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    SubjectRe: [PATCH 1/3] mm: page_counter: remove unneeded atomic ops for low/min
    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@google.com>
    > Reported-by: kernel test robot <oliver.sang@intel.com>
    > ---
    > 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@linux.dev>

    Thanks!

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