Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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 ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com