Re: [PATCH v2 2/3] mm/mglru: add tracepoint for scan_folios()

[Ridong Chen <ridong.chen@linux.dev>]

On 9/17/2026 9:07 AM, Baoquan He wrote:
> On 09/16/26 at 08:17pm, Ridong Chen wrote:
>>
>>
>> On 9/16/2026 10:14 AM, Baoquan He wrote:
>>> On 09/15/26 at 08:17pm, Ridong Chen wrote:
>>>>
>>>>
>>>> On 9/15/2026 7:14 PM, Baoquan He wrote:
>>>>> On 09/11/26 at 06:29pm, Ridong Chen wrote:
>>>>>> From: Ridong Chen <chenridong@xiaomi.com>
>>>>> ...snip...
>>>>>> ---
>>>>>>     include/trace/events/vmscan.h | 63 +++++++++++++++++++++++++++++++++++
>>>>>>     mm/vmscan.c                   |  6 ++++
>>>>>>     2 files changed, 69 insertions(+)
>>>>>>
>>>>>> diff --git a/include/trace/events/vmscan.h b/include/trace/events/vmscan.h
>>>>>> index 8a872990b4be..5defa8f6719c 100644
>>>>>> --- a/include/trace/events/vmscan.h
>>>>>> +++ b/include/trace/events/vmscan.h
>>>>>> @@ -392,6 +392,69 @@ TRACE_EVENT(mm_vmscan_lru_isolate,
>>>>>>     		__print_symbolic(__entry->lru, LRU_NAMES))
>>>>>>     );
>>>>>> +TRACE_EVENT(mm_mglru_scan_folios,
>>>>>> +
>>>>>> +	TP_PROTO(struct lruvec *lruvec,
>>>>>> +		int highest_zoneidx,
>>>>>> +		int order,
>>>>>> +		unsigned long nr_requested,
>>>>>> +		unsigned long nr_scanned,
>>>>>> +		unsigned long nr_sorted,
>>>>>> +		unsigned long nr_skipped,
>>>>>> +		unsigned long nr_taken,
>>>>>> +		int lru,
>>>>>> +		unsigned long max_seq,
>>>>>> +		int tier,
>>>>>> +		unsigned long min_seq),
>>>>>
>>>>
>>>> Thank you for your review.
>>>>
>>>>> Do we need show nid? Guess phone vendor doesn't care about multi-NUMA
>>>>> system.
>>>>>
>>>>
>>>> Eh, nid is not an mglru-specific key. Since there are 12 elements here, it
>>>> can't be added here.
>>>
>>> Hmm, we print memcg_id but not nid, it may not be so clear. How do you
>                                 ^~~~ typo, s/not/no/
> 
>>> differentiate several lruvec-s on one memcg? I am not very familiar with
>>> trace mechanism, do we have a element number limit?
>>>
>>
>> Yes, it will fail to compile if there are more than 12 elements.
>>
>> With AI's help, see
>>
>> include/trace/bpf_probe.h
>> ```
>> 30 #define __CAST1(a,...) __CAST_TO_U64(a)
>>   31 #define __CAST2(a,...) __CAST_TO_U64(a), __CAST1(__VA_ARGS__)
>>   32 #define __CAST3(a,...) __CAST_TO_U64(a), __CAST2(__VA_ARGS__)
>>   33 #define __CAST4(a,...) __CAST_TO_U64(a), __CAST3(__VA_ARGS__)
>>   34 #define __CAST5(a,...) __CAST_TO_U64(a), __CAST4(__VA_ARGS__)
>>   35 #define __CAST6(a,...) __CAST_TO_U64(a), __CAST5(__VA_ARGS__)
>>   36 #define __CAST7(a,...) __CAST_TO_U64(a), __CAST6(__VA_ARGS__)
>>   37 #define __CAST8(a,...) __CAST_TO_U64(a), __CAST7(__VA_ARGS__)
>>   38 #define __CAST9(a,...) __CAST_TO_U64(a), __CAST8(__VA_ARGS__)
>>   39 #define __CAST10(a,...) __CAST_TO_U64(a), __CAST9(__VA_ARGS__)
>>   40 #define __CAST11(a,...) __CAST_TO_U64(a), __CAST10(__VA_ARGS__)
>>   41 #define __CAST12(a,...) __CAST_TO_U64(a), __CAST11(__VA_ARGS__)
>> /* tracepoints with more than 12 arguments will hit build error */
>> ```
>>
>>> Imagine I only want to trace this function, now I can see memcg_id,
>>> zone_idex, while don't know node/lruvec, that's a little pitty. Just my
>>> personal opinion, I could miss anything.
>>>
>>
>> Yeah, you are right. In my personal experience, I always enable
>> trace_mm_vmscan_lru_isolate and trace_mm_vmscan_lru_shrink_inactive, which
>> contain node_id. So we can see how the lruvec is scanned and shrunk, and we
>> can get node_id from trace_mm_vmscan_lru_shrink_inactive. I am not arguing
>> that this is exactly the 'correct' way to use these tracepoints, but I think
>> it is a common usage.
>>
>> I am open to that. If we want to record node_id, we have to remove an
>> element. Maybe tier? But I would strongly recommend record memcg_id, we
>> frequently need to check the reclaim behavior of a memcg, and most of our
> 
> Sorry, that's my typo which could cause misunderstanding. I meant "we print
> memcg_id but no nid, it may not be so clear". I agree with you that
> memcg_id is undispensable.
> 
>> scenarios are non-NUMA.
> 
> Understood, mobile phone mostly is non-NUMA, while server is different.
> About adding nid, I cousulted AI too, it says it's doable.
> 
> It says The 12-argument limit only counts the TP_PROTO arguments (that's
> the BPF __CAST12/bpf_trace_run12 path), not the __entry fields in
> TP_STRUCT__entry. And nid doesn't have to be a 13th argument at all. We
> already pass lruvec, so nid can be derived from it in TP_fast_assign with
> lruvec_pgdat(lruvec)->node_id.
>                                                                                                        
> e.g. for mm_mglru_scan_folios:
>                                                                                                        
>     TP_STRUCT__entry(
>        __field(u64, memcg_id)
>        __field(int, nid)
>        __field(int, highest_zoneidx)
>        __field(int, order)
>        __field(unsigned long, nr_requested)
>        __field(unsigned long, nr_scanned)
>        __field(unsigned long, nr_sorted)
>        __field(unsigned long, nr_skipped)
>        __field(unsigned long, nr_taken)
>        __field(int, lru)
>        __field(unsigned long, max_seq)
>        __field(int, tier)
>        __field(unsigned long, min_seq)
>     ),
>                                                                                                        
>     TP_fast_assign(
>        __entry->memcg_id = mem_cgroup_id(lruvec_memcg(lruvec));
>        __entry->nid      = lruvec_pgdat(lruvec)->node_id;
>        __entry->highest_zoneidx = highest_zoneidx;
>        ...
>     ),
>                                                                                                        
>     TP_printk("memcg_id=%llu nid=%d classzone=%d order=%d ...",
>        __entry->memcg_id,
>        __entry->nid,
>        __entry->highest_zoneidx,
>        ...)
>                                                                                                        
> lruvec_pgdat() comes from include/linux/mmzone.h, which vmscan.h already
> pulls in via linux/mm.h, so no extra include is needed. The same pattern
> applies to mm_mglru_inc_max_seq in patch 3 if you want nid there too.
> I didn't try it though, please feel free to give it a shot.
> 

