Messages in this thread | | | Subject | Re: [LKP] Re: [mm/memcg] bd0b230fe1: will-it-scale.per_process_ops -22.7% regression | From | Waiman Long <> | Date | Wed, 25 Nov 2020 20:34:15 -0500 |
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On 11/25/20 1:24 AM, Feng Tang wrote: > On Fri, Nov 20, 2020 at 07:44:24PM +0800, Feng Tang wrote: >> On Fri, Nov 13, 2020 at 03:34:36PM +0800, Feng Tang wrote: >>>> I would rather focus on a more effective mem_cgroup layout. It is very >>>> likely that we are just stumbling over two counters here. >>>> >>>> Could you try to add cache alignment of counters after memory and see >>>> which one makes the difference? I do not expect memsw to be the one >>>> because that one is used together with the main counter. But who knows >>>> maybe the way it crosses the cache line has the exact effect. Hard to >>>> tell without other numbers. >>> I added some alignments change around the 'memsw', but neither of them can >>> restore the -22.7%. Following are some log showing how the alignments >>> are: >>> >>> tl: memcg=0x7cd1000 memory=0x7cd10d0 memsw=0x7cd1140 kmem=0x7cd11b0 tcpmem=0x7cd1220 >>> t2: memcg=0x7cd0000 memory=0x7cd00d0 memsw=0x7cd0140 kmem=0x7cd01c0 tcpmem=0x7cd0230 >>> >>> So both of the 'memsw' are aligned, but t2's 'kmem' is aligned while >>> t1's is not. >>> >>> I will check more on the perf data about detailed hotspots. >> Some more check updates about it: >> >> Waiman's patch is effectively removing one 'struct page_counter' between >> 'memory' and "memsw'. And the mem_cgroup is: >> >> struct mem_cgroup { >> >> ... >> >> struct page_counter memory; /* Both v1 & v2 */ >> >> union { >> struct page_counter swap; /* v2 only */ >> struct page_counter memsw; /* v1 only */ >> }; >> >> /* Legacy consumer-oriented counters */ >> struct page_counter kmem; /* v1 only */ >> struct page_counter tcpmem; /* v1 only */ >> >> ... >> ... >> >> MEMCG_PADDING(_pad1_); >> >> atomic_t moving_account; >> struct task_struct *move_lock_task; >> >> ... >> }; >> >> >> I do experiments by inserting a 'page_counter' between 'memory' >> and the 'MEMCG_PADDING(_pad1_)', no matter where I put it, the >> benchmark result can be recovered from 145K to 185K, which is >> really confusing, as adding a 'page_counter' right before the >> '_pad1_' doesn't change cache alignment of any members. > I think we finally found the trick :), further debugging shows it > is not related to the alignment inside one cacheline, but the > adjacency of 2 adjacent cacheliens (2N and 2N+1, one pair of 128 bytes). > > For structure mem_cgroup, member 'vmstats_local', 'vmstats_percpu' > sit in one cacheline, while 'vmstats[]' sits in the next cacheline, > and when 'adjacent cacheline prefetch" is enabled, if these 2 lines > sit in one pair (128 btyes), say 2N and 2N+1, then there seems to > be some kind of false sharing, and if they sit in 2 pairs, say > 2N-1 and 2N then it's fine. > > And with the following patch to relayout these members, the regression > is restored and event better. while reducing 64 bytes of sizeof > 'struct mem_cgroup' > > parent_commit Waiman's_commit +relayout patch > > result 187K 145K 200K > > Also, if we disable the hw prefetch feature, the Waiman's commit > and its parent commit will have no performance difference. > > Thanks, > Feng
Finally, misery solved? Well done.
Your patch looks good. It reduces structure size and improves performance.
Acked-by: Waiman Long <longman@redhat.com>
> From 2e63af34fa4853b2dd9669867c37a3cf07f7a505 Mon Sep 17 00:00:00 2001 > From: Feng Tang <feng.tang@intel.com> > Date: Wed, 25 Nov 2020 13:22:21 +0800 > Subject: [PATCH] mm: memcg: relayout structure mem_cgroup to avoid cache > interfereing > > 0day reported one -22.7% regression for will-it-scale page_fault2 > case [1] on a 4 sockets 144 CPU platform, and bisected to it to be > caused by Waiman's optimization (commit bd0b230fe1) of saving one > 'struct page_counter' space for 'struct mem_cgroup'. > > Initially we thought it was due to the cache alignment change introduced > by the patch, but further debug shows that it is due to some hot data > members ('vmstats_local', 'vmstats_percpu', 'vmstats') sit in 2 adjacent > cacheline (2N and 2N+1 cacheline), and when adjacent cache line prefetch > is enabled, it triggers an "extended level" of cache false sharing for > 2 adjacent cache lines. > > So exchange the 2 member blocks, while keeping mostly the original > cache alignment, which can restore and even enhance the performance, > and save 64 bytes of space for 'struct mem_cgroup' (from 2880 to 2816, > with 0day's default RHEL-8.3 kernel config) > > [1]. https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/ > > Fixes: bd0b230fe145 ("mm/memcg: unify swap and memsw page counters") > Reported-by: kernel test robot <rong.a.chen@intel.com> > Signed-off-by: Feng Tang <feng.tang@intel.com> > --- > include/linux/memcontrol.h | 28 ++++++++++++++-------------- > 1 file changed, 14 insertions(+), 14 deletions(-) > > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > index e391e3c..a2d50b0 100644 > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -282,20 +282,6 @@ struct mem_cgroup { > > MEMCG_PADDING(_pad1_); > > - /* > - * set > 0 if pages under this cgroup are moving to other cgroup. > - */ > - atomic_t moving_account; > - struct task_struct *move_lock_task; > - > - /* Legacy local VM stats and events */ > - struct memcg_vmstats_percpu __percpu *vmstats_local; > - > - /* Subtree VM stats and events (batched updates) */ > - struct memcg_vmstats_percpu __percpu *vmstats_percpu; > - > - MEMCG_PADDING(_pad2_); > - > atomic_long_t vmstats[MEMCG_NR_STAT]; > atomic_long_t vmevents[NR_VM_EVENT_ITEMS]; > > @@ -317,6 +303,20 @@ struct mem_cgroup { > struct list_head objcg_list; /* list of inherited objcgs */ > #endif > > + MEMCG_PADDING(_pad2_); > + > + /* > + * set > 0 if pages under this cgroup are moving to other cgroup. > + */ > + atomic_t moving_account; > + struct task_struct *move_lock_task; > + > + /* Legacy local VM stats and events */ > + struct memcg_vmstats_percpu __percpu *vmstats_local; > + > + /* Subtree VM stats and events (batched updates) */ > + struct memcg_vmstats_percpu __percpu *vmstats_percpu; > + > #ifdef CONFIG_CGROUP_WRITEBACK > struct list_head cgwb_list; > struct wb_domain cgwb_domain;
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