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SubjectRe: combinatorial explosion in lockdep

* David Miller <davem@davemloft.net> wrote:

> From: David Miller <davem@davemloft.net>
> Date: Mon, 28 Jul 2008 15:37:02 -0700 (PDT)
>
> > I'm still digging on what exactly makes this happen, but I wanted to
> > get the information out as soon as I had something useful like this.
>
> As a simple hack, I tried mitigating these effects using a
> generation-count based "visited" scheme to bypass traversing
> lock_class chains we've already walked down.
>
> It seems to work.

nice!

Any chance to get the "cat /proc/lockdep*" output, so that we could see
and check the expected behavior of the full graph?

But with 128 cpus and double locking taking in the scheduler, there's
around 127*128/2 possible combinations of cpu rq lock ordering, that's
thousands of chains, and that takes a quadratic number of steps to
iterate via the current algorithm - which would be in the neighborhood
of 127^4 / 4, or about 65 million steps for each new change to the
graph. (but it's late and i'm not sure at all about the numbers so i
could be off by a few orders of magnitude, in either direction)

Plus the irqsafe/softirqsafe layers multiply things too.

Could you please also send the non-stack-iterator changes you did as
well - that's cool stuff too!

Ingo


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