Re: [Qemu-devel] [RFC 0/6] monitor: allow per-monitor thread

[Dr. David Alan Gilbert]

* Fam Zheng (address@hidden) wrote:
> On Tue, 08/22 13:59, Peter Xu wrote:
> > On Tue, Aug 22, 2017 at 12:15:19PM +0800, Fam Zheng wrote:
> > > On Tue, 08/22 10:56, Peter Xu wrote:
> > > > I haven't really encountered (c), but I think it's the migrate_cancel
> > > > command that matters, which should not need BQL as well.
> > > 
> > > There is bdrv_invalidate_cache_all() in migrate_cancel which clearly 
> > > isn't safe.
> > > Is that if block unreachable in this case? If so we should assert, 
> > > otherwise
> > > this command is not okay to run without BQL.
> > 
> > Ah. I see.  Even if so, if that is the only usage of BQL, IMHO we can
> > still mark migrate_cancel as "without-bql=true", instead we take the
> > BQL before calling bdrv_invalidate_cache_all().  Then migrate_cancel
> > can be BQL-free at least when block migration is not active.
> > 
> > > 
> > > Generically, what guarantee the thread-safety of a qmp command when you 
> > > decide
> > > BQL is not needed? In other words, how do you prove commands are safe 
> > > without
> > > BQL? I think almost every command accesses global state, but lock-free 
> > > data
> > > structures are rare AFAICT.
> > 
> > I would suggest we split the problem into at least three parts.  IMHO
> > we need to answer below questions one by one to know what we should do
> > next:
> > 
> > 1. whether we can handle monitor commands outside iothread, or say, in
> >    an isolated thread?
> > 
> >    This is basically what patch 2 does, the "per-monitor threads".
> > 
> >    IMHO this is the very first question to ask.  So now I know that at
> >    least current code cannot do it.  We need to at least do something
> >    to remove/replace the assertion to make this happen.  Can we?  I
> >    don't really know the answer yet.  If this is undoable, we can skip
> >    question 2/3 below and may need to rethink on how to solve the
> >    problem that postcopy recovery encounters.
> > 
> > 2. whether there is any monitor commands can run without BQL?
> > 
> >    This is basically what patch 3/5 does, one for QMP, one for HMP.
> > 
> >    If we can settle question 1, then we can possibly start consider
> >    this question.  This step does not really allow any command to run
> >    without BQL, but we need to know whether it's possible in general,
> >    and if possible, we provide a framework to allow QMP/HMP developers
> >    to specify that.  If you see patch 3/5, the default behavior is
> >    still taking the BQL for all commands.
> > 
> >    IMHO doing this whole thing is generally good in the sense that
> >    this is actually forcing ourselves to break the BQL into smaller
> >    locks.  Take the migration commands for example: migrate_incoming
> >    do not need BQL, and when we write codes around it we know that we
> >    don't need to think about thread-safety.  That's not good IMHO.  I
> >    think it's time we should start consider thread-safety always.
> >    Again, for migrate_incoming to do this, actually we'll possibly at
> >    least need a migration management lock (the smaller lock) to make
> >    sure e.g. the user is not running two migrate_incoming commands in
> >    parallel (after per-monitor threads, it can happen).  But it's
> >    better than BQL, because BQL is for sure too big, so even a guest
> >    page access (as long as it held the BQL) can block migration
> >    commands.
> 
> Yes, this is my point. You cannot just declare a command "BQL-free" without
> adding small locks first, and I think this is actually missing in this series.
> As you said, two per-monitor threads can race if they do migrate_incoming in
> parallel.  This is also the answer to 3.

Some commands really could be lock-free.  For example an 'info status'
should just be a read without any locking.
It might also be possible to make a version of migrate_cancel lock-free;
in particular the critical part is being able to do a shutdown() on the
socket - that's enough to cause any of the migration threads to 
unblock and take their failure path.

Also in the case of COLO; there should be an equivalent place to
do the shutdown() on the migration thread and then set a flag to cause
failover.
(It's currently qmp_x_colo_lost_heartbeat but I think the eventual
version would be more complex).

Dave

> Fam
> 
> > 
> > 3. which monitor commands can be run without BQL?
> > 
> >    This is what patch 4/6 was doing.  It tries to move
> >    migrate_incoming command out as the first candidate BQL-free
> >    command.
> > 
> >    Yes it's hard to say which command can be run without BQL.  So we
> >    need to investigate, possibly modify existing codes to make sure
> >    it's thread-safe, prove validity, then we can add the new ones into
> >    the BQL-free list.
> > 
> >    If after evaluating the pros and cons, we found that one command
> >    can be put into BQL-free but not worth the time for working on it,
> >    we can also keep those commands under BQL.
> > 
> > I assume question 3 is the one you were asking, and I'd say we may
> > need to solve question 1/2 first.  If we are done with 1/2, we just
> > need to spend time on each command to prove whether it is doable to
> > let that command run without BQL, and whether it worths itself to move
> > the command out of BQL.  Then we decide.  Thanks,
> > 
> > -- 
> > Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK