Re: [PATCH v2] mm: page_alloc: dump migrate-failed pages

[Minchan Kim <minchan@kernel.org>]

On Tue, Mar 09, 2021 at 05:32:08PM +0100, Michal Hocko wrote:
> On Tue 09-03-21 08:15:41, Minchan Kim wrote:
> > On Tue, Mar 09, 2021 at 10:32:51AM +0100, Michal Hocko wrote:
> > > On Mon 08-03-21 12:20:47, Minchan Kim wrote:
> > > > alloc_contig_range is usually used on cma area or movable zone.
> > > > It's critical if the page migration fails on those areas so
> > > > dump more debugging message.
> > > 
> > > I disagree with this statement. alloc_contig_range is not a reliable
> > > allocator. Any user, be it CMA or direct users of alloc_contig_range
> > > have to deal with allocation failures. Debugging information can be
> > > still useful but considering migration failures critical is
> > > overstatement to say the least.
> > 
> > Fair enough. Let's change it.
> > 
> > "Currently, debugging CMA allocation failure is too hard
> > due to lacking of page information. alloc_contig_range is
> > proper place to dump them since it has migrate-failed page
> > list."
> 
> "Currently, debugging CMA allocation failures is quite limited. The most
> commong source of these failures seems to be page migration which
> doesn't provide any useful information on the reason of the failure by
> itself. alloc_contig_range can report those failures as it holds a list
> of migrate-failed pages."

Will take it. Thanks.

< snip >

> > > Somebody more familiar with the dynamic debugging infrastructure needs
> > > to have a look but from from a quick look it seems ok.
> > > 
> > > Do we really need all the ugly ifdefery, though? Don't we want to have
> > > this compiled in all the time and just rely on the static branch managed
> > > by the dynamic debugging framework?
> > 
> > I have no further idea to make it simple while we keep the flexibility
> > for arguments and print format.
> > 
> > #if defined(CONFIG_DYNAMIC_DEBUG) || \
> >         (defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
> > static void alloc_contig_dump_pages(struct list_head *page_list)
> > {
> >         static DEFINE_RATELIMIT_STATE(_rs,
> >                                         DEFAULT_RATELIMIT_INTERVAL,
> >                                         DEFAULT_RATELIMIT_BURST);
> > 
> >         DEFINE_DYNAMIC_DEBUG_METADATA(descriptor,
> >                         "migrate failure");
> >         if (DYNAMIC_DEBUG_BRANCH(descriptor) && __ratelimit(&_rs)) {
> >                 struct page *page;
> > 
> >                 WARN(1, "failed callstack");
> >                 list_for_each_entry(page, page_list, lru)
> >                         dump_page(page, "migration failure");
> >         }
> > }
> > #else
> > static inline void alloc_contig_dump_pages(struct list_head *page_list)
> > {
> > }
> > #endif
> 
> First, you would be much better off by droping the rate limitting. I am
> nt really convinced this is really necessary as this is a debugging aid
> enabled on request. A single list can be large enough to swamp logs so
> why bother?

No problem. Just added since David mentioned hugetlb pages are easily
fail to mgirate at this moment.
Yes, We could add the ratelimit if we get complain.

> 
> Also are all those CONFIG_DYNAMIC_DEBUG* ifdefs necessary?  Can we
> simply enable DYNAMIC_DEBUG for page_alloc as I've suggested above?

They are different usecases.

With DYNAMIC_DEBUG_MODULE with CONFIG_DYNAMIC_DEBUG_CORE,
it works for only specific compile flags as you suggested.
(CONFIG_DYNAMIC_DEBUG_CORE is requirement to work DYNAMIC_DEBUG_MODULE.

With CONFIG_DYNAMIC_DEBUG, user could enable/disable every dynamic
debug places without needing DYNAMIC_DEBUG_MODULE flags for source
files.

Both usecase makes sense to me.