Messages in this thread | | | Date | Thu, 27 Jul 2017 10:53:12 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option |
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On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
> 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.
You forgot to ask yourself how you could abuse this.. just spawn more threads.
Per-thread limits are nearly useless, because spawning new threads is cheap.
> 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 ?
So I really don't like SHARED_EXPEDITED, and your use-cases (from later emails) makes me think sys_membarrier() should have a pointer argument to identify the shared mapping.
But even then, iterating the rmap for something that has 1000+ maps isn't going to be nice or fast, even in kernel space.
Another crazy idea is using madvise() for this. The new MADV_MEMBAR could revoke PROT_WRITE and PROT_READ for all extant PTEs. Then the tasks attempting access will fault and the fault handler can figure out if it still needs to issue a MB or not before reinstating the PTE.
That is fully contained to the tasks actually having that map, and doesn't perturb anybody else.
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