Re: [PATCH 3/3] oom, trace: add compaction retry tracepoint

[Vlastimil Babka <vbabka@suse.cz>]

On 12/14/2016 03:53 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
>
> Higher order requests oom debugging is currently quite hard. We do have
> some compaction points which can tell us how the compaction is operating
> but there is no trace point to tell us about compaction retry logic.
> This patch adds a one which will have the following format
>
>             bash-3126  [001] ....  1498.220001: compact_retry: order=9 priority=COMPACT_PRIO_SYNC_LIGHT compaction_result=withdrawn retries=0 max_retries=16 should_retry=0
>
> we can see that the order 9 request is not retried even though we are in
> the highest compaction priority mode becase the last compaction attempt
> was withdrawn. This means that compaction_zonelist_suitable must have
> returned false and there is no suitable zone to compact for this request
> and so no need to retry further.
>
> another example would be
>            <...>-3137  [001] ....    81.501689: compact_retry: order=9 priority=COMPACT_PRIO_SYNC_LIGHT compaction_result=failed retries=0 max_retries=16 should_retry=0
>
> in this case the order-9 compaction failed to find any suitable
> block. We do not retry anymore because this is a costly request
> and those do not go below COMPACT_PRIO_SYNC_LIGHT priority.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  include/trace/events/mmflags.h | 26 ++++++++++++++++++++++++++
>  include/trace/events/oom.h     | 39 +++++++++++++++++++++++++++++++++++++++
>  mm/page_alloc.c                | 22 ++++++++++++++++------
>  3 files changed, 81 insertions(+), 6 deletions(-)
>
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index 7e4cfede873c..aa4caa6914a9 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -187,8 +187,32 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,	"softdirty"	)		\
>  	EM( COMPACT_NO_SUITABLE_PAGE,	"no_suitable_page")	\
>  	EM( COMPACT_NOT_SUITABLE_ZONE,	"not_suitable_zone")	\
>  	EMe(COMPACT_CONTENDED,		"contended")
> +
> +/* High-level compaction status feedback */
> +#define COMPACTION_FAILED	1
> +#define COMPACTION_WITHDRAWN	2
> +#define COMPACTION_PROGRESS	3
> +
> +#define compact_result_to_feedback(result)	\
> +({						\
> + 	enum compact_result __result = result;	\
> +	(compaction_failed(__result)) ? COMPACTION_FAILED : \
> +		(compaction_withdrawn(__result)) ? COMPACTION_WITHDRAWN : COMPACTION_PROGRESS; \
> +})

It seems you forgot to actually use this "function" (sorry, didn't notice 
earlier) so currently it's translating enum compact_result directly into the 
failed/withdrawn/progress strings, which is wrong.

The correct place for the result->feedback conversion should be TP_fast_assign, 
so __entry->result should become __entry->feedback. It's too late in TP_printk, 
as userspace tools (e.g. trace-cmd) won't know the functions that 
compact_result_to_feedback() uses.

