On Tue, Dec 29, 2020 at 10:35:13PM +0800, Feng Tang wrote: > When checking a memory cgroup related performance regression [1], > from the perf c2c profiling data, we found high false sharing for > accessing 'usage' and 'parent'. > > On 64 bit system, the 'usage' and 'parent' are close to each other, > and easy to be in one cacheline (for cacheline size == 64+ B). 'usage' > is usally written, while 'parent' is usually read as the cgroup's > hierarchical counting nature. > > So move the 'parent' to the end of the structure to make sure they > are in different cache lines. > > Following are some performance data with the patch, against > v5.11-rc1, on several generations of Xeon platforms. Most of the > results are improvements, with only one malloc case on one platform > shows a -4.0% regression. Each category below has several subcases > run on different platform, and only the worst and best scores are > listed: > > fio: +1.8% ~ +8.3% > will-it-scale/malloc1: -4.0% ~ +8.9% > will-it-scale/page_fault1: no change > will-it-scale/page_fault2: +2.4% ~ +20.2% > > [1].https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/ > Signed-off-by: Feng Tang <[email protected]> > Cc: Roman Gushchin <[email protected]> > Cc: Johannes Weiner <[email protected]> > --- > include/linux/page_counter.h | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h > index 85bd413..6795913 100644 > --- a/include/linux/page_counter.h > +++ b/include/linux/page_counter.h > @@ -12,7 +12,6 @@ struct page_counter { > unsigned long low; > unsigned long high; > unsigned long max; > - struct page_counter *parent; > > /* effective memory.min and memory.min usage tracking */ > unsigned long emin; > @@ -27,6 +26,14 @@ struct page_counter { > /* legacy */ > unsigned long watermark; > unsigned long failcnt; > + > + /* > + * 'parent' is placed here to be far from 'usage' to reduce > + * cache false sharing, as 'usage' is written mostly while > + * parent is frequently read for cgroup's hierarchical > + * counting nature. > + */ > + struct page_counter *parent; > };
LGTM! Reviewed-by: Roman Gushchin <[email protected]> I wonder if we have the same problem with min/low/high/max? Maybe try to group all mostly-read-only fields (min, low, high, max and parent) and separate them with some padding? Thank you!
