Re: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min

[Roman Gushchin]

On Mon, Aug 22, 2022 at 12:17:35AM +0000, Shakeel Butt wrote:
> For cgroups using low or min protections, the function
> propagate_protected_usage() was doing an atomic xchg() operation
> irrespectively. It only needs to do that operation if the new value of
> protection is different from older one. This patch does that.
> 
> To evaluate the impact of this optimization, on a 72 CPUs machine, we
> ran the following workload in a three level of cgroup hierarchy with top
> level having min and low setup appropriately. More specifically
> memory.min equal to size of netperf binary and memory.low double of
> that.
> 
>  $ netserver -6
>  # 36 instances of netperf with following params
>  $ netperf -6 -H ::1 -l 60 -t TCP_SENDFILE -- -m 10K
> 
> Results (average throughput of netperf):
> Without (6.0-rc1)	10482.7 Mbps
> With patch		14542.5 Mbps (38.7% improvement)
> 
> With the patch, the throughput improved by 38.7%

Nice savings!

> 
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> ---
>  mm/page_counter.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_counter.c b/mm/page_counter.c
> index eb156ff5d603..47711aa28161 100644
> --- a/mm/page_counter.c
> +++ b/mm/page_counter.c
> @@ -17,24 +17,23 @@ static void propagate_protected_usage(struct page_counter *c,
>  				      unsigned long usage)
>  {
>  	unsigned long protected, old_protected;
> -	unsigned long low, min;
>  	long delta;
>  
>  	if (!c->parent)
>  		return;
>  
> -	min = READ_ONCE(c->min);
> -	if (min || atomic_long_read(&c->min_usage)) {
> -		protected = min(usage, min);
> +	protected = min(usage, READ_ONCE(c->min));
> +	old_protected = atomic_long_read(&c->min_usage);
> +	if (protected != old_protected) {
>  		old_protected = atomic_long_xchg(&c->min_usage, protected);
>  		delta = protected - old_protected;
>  		if (delta)
>  			atomic_long_add(delta, &c->parent->children_min_usage);

What if there is a concurrent update of c->min_usage? Then the patched version
can miss an update. I can't imagine a case when it will lead to bad consequences,
so probably it's ok. But not super obvious.
I think the way to think of it is that a missed update will be fixed by the next
one, so it's ok to run some time with old numbers.

Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>

Thanks!