> +
> +#define COMPACTION_FEEDBACK		\
> +	EM(COMPACTION_FAILED,		"failed")	\
> +	EM(COMPACTION_WITHDRAWN,	"withdrawn")	\
> +	EMe(COMPACTION_PROGRESS,	"progress")
> +
> +#define COMPACTION_PRIORITY						\
> +	EM(COMPACT_PRIO_SYNC_FULL,	"COMPACT_PRIO_SYNC_FULL")	\
> +	EM(COMPACT_PRIO_SYNC_LIGHT,	"COMPACT_PRIO_SYNC_LIGHT")	\
> +	EMe(COMPACT_PRIO_ASYNC,		"COMPACT_PRIO_ASYNC")
>  #else
>  #define COMPACTION_STATUS
> +#define COMPACTION_PRIORITY
> +#define COMPACTION_FEEDBACK
>  #endif
>
>  #ifdef CONFIG_ZONE_DMA
> @@ -226,6 +250,8 @@ IF_HAVE_VM_SOFTDIRTY(VM_SOFTDIRTY,	"softdirty"	)		\
>  #define EMe(a, b)	TRACE_DEFINE_ENUM(a);
>
>  COMPACTION_STATUS
> +COMPACTION_PRIORITY
> +COMPACTION_FEEDBACK
>  ZONE_TYPE
>
>  /*
> diff --git a/include/trace/events/oom.h b/include/trace/events/oom.h
> index 9160da7a26a0..e9d690665b7a 100644
> --- a/include/trace/events/oom.h
> +++ b/include/trace/events/oom.h
> @@ -69,6 +69,45 @@ TRACE_EVENT(reclaim_retry_zone,
>  			__entry->no_progress_loops,
>  			__entry->wmark_check)
>  );
> +
> +#ifdef CONFIG_COMPACTION
> +TRACE_EVENT(compact_retry,
> +
> +	TP_PROTO(int order,
> +		enum compact_priority priority,
> +		enum compact_result result,
> +		int retries,
> +		int max_retries,
> +		bool ret),
> +
> +	TP_ARGS(order, priority, result, retries, max_retries, ret),
> +
> +	TP_STRUCT__entry(
> +		__field(	int, order)
> +		__field(	int, priority)
> +		__field(	int, result)
> +		__field(	int, retries)
> +		__field(	int, max_retries)
> +		__field(	bool, ret)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->order = order;
> +		__entry->priority = priority;
> +		__entry->result = result;
> +		__entry->retries = retries;
> +		__entry->max_retries = max_retries;
> +		__entry->ret = ret;
> +	),
> +
> +	TP_printk("order=%d priority=%s compaction_result=%s retries=%d max_retries=%d should_retry=%d",
> +			__entry->order,
> +			__print_symbolic(__entry->priority, COMPACTION_PRIORITY),
> +			__print_symbolic(__entry->result, COMPACTION_FEEDBACK),
> +			__entry->retries, __entry->max_retries,
> +			__entry->ret)
> +);
> +#endif /* CONFIG_COMPACTION */
>  #endif
>
>  /* This part must be outside protection */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 23ca951a8380..150245328882 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3201,6 +3201,9 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  {
>  	int max_retries = MAX_COMPACT_RETRIES;
>  	int min_priority;
> +	bool ret = false;
> +	int retries = *compaction_retries;
> +	enum compact_priority priority = *compact_priority;
>
>  	if (!order)
>  		return false;
> @@ -3222,8 +3225,10 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	 * But do not retry if the given zonelist is not suitable for
>  	 * compaction.
>  	 */
> -	if (compaction_withdrawn(compact_result))
> -		return compaction_zonelist_suitable(ac, order, alloc_flags);
> +	if (compaction_withdrawn(compact_result)) {
> +		ret = compaction_zonelist_suitable(ac, order, alloc_flags);
> +		goto out;
> +	}
>
>  	/*
>  	 * !costly requests are much more important than __GFP_REPEAT
> @@ -3235,8 +3240,10 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  	 */
>  	if (order > PAGE_ALLOC_COSTLY_ORDER)
>  		max_retries /= 4;
> -	if (*compaction_retries <= max_retries)
> -		return true;
> +	if (*compaction_retries <= max_retries) {
> +		ret = true;
> +		goto out;
> +	}
>
>  	/*
>  	 * Make sure there are attempts at the highest priority if we exhausted
> @@ -3245,12 +3252,15 @@ should_compact_retry(struct alloc_context *ac, int order, int alloc_flags,
>  check_priority:
>  	min_priority = (order > PAGE_ALLOC_COSTLY_ORDER) ?
>  			MIN_COMPACT_COSTLY_PRIORITY : MIN_COMPACT_PRIORITY;
> +
>  	if (*compact_priority > min_priority) {
>  		(*compact_priority)--;
>  		*compaction_retries = 0;
> -		return true;
> +		ret = true;
>  	}
> -	return false;
> +out:
> +	trace_compact_retry(order, priority, compact_result, retries, max_retries, ret);
> +	return ret;
>  }
>  #else
>  static inline struct page *
>