:09:38 -0700
> > john stultz <johnstul@us.ibm.com> wrote:
> > 
> > > After talking with some application writers who want very fast, but
> > > not fine-grained timestamps, I decided to try to implement a new
> > > clock_ids to clock_gettime(): CLOCK_REALTIME_COARSE and
> > > CLOCK_MONOTONIC_COARSE which returns the time at the last tick. This
> > > is very fast as we don't have to access any hardware (which can be
> > > very painful if you're using something like the acpi_pm clocksource),
> > > and we can even use the vdso clock_gettime() method to avoid the
> > > syscall. The only trade off is you only get low-res tick grained time
> > > resolution.
> > 
> > Does this tie us to having a tick? I still have hope that we can get
> > rid of the tick even when apps are running .... since with CFS we don't
> > really need the tick for the scheduler anymore for example....
> 
> On the hardware side to make this happen we'd need a platform that has:
> 
>   - cheap, high-res, cross-cpu synced, clocksource
>   - cheap, high-res, clockevents
> 
> Maybe power64, sparc64 and s390x qualify, but certainly nothing on x86
> does.
> 
> Furthermore, on the software side we'd need a few modifications, such as
> doing lazy accounting for things like u/s-time which currently rely on
> the tick and moving the load-balancing into a hrtimer.
> 
> Also, even with the above done, we'd probably want to tinker with the
> clockevent/hrtimer code and possibly use a second per-cpu hardware timer
> for the scheduler, since doing the whole hrtimer rb-tree dance for every
> context switch is simply way too expensive.
> 
> But even with all that manged, there's still other bits that rely on the
> tick -- RCU being one of the more interesting ones.

On alternative to the tick is to inform RCU of each transition to/from
userspace, so that RCU would treat user-mode execution as it currently
does dyntick-idle state.  If there is -never- to be any scheduling-clock
interrupts, then RCU would need to also know about transitions to/from
the idle loop -- which happens automatically if CONFIG_NO_HZ, of course.

But I expect that there would be some additional excitement elsewhere...
And given the large number of transitions to/from userspace, getting all
of them noted in the RCU case might be non-trivial as well.

							Thanx, Paul