linux-kernel - Re: [PATCH v2] mm/vmstat: sum up all possible CPUs instead of using vm_events_fold

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Message-ID: <9d390017-f1c4-44db-864f-cb95b8fd3a9b@arm.com>
Date: Fri, 3 May 2024 15:14:49 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Vlastimil Babka <vbabka@...e.cz>, Barry Song <21cnbao@...il.com>,
 akpm@...ux-foundation.org, linux-mm@...ck.org
Cc: hannes@...xchg.org, linux-kernel@...r.kernel.org, david@...hat.com,
 v-songbaohua@...o.com, willy@...radead.org
Subject: Re: [PATCH v2] mm/vmstat: sum up all possible CPUs instead of using
 vm_events_fold_cpu

On 03/05/2024 14:45, Vlastimil Babka wrote:
> On 5/3/24 11:16 AM, Ryan Roberts wrote:
>> On 03/05/2024 03:09, Barry Song wrote:
>>> @@ -83,8 +83,6 @@ static inline void count_vm_events(enum vm_event_item item, long delta)
>>>  
>>>  extern void all_vm_events(unsigned long *);
>>>  
>>> -extern void vm_events_fold_cpu(int cpu);
>>> -
>>>  #else
>>>  
>>>  /* Disable counters */
>>> @@ -103,9 +101,6 @@ static inline void __count_vm_events(enum vm_event_item item, long delta)
>>>  static inline void all_vm_events(unsigned long *ret)
>>>  {
>>>  }
>>> -static inline void vm_events_fold_cpu(int cpu)
>>> -{
>>> -}
>>>  
>>>  #endif /* CONFIG_VM_EVENT_COUNTERS */
>>>  
>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>> index cd584aace6bf..8b56d785d587 100644
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -5826,14 +5826,6 @@ static int page_alloc_cpu_dead(unsigned int cpu)
>>>  	mlock_drain_remote(cpu);
>>>  	drain_pages(cpu);
>>>  
>>> -	/*
>>> -	 * Spill the event counters of the dead processor
>>> -	 * into the current processors event counters.
>>> -	 * This artificially elevates the count of the current
>>> -	 * processor.
>>> -	 */
>>> -	vm_events_fold_cpu(cpu);
>>> -
>>>  	/*
>>>  	 * Zero the differential counters of the dead processor
>>>  	 * so that the vm statistics are consistent.
>>> diff --git a/mm/vmstat.c b/mm/vmstat.c
>>> index db79935e4a54..aaa32418652e 100644
>>> --- a/mm/vmstat.c
>>> +++ b/mm/vmstat.c
>>> @@ -114,7 +114,7 @@ static void sum_vm_events(unsigned long *ret)
>>>  
>>>  	memset(ret, 0, NR_VM_EVENT_ITEMS * sizeof(unsigned long));
>>>  
>>> -	for_each_online_cpu(cpu) {
>>> +	for_each_possible_cpu(cpu) {
>>
>> One thought comes to mind (due to my lack of understanding exactly what
>> "possible" means): Linux is compiled with a max number of cpus - NR_CPUS - 512
>> for arm64's defconfig. Does all possible cpus include all 512? On an 8 CPU
>> system that would be increasing the number of loops by 64 times.
>>
>> Or perhaps possible just means CPUs that have ever been online?
> 
> IIRC on x86 it comes from some BIOS tables, and some bioses might be not
> providing very realistic numbers, so it can be unnecessary large.

OK thanks for the info.

> 
>> Either way, I guess it's not considered a performance bottleneck because, from
>> memory, the scheduler and many other places are iterating over all possible cpus.
> 
> I doubt anything does it in a fastpath. But this affects only reading
> /proc/vmstat, right? Which is not a fastpath. Also update_balloon_stats()
> which is probably ok as well?

Yep agreed.

> 
> Either way I don't see a clear advantage nor disadvantage of this.

The advantage is just that it deletes 32 lines of code and makes it easier to
understand.

> 
>>>  		struct vm_event_state *this = &per_cpu(vm_event_states, cpu);
>>>  
>>>  		for (i = 0; i < NR_VM_EVENT_ITEMS; i++)
>>> @@ -129,29 +129,10 @@ static void sum_vm_events(unsigned long *ret)
>>>  */
>>>  void all_vm_events(unsigned long *ret)
>>>  {
>>> -	cpus_read_lock();
>>>  	sum_vm_events(ret);
>>> -	cpus_read_unlock();
>>>  }
>>>  EXPORT_SYMBOL_GPL(all_vm_events);
>>>  
>>> -/*
>>> - * Fold the foreign cpu events into our own.
>>> - *
>>> - * This is adding to the events on one processor
>>> - * but keeps the global counts constant.
>>> - */
>>> -void vm_events_fold_cpu(int cpu)
>>> -{
>>> -	struct vm_event_state *fold_state = &per_cpu(vm_event_states, cpu);
>>> -	int i;
>>> -
>>> -	for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
>>> -		count_vm_events(i, fold_state->event[i]);
>>> -		fold_state->event[i] = 0;
>>> -	}
>>> -}
>>> -
>>>  #endif /* CONFIG_VM_EVENT_COUNTERS */
>>>  
>>>  /*
>>
>