Re: [PATCH v5 1/2] percpu: Add percpu_counter_add_local and percpu_counter_sub_local

[Tim Chen]

On Sat, 2022-09-10 at 04:36 +0800, Jiebin Sun wrote:
> The batch size in percpu_counter_add_batch should be very large
> in heavy writing and rare reading case. Add the "_local" version,
> and mostly it will do local adding, reduce the global updating
> and mitigate lock contention in writing.
> 
> Signed-off-by: Jiebin Sun <jiebin.sun@xxxxxxxxx>
> ---
>  include/linux/percpu_counter.h | 38 ++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 01861eebed79..6dd7eaba8527 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -15,6 +15,9 @@
>  #include <linux/types.h>
>  #include <linux/gfp.h>
>  
> +/* percpu_counter batch for local add or sub */
> +#define PERCPU_COUNTER_LOCAL_BATCH	INT_MAX
> +
>  #ifdef CONFIG_SMP
>  
>  struct percpu_counter {
> @@ -56,6 +59,27 @@ static inline void percpu_counter_add(struct percpu_counter *fbc, s64 amount)
>  	percpu_counter_add_batch(fbc, amount, percpu_counter_batch);
>  }
>  
> +/*
> + * Use this function in heavy writing but rare reading case. The large
> + * batch size will reduce the global updating.

Suggest revising the comment, so it is clear we need to use
percpu_counter_sum() to access the counter: 

With percpu_counter_add_local() and percpu_counter_sub_local(),
counts are accumulated in local per cpu counter and not in
fbc->count until local count overflows PERCPU_COUNTER_LOCAL_BATCH.
This makes counter write efficient.

But percpu_counter_sum(), instead of percpu_counter_read(),
needs to be used to add up the counts
from each CPU to account for all the local counts.
So percpu_counter_add_local() and percpu_counter_sub_local()
should be used when a counter is updated frequently and read
rarely.


> + */
> +static inline void
> +percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
> +{
> +	percpu_counter_add_batch(fbc, amount, PERCPU_COUNTER_LOCAL_BATCH);
> +}
> +
> +/*
> + * Similar with percpu_counter_add_local, use it in heavy writing but
> + * rare reading case. The large batch size will reduce the global
> + * updating.
> + */
> +static inline void
> +percpu_counter_sub_local(struct percpu_counter *fbc, s64 amount)
> +{
> +	percpu_counter_add_batch(fbc, -amount, PERCPU_COUNTER_LOCAL_BATCH);
> +}
> +
>  static inline s64 percpu_counter_sum_positive(struct percpu_counter *fbc)
>  {
>  	s64 ret = __percpu_counter_sum(fbc);
> @@ -138,6 +162,20 @@ percpu_counter_add(struct percpu_counter *fbc, s64 amount)
>  	preempt_enable();
>  }
>  
> +/* no smp percpu_counter_add_local is the same with percpu_counter_add */
> +static inline void
> +percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
> +{
> +	percpu_counter_add(fbc, amount);
> +}
> +
> +/* no smp percpu_counter_sub_local is the same with percpu_counter_sub */
> +static inline void
> +percpu_counter_sub_local(struct percpu_counter *fbc, s64 amount)
> +{
> +	percpu_counter_sub(fbc, amount);
> +}
> +
>  static inline void
>  percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
>  {