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    SubjectRe: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option
    On Wed, Jul 26, 2017 at 02:11:46PM -0700, Paul E. McKenney wrote:
    > On Wed, Jul 26, 2017 at 08:37:23PM +0000, Mathieu Desnoyers wrote:
    > > ----- On Jul 26, 2017, at 2:30 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:
    > >
    > > > On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
    > > >> ----- On Jul 26, 2017, at 11:42 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com
    > > >> wrote:
    > > >>
    > > >> > On Wed, Jul 26, 2017 at 09:46:56AM +0200, Peter Zijlstra wrote:
    > > >> >> On Tue, Jul 25, 2017 at 10:50:13PM +0000, Mathieu Desnoyers wrote:
    > > >> >> > This would implement a MEMBARRIER_CMD_PRIVATE_EXPEDITED (or such) flag
    > > >> >> > for expedited process-local effect. This differs from the "SHARED" flag,
    > > >> >> > since the SHARED flag affects threads accessing memory mappings shared
    > > >> >> > across processes as well.
    > > >> >> >
    > > >> >> > I wonder if we could create a MEMBARRIER_CMD_SHARED_EXPEDITED behavior
    > > >> >> > by iterating on all memory mappings mapped into the current process,
    > > >> >> > and build a cpumask based on the union of all mm masks encountered ?
    > > >> >> > Then we could send the IPI to all cpus belonging to that cpumask. Or
    > > >> >> > am I missing something obvious ?
    > > >> >>
    > > >> >> I would readily object to such a beast. You far too quickly end up
    > > >> >> having to IPI everybody because of some stupid shared map or something
    > > >> >> (yes I know, normal DSOs are mapped private).
    > > >> >
    > > >> > Agreed, we should keep things simple to start with. The user can always
    > > >> > invoke sys_membarrier() from each process.
    > > >>
    > > >> Another alternative for a MEMBARRIER_CMD_SHARED_EXPEDITED would be rate-limiting
    > > >> per thread. For instance, we could add a new "ulimit" that would bound the
    > > >> number of expedited membarrier per thread that can be done per millisecond,
    > > >> and switch to synchronize_sched() whenever a thread goes beyond that limit
    > > >> for the rest of the time-slot.
    > > >>
    > > >> A RT system that really cares about not having userspace sending IPIs
    > > >> to all cpus could set the ulimit value to 0, which would always use
    > > >> synchronize_sched().
    > > >>
    > > >> Thoughts ?
    > > >
    > > > The patch I posted reverts to synchronize_sched() in kernels booted with
    > > > rcupdate.rcu_normal=1. ;-)
    > > >
    > > > But who is pushing for multiple-process sys_membarrier()? Everyone I
    > > > have talked to is OK with it being local to the current process.
    > >
    > > I guess I'm probably the guilty one intending to do weird stuff in userspace ;)
    > >
    > > Here are my two use-cases:
    > >
    > > * a new multi-process liburcu flavor, useful if e.g. a set of processes are
    > > responsible for updating a shared memory data structure, and a separate set
    > > of processes read that data structure. The readers can be killed without ill
    > > effect on the other processes. The synchronization could be done by one
    > > multi-process liburcu flavor per reader process "group".
    > >
    > > * lttng-ust user-space ring buffers (shared across processes).
    > >
    > > Both rely on a shared memory mapping for communication between processes, and
    > > I would like to be able to issue a sys_membarrier targeting all CPUs that may
    > > currently touch the shared memory mapping.
    > >
    > > I don't really need a system-wide effect, but I would like to be able to target
    > > a shared memory mapping and efficiently do an expedited sys_membarrier on all
    > > cpus involved.
    > >
    > > With lttng-ust, the shared buffers can spawn across 1000+ processes, so
    > > asking each process to issue sys_membarrier would add lots of unneeded overhead,
    > > because this would issue lots of needless memory barriers.
    > >
    > > Thoughts ?
    >
    > Dealing explicitly with 1000+ processes sounds like no picnic. It instead
    > sounds like a job for synchronize_sched_expedited(). ;-)

    Actually...

    Mathieu, does your use case require unprivileged access to sys_membarrier()?

    Thanx, Paul

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