Re: [linus:master] [mm/mmap] 28c5609fb2: aim9.page_test.ops_per_sec -10.8% regression

[Yin Fengwei]

Hi Liam,

On 5/6/23 14:20, kernel test robot wrote:
> Hello,
> 
> kernel test robot noticed a -10.8% regression of aim9.page_test.ops_per_sec on:
> 
> commit: 28c5609fb236807910ca347ad3e26c4567998526 ("mm/mmap: preallocate maple nodes for brk vma expansion")
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> testcase: aim9
> test machine: 48 threads 2 sockets Intel(R) Xeon(R) CPU E5-2697 v2 @ 2.70GHz (Ivy Bridge-EP) with 112G memory
> parameters:
> 
> 	testtime: 5s
> 	test: all
> 	cpufreq_governor: performance
> 
> If you fix the issue, kindly add following tag
> | Reported-by: kernel test robot <yujie.liu@xxxxxxxxx>
> | Link: https://lore.kernel.org/oe-lkp/202305061457.ac15990c-yujie.liu@xxxxxxxxx
> 

Some finding related:
   eBPF funclatency tool says the latency of function do_brk_flags() doubles
   with the patch 28c5609fb2.

   With the patch 28c5609fb2, the mas_alloc_nodes() is called much more than
   without the patch.

   In my local debugging env, I can see around 17009999 times call to 
   mas_alloc_nodes(). The number is zero without the patch 28c5609fb2.
So we are kind of sure the regression is connected to the patch.


The page_test of AIM9 is doing following work with single thread:
        newbrk = sbrk(1024 * 1024);     /* move up 1 megabyte */                
        while (true) {                   /* while not done */                    
                newbrk = sbrk(-4096 * 16);      /* deallocate some space */     
                for (i = 0; i < 16; i++) {      /* now get it back in pieces */
                        newbrk = sbrk(4096);    /* Get pointer to new space */ 
		}
	}

Is it possible that the sbrk pattern triggers the corner case? Thanks.


Regards
Yin, Fengwei