Re: [PATCH 4/4] powerpc/64s: Support shrinking the SLB for debugging

Michael Ellerman's on January 19, 2019 8:27 pm:

Michal Suchánek [off-list ref] writes:

quoted

On Thu, 17 Jan 2019 23:13:28 +1100
Michael Ellerman [off-list ref] wrote:

quoted

On machines with 1TB segments and a 32-entry SLB it's quite hard to
cause sufficient SLB pressure to trigger bugs caused due to badly
timed SLB faults.

We have seen this in the past and a few years ago added the
disable_1tb_segments command line option to force the use of 256MB
segments. However even this allows some bugs to slip through testing
if the SLB entry in question was recently accessed.

So add a new command line parameter for debugging which shrinks the
SLB to the minimum size we can support. Currently that size is 3, two
bolted SLBs and 1 for dynamic use. This creates the maximal SLB

Doesn't this violate the power of 2 requirement stated in 2/4?

Yes. Good point. This was originally a hack patch in my tree, back when
SLB_NUM_BOLTED was 3 and before Nick even added the slb_used_bitmap, so
back then it was a power of 2 but it also didn't matter :)

I think I'll rework the slb_full_bitmap patch anyway and remove the
power of 2 requirement, so then this patch will be OK.

Thanks, good patch and really will help testing. Sometimes even if you 
can get enough pressure with the 256MB segments, you actually want to 
test 1TB segment code paths too.

Thanks,
Nick