Re: [Qemu-devel] [RFC PATCH 13/26] cpus: only take BQL for sleeping threads

[David Hildenbrand]

On 02.11.2017 12:08, Paolo Bonzini wrote:
> On 31/10/2017 12:26, Pavel Dovgalyuk wrote:
>> From: Alex Bennée <address@hidden>
>>
>> Now the only real need to hold the BQL is for when we sleep on the
>> cpu->halt conditional. The lock is actually dropped while the thread
>> sleeps so the actual window for contention is pretty small. This also
>> means we can remove the special case hack for exclusive work and
>> simply declare that work no longer has an implicit BQL held. This
>> isn't a major problem async work is generally only changing things in
>> the context of its own vCPU. If it needs to work across vCPUs it
>> should be using the exclusive mechanism or possibly taking the lock
>> itself.
>>
>> Signed-off-by: Alex Bennée <address@hidden>
>> Tested-by: Pavel Dovgalyuk <address@hidden>
> 
> At least cpu_throttle_thread would fail with this patch.
> 
> Also I am not sure if the s390 SIGP handlers are ready for this.
> 

We have a global lock to the SIGP "facility". However we need the BQL in
order to inject interrupts into CPUs (otherwise it would trigger an
assert when injecting).

We inject Restart and Stop interrupts from run_on_cpu. This requires the
BQL. So Paolo should be right, this change would break s390x.


> Paolo


-- 

Thanks,

David