linux-kernel - Re: [PATCH] mm/page_alloc: Fix tracepoint mm_page_alloc_zone

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Date:   Wed, 11 May 2022 16:47:23 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Wonhyuk Yang <vvghjk1234@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Baik Song An <bsahn@...i.re.kr>,
        Hong Yeon Kim <kimhy@...i.re.kr>,
        Taeung Song <taeung@...llinux.co.kr>, linuxgeek@...uxgeek.io,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/page_alloc: Fix tracepoint mm_page_alloc_zone_locked()

On Thu, May 12, 2022 at 12:02:30AM +0900, Wonhyuk Yang wrote:
> > The original intent of that tracepoint was to trace when pages were
> > removed from the buddy list. That would suggest this untested patch on
> > top of yours as a simplication;
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 0351808322ba..66a70b898130 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2476,6 +2476,8 @@ struct page *__rmqueue_smallest(struct zone *zone, unsigned int order,
> >                 del_page_from_free_list(page, zone, current_order);
> >                 expand(zone, page, order, current_order, migratetype);
> >                 set_pcppage_migratetype(page, migratetype);
> > +               trace_mm_page_alloc_zone_locked(page, order, migratetype,
> > +                       pcp_allowed_order(order) && migratetype < MIGRATE_PCPTYPES);
> >                 return page;
> >         }
> 
> Interestingly, my first approach was quite similar your suggestion. But I
> noticed that there can be a request whose migration type is MOVABLE
> and alloc_flags doen't have ALLOC_CMA. In that case, page are marked
> as percpu-refill even though it was allocated from buddy-list directly.
> Is there no problem if we just ignore this case?
> 

I assume you are referring to the case where CMA allocations are being
balanced between regular and CMA areas. I think it's relatively harmless
if percpu_refill field is not 100% accurate for that case. There are
also cases like the percpu list is too small to hold a THP and it's not a
percpu_refill either. If 100% accuracy is an issue, I would prefer renaming
it to percpu_eligible or just deleting it instead of adding complexity
for a tracepoint.  The main value of that tracepoint is determining what
percentage of allocations are potentially contending on zone lock at a
particular time.

-- 
Mel Gorman
SUSE Labs