Messages in this thread | | | Date | Thu, 27 Jul 2017 11:22:32 +0100 | From | Will Deacon <> | Subject | Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option |
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On Thu, Jul 27, 2017 at 10:53:12AM +0200, Peter Zijlstra wrote: > 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.
If you did that, wouldn't you need to leave the faulting entries intact until all CPUs with the mm scheduled have either faulted or context-switched? We don't have per-cpu permission bits in the page tables, unfortunately.
Will
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