On 05/27/2011 02:07 PM, Ingo Molnar wrote:
* Ingo Molnar<mingo@xxxxxxx> wrote:
> > This code is very much tied with the kernel scheduler. [...]
>
> It would not be particularly complex to enable user-space to
> request a callback on context switch events.
>
> I was thinking on and off about allowing perf events to generate a
> per sampling event notification signal on specific events, such as
> page faults or context switches.
I was thinking about that on and off so loudly that Peter implemented
it long ago via fasync support on the perf event fd! :-)
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.
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.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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