Re: [PATCH] util/async: make bh_aio_poll() O(1)

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Feb 19, 2020 at 08:05:12PM +0100, Paolo Bonzini wrote:
> Il mer 19 feb 2020, 18:58 Stefan Hajnoczi <stefanha@redhat.com> ha scritto:
> 
> > On Wed, Feb 19, 2020 at 12:09:48PM +0100, Paolo Bonzini wrote:
> > > Really a great idea, though I have some remarks on the implementation
> > below.
> > >
> > > On 19/02/20 11:00, Stefan Hajnoczi wrote:
> > > > + * Each aio_bh_poll() call carves off a slice of the BH list.  This
> > way newly
> > > > + * scheduled BHs are not processed until the next aio_bh_poll()
> > call.  This
> > > > + * concept extends to nested aio_bh_poll() calls because slices are
> > chained
> > > > + * together.
> > >
> > > This is the tricky part so I would expand a bit on why it's needed:
> > >
> > > /*
> > >  * Each aio_bh_poll() call carves off a slice of the BH list, so that
> > >  * newly scheduled BHs are not processed until the next aio_bh_poll()
> > >  * call.  All active aio_bh_poll() calls chained their slices together
> > >  * in a list, so that nested aio_bh_poll() calls process all scheduled
> > >  * bottom halves.
> > >  */
> >
> > Thanks, will fix in v2.
> >
> > > > +struct BHListSlice {
> > > > +    QEMUBH *first_bh;
> > > > +    BHListSlice *next;
> > > > +};
> > > > +
> > >
> > > Using QLIST and QSLIST removes the need to create your own lists, since
> > > you can use QSLIST_MOVE_ATOMIC and QSLIST_INSERT_HEAD_ATOMIC.  For
> > example:
> > >
> > > struct BHListSlice {
> > >     QSLIST_HEAD(, QEMUBH) first_bh;
> > >     QLIST_ENTRY(BHListSlice) next;
> > > };
> > >
> > > ...
> > >
> > >     QSLIST_HEAD(, QEMUBH) active_bh;
> > >     QLIST_HEAD(, BHListSlice) bh_list;
> >
> > I thought about this but chose the explicit tail pointer approach
> > because it lets aio_compute_timeout() and aio_ctx_check() iterate over
> > both the active BH list and slices in a single for loop :).  But
> > thinking about it more, maybe it can still be done by replacing
> > active_bh with a permanently present first BHListSlice element.
> >
> 
> Probably not so easy since the idea was to empty the slices list entirely
> via the FIFO order.
> 
> But since you are rewriting everything anyway, can you add a flag for
> whether there are any non-idle bottom halves anywhere in the list? It need
> not be computed perfectly, because any non-idle bh will cause the idle
> bottom halves to be triggered as well; you can just set in qemu_bh_schedule
> and clear it at the end of aio_bh_poll.
> 
> Then if there is any active bh or slice you consult the flag and use it to
> return the timeout, which will be either 0 or 10ms depending on the flag.
> That's truly O(1). (More precisely, this patch goes from O(#created-bh) to
> O(#scheduled-bh), which of course is optimal for aio_bh_poll but not for
> aio_compute_timeout; making timeout computation O(1) can lower latency a
> bit by decreasing the constant factor).

Yes, good idea.  I'll send a follow-up patch.

Stefan

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