* Peter Xu (pet...@redhat.com) wrote: > On Fri, Aug 25, 2017 at 10:14:12AM +0100, Dr. David Alan Gilbert wrote: > > * Peter Xu (pet...@redhat.com) wrote: > > > On Thu, Aug 24, 2017 at 07:37:32AM +0800, Fam Zheng wrote: > > > > On Wed, 08/23 18:44, Dr. David Alan Gilbert wrote: > > > > > * Peter Xu (pet...@redhat.com) wrote: > > > > > > Introducing this new parameter for QMP commands in general to mark > > > > > > out > > > > > > when the command does not need BQL. Normally QMP command > > > > > > executions are > > > > > > done with the protection of BQL in QEMU. However the truth is that > > > > > > not > > > > > > all the QMP commands require the BQL. > > > > > > > > > > > > This new parameter provides a way to allow QMP commands to run in > > > > > > parallel when possible, without the contention on the BQL. > > > > > > > > > > > > Since the default value of "without-bql" is still false, so now all > > > > > > QMP > > > > > > commands are still protected by BQL still. > > > > > > > > > > > > Signed-off-by: Peter Xu <pet...@redhat.com> > > > > > > > > > > We should define what a 'without-bql' command is allowed to do: > > > > > 'Commands that have without-bql set _may_ be called without the bql > > > > > being taken. They must not take the bql or any other lock that may > > > > > become dependent on the bql.' > > > > > > Sure. > > > > > > > > (Do we need to say anything about RCU?) > > > > > > Could I ask how is RCU related? > > > > My definition above said that anything declared without bql couldn't > > take the bql, so couldn't block on any other thread holding the bql. > > But is our command allowed to use synchronize_rcu or rcu_read_lock > > that could wait for or block other threads doing rcu stuff? > > Because if it did is there any guarantee that it wouldn't block? > > I see. Shall we just ignore RCU for now? Since currently I don't see > a real synchronize_rcu() user yet in QEMU, except the RCU thread. And > rcu_read_lock() should not block itself, so IMHO calling it only in > monitor command handlers should always be fine?
Yes, I think you're right that the rcu_read_lock is OK; just something to keep in mind. Dave > > > > > > > > > > > > > > > > > Also, 'no-bql' is shorter :-) > > > > > > > > Or rather "need-bql" that defaults to true to avoid double negative > > > > (TM) with > > > > "no-bql = false"? > > > > > > Ok let me use "need-bql". :) > > > > Fine by me. > > I'm switching to "need-bql" for QMP only, and used "no-bql" in HMP, > since I failed to find a good way to init mon_cmd_t field to true by > default. > > -- > Peter Xu -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
