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Date: Wed, 26 Jul 2017 14:11:46 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: Peter Zijlstra <peterz@...radead.org>, linux-kernel <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>, dipankar <dipankar@...ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Josh Triplett <josh@...htriplett.org>, Thomas Gleixner <tglx@...utronix.de>, rostedt <rostedt@...dmis.org>, David Howells <dhowells@...hat.com>, Eric Dumazet <edumazet@...gle.com>, fweisbec <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>, Will Deacon <will.deacon@....com> Subject: Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option 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@...ux.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@...ux.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(). ;-) Thanx, Paul
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