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

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Message-ID: <c055203a-4365-4f5e-8276-5c57634abe73@arm.com>
Date: Fri, 3 May 2024 10:16:01 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: 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, vbabka@...e.cz, 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 03:09, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.com>
> 
> When unplugging a CPU, the current code merges its vm_events
> with an online CPU. Because, during summation, it only considers
> online CPUs, which is a crude workaround. By transitioning to
> summing up all possible CPUs, we can eliminate the need for
> vm_events_fold_cpu.
> 
> Suggested-by: Ryan Roberts <ryan.roberts@....com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
>  originally suggested by Ryan while he reviewed mTHP counters
>  patchset[1]; I am also applying this suggestion to vm_events
> 
>  -v2:
>  also drop cpus_read_lock() as we don't care about cpu hotplug any more;
>  -v1:
>  https://lore.kernel.org/linux-mm/20240412123039.442743-1-21cnbao@gmail.com/
> 
>  [1] https://lore.kernel.org/linux-mm/ca73cbf1-8304-4790-a721-3c3a42f9d293@arm.com/
> 
>  include/linux/vmstat.h |  5 -----
>  mm/page_alloc.c        |  8 --------
>  mm/vmstat.c            | 21 +--------------------
>  3 files changed, 1 insertion(+), 33 deletions(-)
> 
> diff --git a/include/linux/vmstat.h b/include/linux/vmstat.h
> index 735eae6e272c..f7eaeb8bfa47 100644
> --- a/include/linux/vmstat.h
> +++ b/include/linux/vmstat.h
> @@ -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?

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.

>  		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 */
>  
>  /*