Messages in this thread | | | Date | Thu, 27 Jul 2017 07:44:39 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option |
| |
On Thu, Jul 27, 2017 at 12:39:36PM +0000, Mathieu Desnoyers wrote: > ----- On Jul 26, 2017, at 9:45 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote: > > > 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()? > > Unfortunately, yes, it does require sys_membarrier to be used from non-root > both for lttng-ust and liburcu multi-process. And as Peter pointed out, stuff > like containers complicates things even for the root case.
Hey, I was hoping! ;-)
Thanx, Paul
| |