* Avi Kivity <[email protected]> wrote:
> > So if you set a notification signal via fcntl(F_SETOWN) on the
> > scheduler context switch event fd, the user-space RCU code will
> > get a signal on every context switch.
>
> Context switches are completely uninteresting for userspace rcu:
>
> rcu_read_lock();
> ---> context switch
>
> have we learned anything from that? no. User code is always
> preemptible and migratable. If rcu_read_lock() prevented migration
> somehow, then we'd know that a context switch means we've started a
> grace period for this thread. But it doesn't, so we don't.
Well, in the next mail i mentioned that we can do migration events as
well, which would be useful: instead of having to keep track of
nr_tasks RCU grace periods we could simplify it down to nr_cpus.
But if we indexed by the TID then we wouldnt need any scheduler
bindings at all - this is the simpler approach.
> What's needed are explicit notifications about grace periods. For
> the vcpu threads, calling KVM_VCPU_RUN seems like a good point.
> For I/O threads, completion of processing of an event is also a
> good point.
Grace period notifications are needed too, obviously.
Thanks,
Ingo
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