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To:     Minchan Kim <minchan@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        John Dias <joaodias@google.com>,
        Michal Hocko <mhocko@suse.com>, Jason Baron <jbaron@akamai.com>
References: <20210311183047.805891-1-minchan@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [PATCH v4] mm: page_alloc: dump migrate-failed pages
Message-ID: <c2bdb4c2-e025-53b2-5c96-6fe413eb242f@redhat.com>
Date:   Thu, 11 Mar 2021 19:53:01 +0100
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On 11.03.21 19:30, Minchan Kim wrote:
> Currently, debugging CMA allocation failures is quite limited.
> The most commong source of these failures seems to be page

s/commong/common/

> 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.
> 
> page refcount, mapcount with page flags on dump_page are
> helpful information to deduce the culprit. Furthermore,
> dump_page_owner was super helpful to find long term pinner
> who initiated the page allocation.

Maybe simply "The information logged by dump_page() has already proven 
helpful for debugging allocation issues, like identifying long-term 
pinnings on ZONE_MOVABLE or MIGRATE_CMA."

> 
> The reason it approach with dynamic debug is the debug message
> could emit lots of noises as alloc_contig_range calls more
> frequently since it's a best effort allocator.

"Let's use the dynamic debugging infrastructure, such that we avoid
flooding the logs and creating a lot of noise on frequent 
alloc_contig_range() calls. This information is helpful for debugging only."

 >>>

> 
> There are two ifdefery conditions to support common dyndbg options:
> 
> - CONFIG_DYNAMIC_DEBUG_CORE && DYNAMIC_DEBUG_MODULE
> It aims for supporting the feature with only specific file
> with adding ccflags.
> 
> - CONFIG_DYNAMIC_DEBUG
> It aims for supporting the feature with system wide globally.
> 
> A simple example to enable the feature:
> 
> Admin could enable the dump like this(by default, disabled)
> 
> 	echo "func alloc_contig_dump_pages +p" > control
> 
> Admin could disable it.
> 
> 	echo "func alloc_contig_dump_pages =_" > control
> 
> Detail goes Documentation/admin-guide/dynamic-debug-howto.rst

<<< I'd drop that completely and only mention:

"For details on dynamic debugging, see 
Documentation/admin-guide/dynamic-debug-howto.rst."

As you have usage in the code itself, I think you don't have to be 
repetitive. The ifdeffery seems to be common (e.g., 
include/linux/netdevice.) for dynamic debugging users, so I don't see 
the need to describe that in detail.

 >>>

> 
> A concern is utility functions in dump_page uses inconsistent
> loglevels.
> 
> __dump_page: KERN_WARNING
> __dump_page_owner: KERN_ALERT
>          stack_trace_print: KERN_DEFAULT
> 
> There are bunch of places to use the inconsistent loglevel
> utility functions(e.g., just grep dump_page/strace_trace_print).
> It's unfortunate but here we are. It could be addressed
> different patchset.

<<< I'd drop that completely and mention

"In the future, we might want to make the loglevels used inside 
dump_page() consistent and eventually rework the way we log the 
information here. See [1]"

Where [1] is a link to the discussion.

> 
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> * from v3 - https://lore.kernel.org/linux-mm/20210310180104.517886-1-minchan@kernel.org
>    * add dyndgb usage comment - akpm
>    * use dumpstack instead of warn_on - david
> 
> * from v2 - https://lore.kernel.org/linux-mm/20210308202047.1903802-1-minchan@kernel.org/
>    * remove ratelimit - mhocko
> 
> * from v1 - https://lore.kernel.org/linux-mm/20210217163603.429062-1-minchan@kernel.org/
>    * use dynamic debugging with system wide instead of per-call site - mhocko
> 
>   mm/page_alloc.c | 31 +++++++++++++++++++++++++++++++
>   1 file changed, 31 insertions(+)
> 

Minor nits:

> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3e4b29ee2b1e..76fc202cb105 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8453,6 +8453,36 @@ static unsigned long pfn_max_align_up(unsigned long pfn)
>   				pageblock_nr_pages));
>   }
>   
> +#if defined(CONFIG_DYNAMIC_DEBUG) || \
> +	(defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))
> +/*
> + * usage)

"usage)" looks wrong here. Did you mean "Usage:"

> + * 	dyndbg_dir="/sys/kernel/debug/dynamic_debug"
> + * To enable:
> + * 	echo "func alloc_contig_dump_pages +p" > $dyndbg_dir/control
> + * To disable:
> + * 	echo "func alloc_contig_dump_pages =_" > $dyndbg_dir/control
> + * For detail, read dynamic-debug-howto.rst

Maybe simply

"See admin-guide/dynamic-debug-howto.rst"

> + */
> +static void alloc_contig_dump_pages(struct list_head *page_list)
> +{
> +	DEFINE_DYNAMIC_DEBUG_METADATA(descriptor,
> +			"migrate failure");

You can fit that into a single line.

> +
> +	if (DYNAMIC_DEBUG_BRANCH(descriptor)) {
> +		struct page *page;
> +
> +		dump_stack();
> +		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
> +
>   /* [start, end) must belong to a single zone. */
>   static int __alloc_contig_migrate_range(struct compact_control *cc,
>   					unsigned long start, unsigned long end)
> @@ -8496,6 +8526,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
>   				NULL, (unsigned long)&mtc, cc->mode, MR_CONTIG_RANGE);
>   	}
>   	if (ret < 0) {
> +		alloc_contig_dump_pages(&cc->migratepages);
>   		putback_movable_pages(&cc->migratepages);
>   		return ret;
>   	}
> 

As I said, for my taste good enough for now. I would certainly preferred 
what Michal suggested (e.g., doing it via debug loglevels), but this 
gets the job done and is not too ugly.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb