[PATCH 0/5] perf/mm: Fix PERF_SAMPLE_*_PAGE_SIZE

[Matthew Wilcox]

On Mon, Nov 16, 2020 at 08:28:23AM -0800, Dave Hansen wrote:
> On 11/16/20 7:54 AM, Matthew Wilcox wrote:
> > It gets even more complicated with CPUs with multiple levels of TLB
> > which support different TLB entry sizes.  My CPU reports:
> > 
> > TLB info
> >  Instruction TLB: 2M/4M pages, fully associative, 8 entries
> >  Instruction TLB: 4K pages, 8-way associative, 64 entries
> >  Data TLB: 1GB pages, 4-way set associative, 4 entries
> >  Data TLB: 4KB pages, 4-way associative, 64 entries
> >  Shared L2 TLB: 4KB/2MB pages, 6-way associative, 1536 entries
> 
> It's even "worse" on recent AMD systems.  Those will coalesce multiple
> adjacent PTEs into a single TLB entry.  I think Alphas did something
> like this back in the day with an opt-in.

I debated mentioning that ;-)  We can detect in software whether that's
_possible_, but we can't detect whether it's *done* it.  I heard it
sometimes takes several faults on the 4kB entries for the CPU to decide
that it's beneficial to use a 32kB TLB entry.  But this is all rumour.

> Anyway, the changelog should probably replace:
> 
> > This enables PERF_SAMPLE_{DATA,CODE}_PAGE_SIZE to report accurate TLB
> > page sizes.
> 
> with something more like:
> 
> This enables PERF_SAMPLE_{DATA,CODE}_PAGE_SIZE to report accurate page
> table mapping sizes.
> 
> That's really the best we can do from software without digging into
> microarchitecture-specific events.

I mean this is perf.  Digging into microarch specific events is what it
does ;-)