Messages in this thread | | | From | "Deng, Pan" <> | Subject | RE: [PATCH v2] sched/task_group: Re-layout structure to reduce false sharing | Date | Mon, 26 Jun 2023 12:51:21 +0000 |
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> -----Original Message----- > From: Lu, Aaron <aaron.lu@intel.com> > Sent: Monday, June 26, 2023 1:48 PM > To: Deng, Pan <pan.deng@intel.com> > Cc: Chen, Tim C <tim.c.chen@intel.com>; peterz@infradead.org; > vincent.guittot@linaro.org; linux-kernel@vger.kernel.org; Li, Tianyou > <tianyou.li@intel.com>; Ma, Yu <yu.ma@intel.com>; Zhu, Lipeng > <lipeng.zhu@intel.com>; Chen, Yu C <yu.c.chen@intel.com>; Tim Chen > <tim.c.chen@linux.intel.com> > Subject: Re: [PATCH v2] sched/task_group: Re-layout structure to reduce false > sharing > > On Wed, Jun 21, 2023 at 04:14:25PM +0800, Deng Pan wrote: > > When running UnixBench/Pipe-based Context Switching case, we observed > > high false sharing for accessing ‘load_avg’ against rt_se and rt_rq > > when config CONFIG_RT_GROUP_SCHED is turned on. > > > > Pipe-based Context Switching case is a typical sleep/wakeup scenario, > > in which load_avg is frequenly loaded and stored, at the meantime, > > rt_se and rt_rq are frequently loaded. Unfortunately, they are in the > > same cacheline. > > > > This change re-layouts the structure: > > 1. Move rt_se and rt_rq to a 2nd cacheline. > > 2. Keep ‘parent’ field in the 2nd cacheline since it's also accessed > > very often when cgroups are nested, thanks Tim Chen for providing the > > insight. > > > > Tested on Intel Icelake 2 sockets 80C/160T platform, based on v6.4-rc5. > > > > With this change, Pipe-based Context Switching 160 parallel score is > > improved ~9.6%, perf record tool reports rt_se and rt_rq access cycles > > are reduced from ~14.5% to ~0.3%, perf c2c tool shows the > > false-sharing is resolved as expected: > > I also give it a run on an Icelake and saw similar things when > CONFIG_RT_GROUP_SCHED is on. > > For hackbench/pipe/thread, set_task_cpu() dropped from 1.67% to 0.51% > according to perf cycle; for netperf/nr_client=nr_cpu/UDP_RR, > set_task_cpu() dropped from 5.06% to 1.08%.
Thank you very much Aaron!
> > The patch looks good to me, just a nit below. > > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index > > ec7b3e0a2b20..4fbd4b3a4bdd 100644 > > --- a/kernel/sched/sched.h > > +++ b/kernel/sched/sched.h > > @@ -389,6 +389,19 @@ struct task_group { #endif #endif > > > > + struct rcu_head rcu; > > + struct list_head list; > > + > > + struct list_head siblings; > > + struct list_head children; > > + > > + /* > > + * To reduce false sharing, current layout is optimized to make > > + * sure load_avg is in a different cacheline from parent, rt_se > > + * and rt_rq. > > + */ > > + struct task_group *parent; > > + > > I wonder if we can simply do: > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index > ec7b3e0a2b20..31b73e8d9568 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -385,7 +385,9 @@ struct task_group { > * it in its own cacheline separated from the fields above which > * will also be accessed at each tick. > */ > - atomic_long_t load_avg ____cacheline_aligned; > + struct { > + atomic_long_t load_avg; > + } ____cacheline_aligned_in_smp; > #endif > #endif > > This way it can make sure there is no false sharing with load_avg no matter > how the layout of this structure changes in the future. > > Your patch has the advantage of not adding any more padding, thus saves > some space; the example code above has the advantage of no need to worry > about future changes that might break the expected alignment, but it does > make the structure size a little larger(704 -> 768). >
Right, the original purpose is to keep the structure size, actually I don't have a strong preference here whether keep the size or the maintainability, would like to know maintainer's insight :)
Thanks Pan
> Thanks, > Aaron > > > #ifdef CONFIG_RT_GROUP_SCHED > > struct sched_rt_entity **rt_se; > > struct rt_rq **rt_rq; > > @@ -396,13 +409,6 @@ struct task_group { > > struct rt_bandwidth rt_bandwidth; > > #endif > > > > - struct rcu_head rcu; > > - struct list_head list; > > - > > - struct task_group *parent; > > - struct list_head siblings; > > - struct list_head children; > > - > > #ifdef CONFIG_SCHED_AUTOGROUP > > struct autogroup *autogroup; > > #endif > > -- > > 2.39.3 > >
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