On Tue, Dec 29, 2020 at 08:56:42AM -0800, Roman Gushchin wrote: > 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 <feng.tang@xxxxxxxxx> > > Cc: Roman Gushchin <guro@xxxxxx> > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > > --- > > 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 <guro@xxxxxx> Thanks for the review! > 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? Yep, we thought about it too. From current perf c2c profiling data, I haven't noticed obvious hot spots of false sharing for min/low/high/max (which are read mostly). For padding, we had some proposal before, current page_counter for 64 bits platform is 112 bytes, padding to 2 cacheline will only cost 16 bytes more. If this is fine, I can send another patch or folder it to this one. Thanks, Feng > Thank you!
