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Date: Thu, 11 Mar 2021 19:53:01 +0100
From: David Hildenbrand <david@...hat.com>
To: Minchan Kim <minchan@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
John Dias <joaodias@...gle.com>,
Michal Hocko <mhocko@...e.com>, Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH v4] mm: page_alloc: dump migrate-failed pages
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@...nel.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@...hat.com>
--
Thanks,
David / dhildenb
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