Re: [mm-unstable v7 11/18] mm/madvise: add huge_memory:mm_madvise_collapse tracepoint

[Zach O'Keefe <zokeefe@google.com>]

Hey Yang,

Thanks for taking the time to review this series again.

On Jul 11 14:32, Yang Shi wrote:
> On Wed, Jul 6, 2022 at 5:06 PM Zach O'Keefe <zokeefe@google.com> wrote:
> >
> > Add a tracepoint to expose mm, address, and enum scan_result of each
> > hugepage attempted to be collapsed by call to madvise(MADV_COLLAPSE).
> 
> Is this necessary? Isn't mm_khugepaged_scan_pmd tracepoint good
> enough? It doesn't have "address", but you should be able to calculate
> address from it with syscall trace together.
> 

I've also found this useful debugging along the file path. Perhaps the answer to
that is: add tracepoints to the file path - and we should probably do that - but
the other issue is that turning on these tracepoints (for the purposes of
debugging MADV_COLLAPSE) generates a lot of noise from khugepaged that is hard
to separate out. Augmenting existing tracepoints with .is_khugepaged data
incurred the risks associated with altering an existing kernel API. WDYT?

Thanks again,
Zach


> 
> >
> > Signed-off-by: Zach O'Keefe <zokeefe@google.com>
> > ---
> >  include/trace/events/huge_memory.h | 22 ++++++++++++++++++++++
> >  mm/khugepaged.c                    |  2 ++
> >  2 files changed, 24 insertions(+)
> >
> > diff --git a/include/trace/events/huge_memory.h b/include/trace/events/huge_memory.h
> > index 55392bf30a03..38d339ffdb16 100644
> > --- a/include/trace/events/huge_memory.h
> > +++ b/include/trace/events/huge_memory.h
> > @@ -167,5 +167,27 @@ TRACE_EVENT(mm_collapse_huge_page_swapin,
> >                 __entry->ret)
> >  );
> >
> > +TRACE_EVENT(mm_madvise_collapse,
> > +
> > +       TP_PROTO(struct mm_struct *mm, unsigned long addr, int result),
> > +
> > +       TP_ARGS(mm, addr, result),
> > +
> > +       TP_STRUCT__entry(__field(struct mm_struct *, mm)
> > +                        __field(unsigned long, addr)
> > +                        __field(int, result)
> > +       ),
> > +
> > +       TP_fast_assign(__entry->mm = mm;
> > +                      __entry->addr = addr;
> > +                      __entry->result = result;
> > +       ),
> > +
> > +       TP_printk("mm=%p addr=%#lx result=%s",
> > +                 __entry->mm,
> > +                 __entry->addr,
> > +                 __print_symbolic(__entry->result, SCAN_STATUS))
> > +);
> > +
> >  #endif /* __HUGE_MEMORY_H */
> >  #include <trace/define_trace.h>
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index e0d00180512c..0207fc0a5b2a 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2438,6 +2438,8 @@ int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> >                 if (!mmap_locked)
> >                         *prev = NULL;  /* Tell caller we dropped mmap_lock */
> >
> > +               trace_mm_madvise_collapse(mm, addr, result);
> > +
> >                 switch (result) {
> >                 case SCAN_SUCCEED:
> >                 case SCAN_PMD_MAPPED:
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >