Re: [PATCH 1/4] tracing, page-allocator: Add trace events for page allocation and page freeing

Mel Gorman
2009-08-05T09:40:30Z

On Wed, Aug 05, 2009 at 06:13:09PM +0900, KOSAKI Motohiro wrote:
> Hi
>
> sorry for the delayed review.
>
> > This patch adds trace events for the allocation and freeing of pages,
> > including the freeing of pagevecs. Using the events, it will be known what
> > struct page and pfns are being allocated and freed and what the call site
> > was in many cases.
> >
> > The page alloc tracepoints be used as an indicator as to whether the workload
> > was heavily dependant on the page allocator or not. You can make a guess based
> > on vmstat but you can't get a per-process breakdown. Depending on the call
> > path, the call_site for page allocation may be __get_free_pages() instead
> > of a useful callsite. Instead of passing down a return address similar to
> > slab debugging, the user should enable the stacktrace and seg-addr options
> > to get a proper stack trace.
> >
> > The pagevec free tracepoint has a different usecase. It can be used to get
> > a idea of how many pages are being dumped off the LRU and whether it is
> > kswapd doing the work or a process doing direct reclaim.
> >
> > Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> > Acked-by: Rik van Riel <riel@redhat.com>
> > ---
> > include/trace/events/kmem.h | 86 +++++++++++++++++++++++++++++++++++++++++++
> > mm/page_alloc.c | 6 ++-
> > 2 files changed, 91 insertions(+), 1 deletions(-)
> >
> > diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> > index 1493c54..57bf13c 100644
> > --- a/include/trace/events/kmem.h
> > +++ b/include/trace/events/kmem.h
> > @@ -225,6 +225,92 @@ TRACE_EVENT(kmem_cache_free,
> >
> > TP_printk("call_site=%lx ptr=%p", __entry->call_site, __entry->ptr)
> > );
> > +
> > +TRACE_EVENT(mm_page_free_direct,
> > +
> > + TP_PROTO(unsigned long call_site, const void *page, unsigned int order),
> > +
> > + TP_ARGS(call_site, page, order),
> > +
> > + TP_STRUCT__entry(
> > + __field( unsigned long, call_site )
> > + __field( const void *, page )
>
> Why void? Is there any benefit?
>

No real benefit, I'll switch to struct page *. I thought at one point it was
failing to compile as struct page * was not in scope but that must have been
my imagination.

> > + __field( unsigned int, order )
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->call_site = call_site;
> > + __entry->page = page;
> > + __entry->order = order;
> > + ),
> > +
> > + TP_printk("call_site=%lx page=%p pfn=%lu order=%d",
> > + __entry->call_site,
> > + __entry->page,
> > + page_to_pfn((struct page *)__entry->page),
> > + __entry->order)
> > +);
> > +
> > +TRACE_EVENT(mm_pagevec_free,
> > +
> > + TP_PROTO(unsigned long call_site, const void *page, int order, int cold),
> > +
> > + TP_ARGS(call_site, page, order, cold),
> > +
> > + TP_STRUCT__entry(
> > + __field( unsigned long, call_site )
> > + __field( const void *, page )
> > + __field( int, order )
> > + __field( int, cold )
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->call_site = call_site;
> > + __entry->page = page;
> > + __entry->order = order;
> > + __entry->cold = cold;
> > + ),
> > +
> > + TP_printk("call_site=%lx page=%p pfn=%lu order=%d cold=%d",
> > + __entry->call_site,
> > + __entry->page,
> > + page_to_pfn((struct page *)__entry->page),
> > + __entry->order,
> > + __entry->cold)
> > +);
> > +
> > +TRACE_EVENT(mm_page_alloc,
> > +
> > + TP_PROTO(unsigned long call_site, const void *page, unsigned int order,
> > + gfp_t gfp_flags, int migratetype),
> > +
> > + TP_ARGS(call_site, page, order, gfp_flags, migratetype),
> > +
> > + TP_STRUCT__entry(
> > + __field( unsigned long, call_site )
> > + __field( const void *, page )
> > + __field( unsigned int, order )
> > + __field( gfp_t, gfp_flags )
> > + __field( int, migratetype )
> > + ),
> > +
> > + TP_fast_assign(
> > + __entry->call_site = call_site;
> > + __entry->page = page;
> > + __entry->order = order;
> > + __entry->gfp_flags = gfp_flags;
> > + __entry->migratetype = migratetype;
> > + ),
> > +
> > + TP_printk("call_site=%lx page=%p pfn=%lu order=%d migratetype=%d gfp_flags=%s",
> > + __entry->call_site,
> > + __entry->page,
> > + page_to_pfn((struct page *)__entry->page),
> > + __entry->order,
> > + __entry->migratetype,
> > + show_gfp_flags(__entry->gfp_flags))
> > +);
> > +
> > #endif /* _TRACE_KMEM_H */
> >
> > /* This part must be outside protection */
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index d052abb..843bdec 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1905,6 +1905,7 @@ __alloc_pages_nodemask(gfp_t gfp_mask, unsigned int order,
> > zonelist, high_zoneidx, nodemask,
> > preferred_zone, migratetype);
> >
> > + trace_mm_page_alloc(_RET_IP_, page, order, gfp_mask, migratetype);
> > return page;
> > }
>
> In almost case, __alloc_pages_nodemask() is called from alloc_pages_current().
> Can you add call_site argument? (likes slab_alloc)
>

In the NUMA case, this will be true but addressing it involves passing down
an additional argument in the non-tracing case which I wanted to avoid.
As the stacktrace option is available to ftrace, I think I'll drop call_site
altogether as anyone who really needs that information has options.

> > EXPORT_SYMBOL(__alloc_pages_nodemask);
> > @@ -1945,13 +1946,16 @@ void __pagevec_free(struct pagevec *pvec)
> > {
> > int i = pagevec_count(pvec);
> >
> > - while (--i >= 0)
> > + while (--i >= 0) {
> > + trace_mm_pagevec_free(_RET_IP_, pvec->pages[i], 0, pvec->cold);
> > free_hot_cold_page(pvec->pages[i], pvec->cold);
> > + }
> > }
>
> This _RET_IP_ assume pagevec_free() is inlined function. Then,
> pagevec_free() sould also change always_inline?
>

There is an assumption being made about the inlining all right.

> Yeah, I agree this is theoretical issue. but it improve readability and
> studying author's intention.
>

If call_site persists, I'll do this but the next version of the patchset
is likely to drop call_site.

> > void __free_pages(struct page *page, unsigned int order)
> > {
> > if (put_page_testzero(page)) {
> > + trace_mm_page_free_direct(_RET_IP_, page, order);
> > if (order == 0)
> > free_hot_page(page);
> > else
>
> This patch covered free_pages() and __pagevec_free() case.
> but it doesn't cover free_hot_page() direct call.
>
> (Fortunately, there is no free_cold_page() caller)
>

Good spot. free_cold_page() is dead code but I'll duplicate the
trace_mm_page_free_direct event for now and look at cleaning out
free_cold_page(). Thanks

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab