Re: [PATCH v1] sched/numa: Add tracepoint to track NUMA migration cost

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 29 Oct 2025 21:22:55 +0800
Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> From: Jiayuan Chen <jiayuan.chen@shopee.com>
> 
> In systems with multiple NUMA nodes, memory imbalance between nodes often
> occurs.  To address this, we typically tune parameters like scan_size_mb or
> scan_period_{min,max}_ms to allow processes to migrate pages between NUMA
> nodes.
> 
> Currently, the migration task task_numa_work() holds the mmap_lock during
> the entire migration process, which can significantly impact process
> performance, especially for memory operations. This patch introduces a new
> tracepoint that records the migration duration, along with the number of
> scanned pages and migrated pages. These metrics can be used to calculate
> efficiency metrics similar to %vmeff in 'sar -B'.
> 
> These metrics help evaluate whether the adjusted NUMA balancing parameters
> are properly tuned.
> 
> Here's an example bpftrace script:
> ```bash
> 
> bpftrace -e '
> tracepoint:sched:sched_numa_balance_start
> {
>     @start_time[cpu] = nsecs;
> }
> 
> tracepoint:sched:sched_numa_balance_end {
>     if (@start_time[cpu] > 0) {
>         $cost = nsecs - @start_time[cpu];
>         printf("task '%s' migrate cost %lu, scanned %lu, migrated %lu\n",
>                args.comm, $cost, args.scanned, args.migrated);
>     }
> }
> '

BTW, you don't need bpf for this either:

  # trace-cmd sqlhist -e -n numa_balance SELECT end.comm, TIMESTAMP_DELTA_USECS as cost, \
            end.scanned, end.migrated FROM sched_numa_balance_start AS start \
            JOIN sched_numa_balance_end AS end ON start.common_pid = end.common_pid

  # trace-cmd start -e numa_balance

[ I'd show the output, but my test boxes don't have NUMA ]

You could also make a histogram with it:

  # trace-cmd sqlhist -e SELECT start.comm, 'CAST(start.cost AS BUCKETS=50)' FROM numa_balance AS start

And then cat /sys/kernel/tracing/events/synthetic/numa_balance/hist

Just to give you an idea.


> ```
> Sample output:
> Attaching 2 probes...
> task 'rs:main Q:Reg' migrate cost 5584655, scanned 24516, migrated 22373
> task 'systemd-journal' migrate cost 123191, scanned 6308, migrated 0
> task 'wrk' migrate cost 894026, scanned 5842, migrated 5841
> 
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
>  include/trace/events/sched.h | 60 ++++++++++++++++++++++++++++++++++++
>  kernel/sched/fair.c          | 14 +++++++--
>  2 files changed, 72 insertions(+), 2 deletions(-)
> 
> diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
> index 7b2645b50e78..e24bf700a614 100644
> --- a/include/trace/events/sched.h
> +++ b/include/trace/events/sched.h
> @@ -804,6 +804,66 @@ TRACE_EVENT(sched_skip_cpuset_numa,
>  		  __entry->ngid,
>  		  MAX_NUMNODES, __entry->mem_allowed)
>  );
> +
> +TRACE_EVENT(sched_numa_balance_start,
> +
> +	TP_PROTO(struct task_struct *tsk),
> +
> +	TP_ARGS(tsk),
> +
> +	TP_STRUCT__entry(
> +		__array(char,	comm, TASK_COMM_LEN)

Please use __string() and not __array(). I'm trying to get rid of these for
task comm.