Good. It works, I just tested it. Will add nid.

> 
>>
> 
>>>>
>>>>
>>>>> And I checked v1 thread, seems Steven suggested to move the two int to
>>>>> avoid extra padding.
>>>>>
>>>>
>>>> Yeah, I will update it. I updated v2 to fix Sashiko's issue, before Steven
>>>> had any comment.
>>>>
>>>>
>>>>>> +
>>>>>> +	TP_ARGS(lruvec, highest_zoneidx, order, nr_requested, nr_scanned,
>>>>>> +		nr_sorted, nr_skipped, nr_taken, lru, max_seq, tier, min_seq),
>>>>>> +
>>>>>> +	TP_STRUCT__entry(
>>>>>> +		__field(u64, memcg_id)
>>>>>> +		__field(int, highest_zoneidx)
>>>>>> +		__field(int, order)
>>>>>> +		__field(unsigned long, nr_requested)
>>>>>> +		__field(unsigned long, nr_scanned)
>>>>>> +		__field(unsigned long, nr_sorted)
>>>>>> +		__field(unsigned long, nr_skipped)
>>>>>> +		__field(unsigned long, nr_taken)
>>>>>> +		__field(int, lru)
>>>>>> +		__field(unsigned long, max_seq)
>>>>>> +		__field(int, tier)
>>>>>> +		__field(unsigned long, min_seq)
>>>>>> +	),
>>>>>> +
>>>>>> +	TP_fast_assign(
>>>>>> +		__entry->memcg_id = mem_cgroup_id(lruvec_memcg(lruvec));
>>>>>> +		__entry->highest_zoneidx = highest_zoneidx;
>>>>>> +		__entry->order = order;
>>>>>> +		__entry->nr_requested = nr_requested;
>>>>>> +		__entry->nr_scanned = nr_scanned;
>>>>>> +		__entry->nr_sorted = nr_sorted;
>>>>>> +		__entry->nr_skipped = nr_skipped;
>>>>>> +		__entry->nr_taken = nr_taken;
>>>>>> +		__entry->lru = lru;
>>>>>> +		__entry->max_seq = max_seq;
>>>>>> +		__entry->tier = tier;
>>>>>> +		__entry->min_seq = min_seq;
>>>>>> +	),
>>>>>> +
>>>>>> +	TP_printk("memcg_id=%llu classzone=%d order=%d nr_requested=%lu nr_scanned=%lu nr_sorted=%lu nr_skipped=%lu nr_taken=%lu lru=%s max_seq=%lu tier=%d min_seq=%lu",
>>>>>> +		__entry->memcg_id,
>>>>>> +		__entry->highest_zoneidx,
>>>>>> +		__entry->order,
>>>>>> +		__entry->nr_requested,
>>>>>> +		__entry->nr_scanned,
>>>>>> +		__entry->nr_sorted,
>>>>>> +		__entry->nr_skipped,
>>>>>> +		__entry->nr_taken,
>>>>>> +		__print_symbolic(__entry->lru, LRU_NAMES),
>>>>>> +		__entry->max_seq,
>>>>>> +		__entry->tier,
>>>>>> +		__entry->min_seq)
>>>>>> +);
>>>>>> +
>>>>>>     TRACE_EVENT(mm_vmscan_write_folio,
>>>>>>     	TP_PROTO(struct folio *folio),
>>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>>>> index 2554a6513aa8..67f59aa73fb9 100644
>>>>>> --- a/mm/vmscan.c
>>>>>> +++ b/mm/vmscan.c
>>>>>> @@ -4876,6 +4876,12 @@ static int scan_folios(unsigned long nr_to_scan, struct lruvec *lruvec,
>>>>>>     	trace_mm_vmscan_lru_isolate(sc->reclaim_idx, sc->order, nr_to_scan,
>>>>>>     				scanned, skipped, isolated,
>>>>>>     				type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON);
>>>>>> +	trace_mm_mglru_scan_folios(lruvec,
>>>>>> +				   sc->reclaim_idx, sc->order, nr_to_scan,
>>>>>> +				   scanned, sorted, skipped, isolated,
>>>>>> +				   type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON,
>>>>>> +				   lrugen->max_seq, tier,
>>>>>> +				   lrugen->min_seq[type]);
>>>>>
>>>>> And seems Steven also suggested to calculate lru, max_seq, min_seq in
>>>>> TP_fast_assign() in v1 which could be missed.
>>>>>
>>>>
>>>> Thanks, I will update it following Steven's suggestions.
>>>>
>>>>
>>>>>            TP_fast_assign(
>>>>>                    __entry->memcg_id = mem_cgroup_id(lruvec_memcg(lruvec));
>>>>>                    __entry->highest_zoneidx = sc->reclaim_idx;
>>>>>                    __entry->order = sc->order;
>>>>>                    __entry->nr_requested = nr_requested;
>>>>>                    __entry->nr_scanned = nr_scanned;
>>>>>                    __entry->nr_sorted = nr_sorted;
>>>>>                    __entry->nr_skipped = nr_skipped;
>>>>>                    __entry->nr_taken = nr_taken;
>>>>>                    __entry->lru = type ? LRU_INACTIVE_FILE : LRU_INACTIVE_ANON;
>>>>>                    __entry->max_seq = lrugen->max_seq;
>>>>>                    __entry->tier = tier;
>>>>>                    __entry->min_seq = lrugen->min_seq;
>>>>>            ),
>>>>>
>>>>
>>>> However, 'sc' is an internal structure, so sc->order cannot be accessed in
>>>> TP_fast_assign(), and I don't think it is worth making scan_control an
>>>> external structure just for this.
>>>
>>> Agree.
>>>
>>>>
>>>>>
>>>>>>     	*isolatedp = isolated;
>>>>>>     	return scanned;
>>>>>> -- 
>>>>>> 2.34.1
>>>>>>
>>>>
>>>> -- 
>>>> Best regards
>>>> Ridong
>>>>
>>
>> -- 
>> Best regards
>> Ridong
>>

-- 
Best regards
Ridong