Re: [PATCH] mm: vmscan: add tracepoints for node reclaim

[Yafang Shao <laoar.shao@gmail.com>]

On Thu, Feb 28, 2019 at 6:44 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 2/28/19 11:34 AM, Yafang Shao wrote:
> > On Thu, Feb 28, 2019 at 6:21 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >>
> >> On 2/28/19 9:14 AM, Yafang Shao wrote:
> >>> In the page alloc fast path, it may do node reclaim, which may cause
> >>> latency spike.
> >>> We should add tracepoint for this event, and also mesure the latency
> >>> it causes.
> >>>
> >>> So bellow two tracepoints are introduced,
> >>>       mm_vmscan_node_reclaim_begin
> >>>       mm_vmscan_node_reclaim_end
> >>>
> >>> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> >>> ---
> >>>  include/trace/events/vmscan.h | 48 +++++++++++++++++++++++++++++++++++++++++++
> >>>  mm/vmscan.c                   | 13 +++++++++++-
> >>>  2 files changed, 60 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
> >>> index a1cb913..9310d5b 100644
> >>> --- a/include/trace/events/vmscan.h
> >>> +++ b/include/trace/events/vmscan.h
> >>> @@ -465,6 +465,54 @@
> >>>               __entry->ratio,
> >>>               show_reclaim_flags(__entry->reclaim_flags))
> >>>  );
> >>> +
> >>> +TRACE_EVENT(mm_vmscan_node_reclaim_begin,
> >>> +
> >>> +     TP_PROTO(int nid, int order, int may_writepage,
> >>> +             gfp_t gfp_flags, int zid),
> >>> +
> >>> +     TP_ARGS(nid, order, may_writepage, gfp_flags, zid),
> >>> +
> >>> +     TP_STRUCT__entry(
> >>> +             __field(int, nid)
> >>> +             __field(int, order)
> >>> +             __field(int, may_writepage)
> >>
> >> For node reclaim may_writepage is statically set in node_reclaim_mode,
> >> so I'm not sure it's worth including it.
> >>
> >>> +             __field(gfp_t, gfp_flags)
> >>> +             __field(int, zid)
> >>
> >> zid seems wasteful and misleading as it's simply derived by
> >> gfp_zone(gfp_mask), so I would drop it.
> >>
> >
> > I agree with you that may_writepage and zid is wasteful, but I found
> > they are in other tracepoints in this file,
> > so I place them in this tracepoint as well.
>
> I see zid only in kswapd waking tracepoints? That's different kind of
> event.
>

Pls. see mm_vmscan_wakeup_kswapd and  classzone_idx in
mm_vmscan_direct_reclaim_begin_template.

mm_vmscan_direct_reclaim_begin_template:
    "order=%d may_writepage=%d gfp_flags=%s classzone_idx=%d"

mm_vmscan_wakeup_kswapd:
    "nid=%d zid=%d order=%d gfp_flags=%s"








> > Seems we'd better drop them from other tracepoints as well ?
>
> Hmm seems may_writepage in other tracepoints depends on laptop_mode
> which is also a static setting. do_try_to_free_pages() can override it
> due to priority, but that doesn't affect the tracepoints. If they are to
> be dropped, it would be a separate patch though.

OK.

Thanks
Yafang