On Thu 25-08-22 00:05:04, Shakeel Butt wrote:
> For cgroups using low or min protections, the function
> propagate_protected_usage() was doing an atomic xchg() operation
> irrespectively. We can optimize out this atomic operation for one
> specific scenario where the workload is using the protection (i.e.
> min > 0) and the usage is above the protection (i.e. usage > min).
>
> This scenario is actually very common where the users want a part of
> their workload to be protected against the external reclaim. Though this
> optimization does introduce a race when the usage is around the
> protection and concurrent charges and uncharged trip it over or under
> the protection. In such cases, we might see lower effective protection
> but the subsequent charge/uncharge will correct it.
Thanks this is much more useful
> 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 to see if this optimization
> is effective for the mentioned case.
>
> $ 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%
>
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Acked-by: Soheil Hassas Yeganeh <soheil@xxxxxxxxxx>
> Reviewed-by: Feng Tang <feng.tang@xxxxxxxxx>
> Acked-by: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!
> ---
> Changes since v1:
> - Commit message update with more detail on which scenario is getting
> optimized and possible race condition.
>
> 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);
> }
>
> - low = READ_ONCE(c->low);
> - if (low || atomic_long_read(&c->low_usage)) {
> - protected = min(usage, low);
> + protected = min(usage, READ_ONCE(c->low));
> + old_protected = atomic_long_read(&c->low_usage);
> + if (protected != old_protected) {
> old_protected = atomic_long_xchg(&c->low_usage, protected);
> delta = protected - old_protected;
> if (delta)
> --
> 2.37.1.595.g718a3a8f04-goog
--
Michal Hocko
SUSE Labs
