Messages in this thread | | | Date | Wed, 26 Jul 2017 16:16:09 +0900 | From | Byungchul Park <> | Subject | Re: [PATCH v7 08/16] lockdep: Avoid adding redundant direct links of crosslocks |
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On Tue, Jul 25, 2017 at 05:41:36PM +0200, Peter Zijlstra wrote: > On Wed, May 24, 2017 at 05:59:41PM +0900, Byungchul Park wrote: > > We can skip adding a dependency 'AX -> B', in case that we ensure 'AX -> > > the previous of B in hlocks' to be created, where AX is a crosslock and > > B is a typical lock. Remember that two adjacent locks in hlocks generate > > a dependency like 'prev -> next', that is, 'the previous of B in hlocks > > -> B' in this case. > > > > For example: > > > > in hlocks[] > > ------------ > > ^ A (gen_id: 4) --+ > > | | previous gen_id > > | B (gen_id: 3) <-+ > > | C (gen_id: 3) > > | D (gen_id: 2) > > oldest | E (gen_id: 1) > > > > in xhlocks[] > > ------------ > > ^ A (gen_id: 4, prev_gen_id: 3(B's gen id)) > > | B (gen_id: 3, prev_gen_id: 3(C's gen id)) > > | C (gen_id: 3, prev_gen_id: 2(D's gen id)) > > | D (gen_id: 2, prev_gen_id: 1(E's gen id)) > > oldest | E (gen_id: 1, prev_gen_id: NA) > > > > On commit for a crosslock AX(gen_id = 3), it's engough to add 'AX -> C', > > but adding 'AX -> B' and 'AX -> A' is unnecessary since 'AX -> C', 'C -> > > B' and 'B -> A' cover them, which are guaranteed to be generated. > > > > This patch intoduces a variable, prev_gen_id, to avoid adding this kind > > of redundant dependencies. In other words, the previous in hlocks will > > anyway handle it if the previous's gen_id >= the crosslock's gen_id. > > > > Didn't we talk about an alternative to this?
Yes, we did. You said the optimazation was unnecessary, and I was not sure if it's true, so added it at this time.
But *I will exclude this from next spin*.
> > /me goes dig > > https://lkml.kernel.org/r/20170303091338.GH6536@twins.programming.kicks-ass.net > > There and replies. > > So how much does this save vs avoiding redundant links?
No different on my qemu machine. The answer was:
https://lkml.org/lkml/2017/3/14/103
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