> +		__field(pid_t,	pid)
> +		__field(pid_t,	tgid)
> +		__field(pid_t,	ngid)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, tsk->comm, TASK_COMM_LEN);
> +		__entry->pid		 = task_pid_nr(tsk);
> +		__entry->tgid		 = task_tgid_nr(tsk);
> +		__entry->ngid		 = task_numa_group_id(tsk);
> +	),
> +
> +	TP_printk("comm=%s pid=%d tgid=%d ngid=%d",
> +		  __entry->comm,
> +		  __entry->pid,
> +		  __entry->tgid,
> +		  __entry->ngid)
> +);
> +
> +TRACE_EVENT(sched_numa_balance_end,
> +
> +	TP_PROTO(struct task_struct *tsk, unsigned long scanned, unsigned long migrated),
> +
> +	TP_ARGS(tsk, scanned, migrated),
> +
> +	TP_STRUCT__entry(
> +		__array(char,		comm, TASK_COMM_LEN)
> +		__field(pid_t,		pid)
> +		__field(pid_t,		tgid)
> +		__field(pid_t,		ngid)
> +		__field(unsigned long,	migrated)
> +		__field(unsigned long,	scanned)
> +	),
> +
> +	TP_fast_assign(
> +		memcpy(__entry->comm, tsk->comm, TASK_COMM_LEN);
> +		__entry->pid		 = task_pid_nr(tsk);
> +		__entry->tgid		 = task_tgid_nr(tsk);
> +		__entry->ngid		 = task_numa_group_id(tsk);
> +		__entry->migrated	 = migrated;
> +		__entry->scanned	 = scanned;
> +	),
> +
> +	TP_printk("comm=%s pid=%d tgid=%d ngid=%d scanned=%lu migrated=%lu",
> +		  __entry->comm,
> +		  __entry->pid,
> +		  __entry->tgid,
> +		  __entry->ngid,
> +		  __entry->scanned,
> +		  __entry->migrated)
> +);
>  #endif /* CONFIG_NUMA_BALANCING */
>  
>  /*
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 25970dbbb279..173c9c8397e2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3294,6 +3294,9 @@ static void task_numa_work(struct callback_head *work)
>  	struct vm_area_struct *vma;
>  	unsigned long start, end;
>  	unsigned long nr_pte_updates = 0;
> +	unsigned long nr_scanned = 0;
> +	unsigned long total_migrated = 0;
> +	unsigned long total_scanned = 0;
>  	long pages, virtpages;
>  	struct vma_iterator vmi;
>  	bool vma_pids_skipped;
> @@ -3359,6 +3362,7 @@ static void task_numa_work(struct callback_head *work)
>  	if (!mmap_read_trylock(mm))
>  		return;
>  
> +	trace_sched_numa_balance_start(p);
>  	/*
>  	 * VMAs are skipped if the current PID has not trapped a fault within
>  	 * the VMA recently. Allow scanning to be forced if there is no
> @@ -3477,6 +3481,10 @@ static void task_numa_work(struct callback_head *work)
>  			end = min(end, vma->vm_end);
>  			nr_pte_updates = change_prot_numa(vma, start, end);
>  
> +			nr_scanned = (end - start) >> PAGE_SHIFT;
> +			total_migrated += nr_pte_updates;
> +			total_scanned += nr_scanned;
> +

This will require the scheduler maintainers agreeing on this for acceptance.

Will kprobes not due?

-- Steve


>  			/*
>  			 * Try to scan sysctl_numa_balancing_size worth of
>  			 * hpages that have at least one present PTE that
> @@ -3486,8 +3494,8 @@ static void task_numa_work(struct callback_head *work)
>  			 * areas faster.
>  			 */
>  			if (nr_pte_updates)
> -				pages -= (end - start) >> PAGE_SHIFT;
> -			virtpages -= (end - start) >> PAGE_SHIFT;
> +				pages -= nr_scanned;
> +			virtpages -= nr_scanned;
>  
>  			start = end;
>  			if (pages <= 0 || virtpages <= 0)
> @@ -3528,6 +3536,8 @@ static void task_numa_work(struct callback_head *work)
>  		mm->numa_scan_offset = start;
>  	else
>  		reset_ptenuma_scan(p);
> +
> +	trace_sched_numa_balance_end(p, total_scanned, total_migrated);
>  	mmap_read_unlock(mm);
>  
>  	/*