RE: [Xen-devel] Why is 'emulate' as good as writable PT's?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> I was wondering, perhaps we are not just triggering writable
pagetables
> when we shouldn't, but maybe we are flushing them back too early.  I
> added some xen perf counters to get an idea of why we are flushing
back
> wtpt's (run on SDET again):

Are these numbers taken on a uniprocessor guest (or dom0?)

>                         modified:  0 <=10  <=20  <=30  <=40  <=50
>  1 writable pt updates    T=1086   0  612   194   111    49    85
>  2 ptwr_flush: called from ptwr_emulated_update because wtpt exists
T=0
>  3 ptwr_flush: called from ptwr_do_page_fault because wtpt is already
> used T=338
>  4 ptwr_flush: called from spurious_page_fault  T=0
>  5 ptwr_flush: called from fixup_page_fault  T=0
>  6 ptwr_flush: called from cleanup_wpt, do_mmuext_op (active) T=467
>  7 ptwr_flush: called from cleanup_wpt, do_mmuext_op (inactive) T=0
>  8 ptwr_flush: called from cleanup_wpt, update_va_mapping (active)
T=280
>  9 ptwr_flush: called from cleanup_wpt, update_va_mapping (inactive)
T=0
> 10 ptwr_flush: called from cleanup_wpt, do_mmu_update (active) T=1
> 11 ptwr_flush: called from cleanup_wpt, do_mmu_update (inactive) T=0


> line 3:  I think we can just goto emulate instead of flushing back the
> wtpt here, right?  I've tried this, but no real difference in
> performance.  Could we increase the number of wtpt's we keep track of,
> so we don't have to flush back or emulate?

This will happen as part of a fork when we move on to the next page in
the PT. It should be harmless unless we're flopping back and forth.

> line 6: We seem to call cleanup_writable_pagetables unconditionally
> here, and if either of the active or inactive pages are used, they get
> flushed back.  Do we always need to do this?

What's the op? is it a TLB flush, invplg, or cr3 load?
 
> line 8: Also call cleanup_writable_pagetables unconditionally here.
Do
> the wtpt's always need this to happen?  Is is possible the
> update_va_mapping call is for an address space which does not affect
the
> wtpt?

It's interesting to understand what the interaction is here. I'd like to
know 

> line 10: Not seeing many flushes here, so I guess it's not an issue.
> 
> Sorry if these questions seem odd.  There's a good chance I am not
> "getting it" :)

This is useful work. It's been on our todo list to re-profile this on
newer kernels. Once upon a time we had it quite nicely tuned...

Could you find out all the kernel EIPs that are triggering writeable
pagetables with any frequency and list them for us. It might be good to
turn everything into using update mmuop and then just turn on direct
writes just for the fork case which is where we know need it.

Thanks,
Ian








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