Re: [RFC PATCH 16/20] famfs: Add fault counters

[Dan Williams]

Dave Hansen wrote:
> On 2/23/24 12:39, John Groves wrote:
> >> We had similar unit test regression concerns with fsdax where some
> >> upstream change silently broke PMD faults. The solution there was trace
> >> points in the fault handlers and a basic test that knows apriori that it
> >> *should* be triggering a certain number of huge faults:
> >>
> >> https://github.com/pmem/ndctl/blob/main/test/dax.sh#L31
> > Good approach, thanks Dan! My working assumption is that we'll be able to make
> > that approach work in the famfs tests. So the fault counters should go away
> > in the next version.
> 
> I do really suspect there's something more generic that should be done
> here.  Maybe we need a generic 'huge_faults' perf event to pair up with
> the good ol' faults that we already have:
> 
> # perf stat -e faults /bin/ls
> 
>  Performance counter stats for '/bin/ls':
> 
>                104      faults
> 
> 
>        0.001499862 seconds time elapsed
> 
>        0.001490000 seconds user
>        0.000000000 seconds sys

Certainly something like that would have satisified this sanity test use
case. I will note that mm_account_fault() would need some help to figure
out the size of the page table entry that got installed. Maybe
extensions to vm_fault_reason to add VM_FAULT_P*D? That compliments
VM_FAULT_FALLBACK to indicate whether, for example, the fallback went
from PUD to PMD, or all the way back to PTE.

Then use cases like this could just add a dynamic probe in
mm_account_fault(). No real need for a new tracepoint unless there was a
use case for this outside of regression testing fault handlers, right?