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

[Fam Zheng]

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.

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