----- On Jul 26, 2017, at 2:30 PM, Paul E. McKenney paul...@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 >> paul...@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 ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com