Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts""

[Avi Kivity]

On 10/09/2012 03:50 PM, Paolo Bonzini wrote:
> Il 09/10/2012 15:21, Avi Kivity ha scritto:
>> On 10/09/2012 03:11 PM, Paolo Bonzini wrote:
>>>> But no, it's actually impossible.  Hotplug may be triggered from a vcpu
>>>> thread, which clearly it can't be stopped.
>>>
>>> Hotplug should always be asynchronous (because that's how hardware
>>> works), so it should always be possible to delegate the actual work to a
>>> non-VCPU thread.  Or not?
>> 
>> The actual device deletion can happen from a different thread, as long
>> as you isolate the device before.  That's part of the garbage collector
>> idea.
>> 
>> vcpu thread:
>>   rcu_read_lock
>>   lookup
>>   dispatch
>>     mmio handler
>>       isolate
>>       queue(delete_work)
>>   rcu_read_unlock
>> 
>> worker thread:
>>   process queue
>>     delete_work
>>       synchronize_rcu() / stop_machine()
>>       acquire qemu lock
>>       delete object
>>       drop qemu lock
>> 
>> Compared to the garbage collector idea, this drops fined-grained locking
>> for the qdev tree, a significant advantage.  But it still suffers from
>> dispatching inside the rcu critical section, which is something we want
>> to avoid.
> 
> But we are not Linux, and I think the tradeoffs are different for RCU in
> Linux vs. QEMU.
> 
> For CPUs in the kernel, running user code is just one way to get things
> done; QEMU threads are much more event driven, and their whole purpose
> is to either run the guest or sleep, until "something happens" (VCPU
> exit or readable fd).  In other words, QEMU threads should be able to
> stay most of the time in KVM_RUN or select() for any workload (to some
> approximation).

If you're streaming data (the saturated iothread from that other thread)
or live migrating or have a block job with fast storage, this isn't
necessarily true.  You could make sure each thread polls the rcu state
periodically though.

> Not just that: we do not need to minimize RCU critical sections, because
> anyway we want to minimize the time spent in QEMU, period.
> 
> So I believe that to some approximation, in QEMU we can completely
> ignore everything else, and behave as if threads were always under
> rcu_read_lock(), except if in KVM_RUN/select.  KVM_RUN and select are
> what Paul McKenney calls extended quiescent states, and in fact the
> following mapping works:
> 
>     rcu_extended_quiesce_start()     -> rcu_read_unlock();
>     rcu_extended_quiesce_end()       -> rcu_read_lock();
>     rcu_read_lock/unlock()           -> nop
> 
> This in turn means that dispatching inside the RCU critical section is
> not really bad.

I believe you still cannot synchronize_rcu() while in an rcu critical
section per the rcu documentation, even when lock/unlock map to nops.
Of course we can violate that and it wouldn't know a thing, but I prefer
to stick to the established pattern.


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