Re: [PATCH] mm,oom: add tracepoints for oom reaper-related events

[Michal Hocko <mhocko@kernel.org>]

On Tue 30-05-17 13:05:32, Roman Gushchin wrote:
> Add tracepoints to simplify the debugging of the oom reaper code.
> 
> Trace the following events:
> 1) a process is marked as an oom victim,
> 2) a process is added to the oom reaper list,
> 3) the oom reaper starts reaping process's mm,
> 4) the oom reaper finished reaping,
> 5) the oom reaper skips reaping.

I am not against but could you explain why the current printks are not
sufficient? We do not have any explicit printk for the 2) and 3) but
are those really necessary?

In other words could you describe the situation when you found these
tracepoints more useful than what the kernel log offers already?

> Signed-off-by: Roman Gushchin <guro@fb.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
> Cc: kernel-team@fb.com
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-mm@kvack.org
> ---
>  include/trace/events/oom.h | 80 ++++++++++++++++++++++++++++++++++++++++++++++
>  mm/oom_kill.c              |  7 ++++
>  2 files changed, 87 insertions(+)
> 
> diff --git a/include/trace/events/oom.h b/include/trace/events/oom.h
> index 38baeb2..c3c19d4 100644
> --- a/include/trace/events/oom.h
> +++ b/include/trace/events/oom.h
> @@ -70,6 +70,86 @@ TRACE_EVENT(reclaim_retry_zone,
>  			__entry->wmark_check)
>  );
>  
> +TRACE_EVENT(mark_victim,
> +	TP_PROTO(int pid),
> +
> +	TP_ARGS(pid),
> +
> +	TP_STRUCT__entry(
> +		__field(int, pid)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pid = pid;
> +	),
> +
> +	TP_printk("pid=%d", __entry->pid)
> +);
> +
> +TRACE_EVENT(wake_reaper,
> +	TP_PROTO(int pid),
> +
> +	TP_ARGS(pid),
> +
> +	TP_STRUCT__entry(
> +		__field(int, pid)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pid = pid;
> +	),
> +
> +	TP_printk("pid=%d", __entry->pid)
> +);
> +
> +TRACE_EVENT(start_task_reaping,
> +	TP_PROTO(int pid),
> +
> +	TP_ARGS(pid),
> +
> +	TP_STRUCT__entry(
> +		__field(int, pid)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pid = pid;
> +	),
> +
> +	TP_printk("pid=%d", __entry->pid)
> +);
> +
> +TRACE_EVENT(finish_task_reaping,
> +	TP_PROTO(int pid),
> +
> +	TP_ARGS(pid),
> +
> +	TP_STRUCT__entry(
> +		__field(int, pid)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pid = pid;
> +	),
> +
> +	TP_printk("pid=%d", __entry->pid)
> +);
> +
> +TRACE_EVENT(skip_task_reaping,
> +	TP_PROTO(int pid),
> +
> +	TP_ARGS(pid),
> +
> +	TP_STRUCT__entry(
> +		__field(int, pid)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->pid = pid;
> +	),
> +
> +	TP_printk("pid=%d", __entry->pid)
> +);
> +
>  #ifdef CONFIG_COMPACTION
>  TRACE_EVENT(compact_retry,
>  
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 04c9143..409b685 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -490,6 +490,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  
>  	if (!down_read_trylock(&mm->mmap_sem)) {
>  		ret = false;
> +		trace_skip_task_reaping(tsk->pid);
>  		goto unlock_oom;
>  	}
>  
> @@ -500,9 +501,12 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  	 */
>  	if (!mmget_not_zero(mm)) {
>  		up_read(&mm->mmap_sem);
> +		trace_skip_task_reaping(tsk->pid);
>  		goto unlock_oom;
>  	}
>  
> +	trace_start_task_reaping(tsk->pid);
> +
>  	/*
>  	 * Tell all users of get_user/copy_from_user etc... that the content
>  	 * is no longer stable. No barriers really needed because unmapping
> @@ -544,6 +548,7 @@ static bool __oom_reap_task_mm(struct task_struct *tsk, struct mm_struct *mm)
>  	 * put the oom_reaper out of the way.
>  	 */
>  	mmput_async(mm);
> +	trace_finish_task_reaping(tsk->pid);
>  unlock_oom:
>  	mutex_unlock(&oom_lock);
>  	return ret;
> @@ -615,6 +620,7 @@ static void wake_oom_reaper(struct task_struct *tsk)
>  	tsk->oom_reaper_list = oom_reaper_list;
>  	oom_reaper_list = tsk;
>  	spin_unlock(&oom_reaper_lock);
> +	trace_wake_reaper(tsk->pid);
>  	wake_up(&oom_reaper_wait);
>  }
>  
> @@ -666,6 +672,7 @@ static void mark_oom_victim(struct task_struct *tsk)
>  	 */
>  	__thaw_task(tsk);
>  	atomic_inc(&oom_victims);
> +	trace_mark_victim(tsk->pid);
>  }
>  
>  /**
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs