Messages in this thread | | | Date | Fri, 20 Nov 2020 19:44:24 +0800 | From | Feng Tang <> | Subject | Re: [LKP] Re: [mm/memcg] bd0b230fe1: will-it-scale.per_process_ops -22.7% regression |
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On Fri, Nov 13, 2020 at 03:34:36PM +0800, Feng Tang wrote: > On Thu, Nov 12, 2020 at 03:16:54PM +0100, Michal Hocko wrote: > > > > > I add one phony page_counter after the union and re-test, the regression > > > > > reduced to -1.2%. It looks like the regression caused by the data structure > > > > > layout change. > > > > > > > > Thanks for double checking. Could you try to cache align the > > > > page_counter struct? If that helps then we should figure which counters > > > > acks against each other by adding the alignement between the respective > > > > counters. > > > > > > We tried below patch to make the 'page_counter' aligned. > > > > > > diff --git a/include/linux/page_counter.h b/include/linux/page_counter.h > > > index bab7e57..9efa6f7 100644 > > > --- a/include/linux/page_counter.h > > > +++ b/include/linux/page_counter.h > > > @@ -26,7 +26,7 @@ struct page_counter { > > > /* legacy */ > > > unsigned long watermark; > > > unsigned long failcnt; > > > -}; > > > +} ____cacheline_internodealigned_in_smp; > > > > > > and with it, the -22.7% peformance change turns to a small -1.7%, which > > > confirms the performance bump is caused by the change to data alignment. > > > > > > After the patch, size of 'page_counter' increases from 104 bytes to 128 > > > bytes, and the size of 'mem_cgroup' increases from 2880 bytes to 3008 > > > bytes(with our kernel config). Another major data structure which > > > contains 'page_counter' is 'hugetlb_cgroup', whose size will change > > > from 912B to 1024B. > > > > > > Should we make these page_counters aligned to reduce cacheline conflict? > > > > 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.
Thanks, Feng
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