Re: Regression in SYS_membarrier expedited

[Rich Felker]

On Sun, Feb 17, 2019 at 04:34:45PM -0500, Mathieu Desnoyers wrote:
> ----- On Feb 17, 2019, at 1:48 PM, Rich Felker dalias@xxxxxxxx wrote:
> 
> > commit a961e40917fb14614d368d8bc9782ca4d6a8cd11 made it so that the
> > MEMBARRIER_CMD_PRIVATE_EXPEDITED command cannot be used without first
> > registering intent to use it. However, registration is an expensive
> > operation since commit 3ccfebedd8cf54e291c809c838d8ad5cc00f5688, which
> > added synchronize_sched() to it; this means it's no longer possible to
> > lazily register intent at first use, and it's unreasonably expensive
> > to preemptively register intent for possibly extremely-short-lived
> > processes that will never use it. (My usage case is in libc (musl),
> > where I can't know if the process will be short- or long-lived;
> > unnecessary and potentially expensive syscalls can't be made
> > preemptively, only lazily at first use.)
> > 
> > Can we restore the functionality of MEMBARRIER_CMD_PRIVATE_EXPEDITED
> > to work even without registration? The motivation of requiring
> > registration seems to be:
> > 
> >    "Registering at this time removes the need to interrupt each and
> >    every thread in that process at the first expedited
> >    sys_membarrier() system call."
> > 
> > but interrupting every thread in the process is exactly what I expect,
> > and is not a problem. What does seem like a big problem is waiting for
> > synchronize_sched() to synchronize with an unboundedly large number of
> > cores (vs only a few threads in the process), especially in the
> > presence of full_nohz, where it seems like latency would be at least a
> > few ms and possibly unbounded.
> > 
> > Short of a working SYS_membarrier that doesn't require expensive
> > pre-registration, I'm stuck just implementing it in userspace with
> > signals...
> 
> Hi Rich,
> 
> Let me try to understand the scenario first.
> 
> musl libc support for using membarrier private expedited
> would require to first register membarrier private expedited for
> the process at musl library init (typically after exec). At that stage, the
> process is still single-threaded, right ? So there is no reason
> to issue a synchronize_sched() (or now synchronize_rcu() in newer
> kernels):
> 
> membarrier_register_private_expedited()
> 
>         if (!(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)) {
>                 /*
>                  * Ensure all future scheduler executions will observe the
>                  * new thread flag state for this process.
>                  */
>                 synchronize_rcu();
>         }
> 
> So considering that pre-registration carefully done before the process
> becomes multi-threaded just costs a system call (and not a synchronize_sched()),
> does it make the pre-registration approach more acceptable ?

It does get rid of the extreme cost, but I don't think it would be
well-received by users who don't like random unnecessary syscalls at
init time (each adding a few us of startup time cost). If it's so
cheap, why isn't it just the default at kernel-side process creation?
Why is there any requirement of registration to begin with? Reading
the code, it looks like all it does is set a flag, and all this flag
is used for is erroring-out if it's not set.

Rich