Re: [bisected] pre-3.16 regression on open() scalability

[Dave Hansen <dave.hansen@intel.com>]

On 06/17/2014 05:18 PM, Paul E. McKenney wrote:
> So if I understand correctly, a goodly part of the regression is due not
> to the overhead added to cond_resched(), but rather because grace periods
> are now happening faster, thus incurring more overhead.  Is that correct?

Yes, that's the theory at least.

> If this is the case, could you please let me know roughly how sensitive is
> the performance to the time delay in RCU_COND_RESCHED_EVERY_THIS_JIFFIES?

This is the previous kernel, plus RCU tracing, so it's not 100%
apples-to-apples (and it peaks a bit lower than the other kernel).  But
here's the will-it-scale open1 throughput on the y axis vs
RCU_COND_RESCHED_EVERY_THIS_JIFFIES on x:

	http://sr71.net/~dave/intel/jiffies-vs-openops.png

This was a quick and dirty single run with very little averaging, so I
expect there to be a good amount of noise.  I ran it from 1->100, but it
seemed to peak at about 30.

> The patch looks promising.  I will probably drive the time-setup deeper
> into the guts of RCU, which should allow moving the access to jiffies
> and the comparison off of the fast path as well, but this appears to
> me to be good and sufficient for others encountering this same problem
> in the meantime.

Yeah, the more overhead we can push out of cond_resched(), the better.
I had no idea how much we call it!