On Tue, Feb 17, 2015 at 07:36:36PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 17, 2015 at 08:05:32AM -0800, Paul E. McKenney wrote:
> > > FWIW, we should probably update that table to include control
> > > dependencies too; we didn't (formally) have those back then I think.
> > >
> > > The blob under SMP BARRIER PAIRING does not mention pairing with control
> > > dependencies; and I'm rather sure I've done so.
> >
> > Yep, they should pair as well, though the pairing is limited.
> > No transitivity, of course.
> >
> > So the straightforward approach requires eighteen bits per cell, though
> > some of them are a bit, ummm, "unusual".
>
> Right, I think the idea was to not mark with 'X' when very unusual,
> otherwise you do indeed obtain the below 'trivial' matrix.
>
> > Sixteen of these are given by
> > Scenarios 0-15 in http://lwn.net/Articles/573436/, with the barrier on
> > the side corresponding to the first column and the barrier on the top
> > corresponding to the second column. The seventeenth bit says whether
> > you get transitivity chaining after the top access, assuming that it
> > happens later. The eighteenth bit says whether you get transitivity
> > chaining before the side access, assuming that it happens earlier.
> >
> > The following is a rough first guess, filling in only the diagonal.
> > Some of the entries are no doubt wrong, and getting them right requires
> > something like 7*7*18 test cases, which will take some time. So, is
> > something like this really helpful?
>
>
> > | mb | wmb | rmb | rbd | acq | rel | ctl |
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > mb | 3ffff | X | X | X | X | X | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > wmb | X | 01000 | X | X | X | X | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > rmb | X | X | 00000 | X | X | X | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > rbd | X | X | X | 00000 | X | X | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > acq | X | X | X | X | 00020 | X | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > rel | X | X | X | X | X | 0cc00 | X +
> > -----+-------+-------+-------+-------+-------+-------+-------+
> > ctl | X | X | X | X | X | X | 00020 +
> > -----+-------+-------+-------+-------+-------+-------+-------+
>
> So maybe make two tables; one with 'obvious' pairings, which would
> include things like mb - {mb,rmb,wmb}; rmb-wmb; acq-rel; ctl-mb; etc.
>
> That table is for people to quickly check 'easy'; like yes wmb-rbd makes
> sense and rmb-rbd doesn't appear to make sense, I need more reading up.
>
> After that do the 'funny' table, which will explain further possible
> pairings in more detail, like the rmb-rbd pairing.
>
> I'm not entirely sure we want to do the 7*7*18 state table, that's a lot
> of work to exhaustively generate. We should be lazy and demand fill when
> people come to us.
I could do a table per communication style. For example, message
passing looks like this (give or take likely errors in the table):
Side CPU Top CPU
-------- -------
X = 1; r1 = Y;
<some barrier> <some barrier>
Y = 1; r2 = X;
assert(r1 == 0 || r2 == 1);
| mb | wmb | rmb | rbd | acq | rel | ctl |
-----+-------+-------+-------+-------+-------+-------+-------+
mb | Y | | Y | y | Y | | Y +
-----+-------+-------+-------+-------+-------+-------+-------+
wmb | Y | | Y | y | Y | | Y +
-----+-------+-------+-------+-------+-------+-------+-------+
rmb | | | | | | | +
-----+-------+-------+-------+-------+-------+-------+-------+
rbd | | | | | | | +
-----+-------+-------+-------+-------+-------+-------+-------+
acq | | | | | | | +
-----+-------+-------+-------+-------+-------+-------+-------+
rel | Y | | Y | y | Y | | Y +
-----+-------+-------+-------+-------+-------+-------+-------+
ctl | | | | | | | +
-----+-------+-------+-------+-------+-------+-------+-------+
Here "Y" says that the barrier pair works, "y" says that it can
work but requires an artificial dependency, and " " says that
it does not work.
Thanx, Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
