Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Jul 10, 2018 at 10:12:29AM -0700, Paul E. McKenney wrote:
[..]
> > > > The other question I have is about the whole "nohz-full doesn't work" thing.
> > > > I didn't fully understand why. RCU is already tracking the state of nohz-full
> > > > CPUs because the rcu dynticks code in (kernel/rcu/tree.c) monitors
> > > > transitions to and from usermode even if the timer tick is turned off. So why
> > > > would it not work?
> > > 
> > > In the nohz_full case, there is no need for sys_membarrier()'s call to
> > > synchronize_sched() to interact directly with the nohz_full CPU.  It
> > > can instead look at the target CPU's dyntick-idle state, and that state
> > > would potentially have been set in the dim distant past, thus having
> > > no effect on the target CPU's current execution.
> > 
> > In nohz-idle case though, there's nothing to promote the barrier() to
> > smp_mb() if you were to purely look at the dynticks-idle state on the
> > nohz-full CPU executing in user mode?
> > 
> > So then it makes sense to me now that nohz-full needs something to IPI that
> > CPU inorder to enforce the needed memory barrier and pure synchronize_sched()
> > wouldn't work. So then makes me think the expedited versions of
> > synchronize_sched should be able to do the job but I could off on a different
> > track..
> 
> The problem is that the expedited versions also check the dyntick-idle
> state and don't touch idle (or nohz_full usermode) CPUs.  This is by
> design for the battery-powered embedded use case.  ;-)

Oh ok! ;)

I guess there's also a MEMBARRIER_CMD_GLOBAL_EXPEDITED which seems to IPI
CPUs (I'm guessing regardless of dynticks state) and execute smp_mb within
the IPI so userspace can fallback to using that incase MEMBARRIER_CMD_GLOBAL
returns -EINVAL.

thanks!

-Joel