Re: [RFC PATCH V0 09/10] trace/kmmscand: Add tracing of scanning and migration

[Steven Rostedt]

On Sun, 1 Dec 2024 15:38:17 +0000
Raghavendra K T <raghavendra.kt@xxxxxxx> wrote:

> Add tracing support to track
>  - start and end of scanning.
>  - migration.
> 
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> CC: linux-trace-kernel@xxxxxxxxxxxxxxx
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxx>
> ---
>  include/trace/events/kmem.h | 99 +++++++++++++++++++++++++++++++++++++
>  mm/kmmscand.c               | 12 ++++-
>  2 files changed, 110 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
> index b37eb0a7060f..80978d85300d 100644
> --- a/include/trace/events/kmem.h
> +++ b/include/trace/events/kmem.h
> @@ -9,6 +9,105 @@
>  #include <linux/tracepoint.h>
>  #include <trace/events/mmflags.h>
>  
> +TRACE_EVENT(kmem_mm_enter,
> +
> +	TP_PROTO(struct task_struct *t,
> +		 struct mm_struct *mm),
> +
> +	TP_ARGS(t, mm),
> +
> +	TP_STRUCT__entry(
> +		__array(	char, comm, TASK_COMM_LEN	)

Is there a reason to record "comm"? There's other ways to retrieve it than
to always write it to the ring buffer.

> +		__field(	struct mm_struct *, mm		)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
> +		__entry->mm = mm;
> +	),
> +
> +	TP_printk("kmmscand: mm_enter comm =%s mm=%p", __entry->comm, __entry->mm)
> +);
> +
> +TRACE_EVENT(kmem_scan_mm_start,
> +
> +	TP_PROTO(struct task_struct *t,
> +		 struct mm_struct *mm),
> +
> +	TP_ARGS(t, mm),
> +
> +	TP_STRUCT__entry(
> +		__array(	char, comm, TASK_COMM_LEN	)
> +		__field(	struct mm_struct *, mm		)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
> +		__entry->mm = mm;
> +	),
> +
> +	TP_printk("kmmscand: scan_mm_start comm =%s mm=%p", __entry->comm, __entry->mm)

No need to write the event name into the TP_printk(). That's redundant.

Also, the above two events are pretty much identical. Please use
DECLARE_EVENT_CLASS().

> +);
> +
> +TRACE_EVENT(kmem_scan_mm_end,
> +
> +	TP_PROTO(struct task_struct *t,
> +		 struct mm_struct *mm,
> +		 unsigned long start,
> +		 unsigned long total,
> +		 unsigned long scan_size,
> +		 unsigned long scan_period),
> +
> +	TP_ARGS(t, mm, start, total, scan_period, scan_size),
> +
> +	TP_STRUCT__entry(
> +		__array(	char, comm, TASK_COMM_LEN	)

Again, why comm?

> +		__field(	struct mm_struct *, mm		)
> +		__field(	unsigned long,   start		)
> +		__field(	unsigned long,   total		)
> +		__field(	unsigned long,   scan_period	)
> +		__field(	unsigned long,   scan_size	)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
> +		__entry->mm = mm;
> +		__entry->start = start;
> +		__entry->total = total;
> +		__entry->scan_period = scan_period;
> +		__entry->scan_size = scan_size;
> +	),
> +
> +	TP_printk("kmmscand: scan_mm_end comm =%s mm=%p, start = %ld, total = %ld, scan_period = %ld, scan_size = %ld",
> +			__entry->comm, __entry->mm, __entry->start,
> +			__entry->total, __entry->scan_period, __entry->scan_size)
> +);
> +
> +TRACE_EVENT(kmem_scan_mm_migrate,
> +
> +	TP_PROTO(struct task_struct *t,
> +		 struct mm_struct *mm,
> +		 int rc),
> +
> +	TP_ARGS(t, mm, rc),
> +
> +	TP_STRUCT__entry(
> +		__array(	char, comm, TASK_COMM_LEN	)
> +		__field(	struct mm_struct *, mm		)
> +		__field(	int,   rc			)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, t->comm, TASK_COMM_LEN);
> +		__entry->mm = mm;
> +		__entry->rc = rc;
> +	),
> +
> +	TP_printk("kmmscand: scan_mm_migrate comm =%s mm=%p rc=%d", __entry->comm,
> +			__entry->mm, __entry->rc)
> +);
> +
> +
>  TRACE_EVENT(kmem_cache_alloc,
>  
>  	TP_PROTO(unsigned long call_site,
> diff --git a/mm/kmmscand.c b/mm/kmmscand.c
> index 682c0523c0b4..70f588a210dd 100644
> --- a/mm/kmmscand.c
> +++ b/mm/kmmscand.c
> @@ -668,8 +668,10 @@ static void kmmscand_migrate_folio(void)
>  			WRITE_ONCE(kmmscand_cur_migrate_mm, info->mm);
>  			spin_unlock(&kmmscand_migrate_lock);
>  
> -			if (info->mm)
> +			if (info->mm) {
>  				ret = kmmscand_promote_folio(info);
> +				trace_kmem_scan_mm_migrate(info->mm->owner, info->mm, ret);
> +			}
>  
>  			/* TBD: encode migrated count here, currently assume folio_nr_pages */
>  			if (!ret)
> @@ -828,6 +830,9 @@ static unsigned long kmmscand_scan_mm_slot(void)
>  		goto outerloop;
>  	}
>  
> +	if (mm->owner)
> +		trace_kmem_scan_mm_start(mm->owner, mm);
> +
>  	now = jiffies;
>  	/*
>  	 * Dont scan if :
> @@ -868,6 +873,10 @@ static unsigned long kmmscand_scan_mm_slot(void)
>  
>  	update_mmslot_info = true;
>  
> +	if (mm->owner)
> +		trace_kmem_scan_mm_end(mm->owner, mm, address, total,
> +					mm_slot_scan_period, mm_slot_scan_size);

Please do not add conditions that is used just for calling a tracepoint.
That takes away the "nop" of the function. You can either use
TRACE_EVENT_CONDITION() or DEFINE_EVENT_CONDITION(), or you can hard code
it here:

	if (trace_kmem_scan_mm_end_enabled()) {
		if (mm->owner)
			trace_kmem_scan_mm_end(mm->owner, mm, address, total,
						mm_slot_scan_period, mm_slot_scan_size);
	}

But since it is a single condition, I would prefer the *_CONDITION() macros
above.

-- Steve

> +
>  	count_kmmscand_mm_scans();
>  
>  outerloop:
> @@ -1020,6 +1029,7 @@ void __kmmscand_enter(struct mm_struct *mm)
>  	spin_unlock(&kmmscand_mm_lock);
>  
>  	mmgrab(mm);
> +	trace_kmem_mm_enter(mm->owner, mm);
>  	if (wakeup)
>  		wake_up_interruptible(&kmmscand_wait);
>  }