linux-kernel - Re: [PATCH v2 00/39] Memory allocation profiling

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Date:   Tue, 24 Oct 2023 11:29:03 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     akpm@...ux-foundation.org, kent.overstreet@...ux.dev,
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Subject: Re: [PATCH v2 00/39] Memory allocation profiling

On Tue, Oct 24, 2023 at 06:45:57AM -0700, Suren Baghdasaryan wrote:
> Updates since the last version [1]
> - Simplified allocation tagging macros;
> - Runtime enable/disable sysctl switch (/proc/sys/vm/mem_profiling)
> instead of kernel command-line option;
> - CONFIG_MEM_ALLOC_PROFILING_BY_DEFAULT to select default enable state;
> - Changed the user-facing API from debugfs to procfs (/proc/allocinfo);
> - Removed context capture support to make patch incremental;
> - Renamed uninstrumented allocation functions to use _noprof suffix;
> - Added __GFP_LAST_BIT to make the code cleaner;
> - Removed lazy per-cpu counters; it turned out the memory savings was
> minimal and not worth the performance impact;

Hello Suren,

> Performance overhead:
> To evaluate performance we implemented an in-kernel test executing
> multiple get_free_page/free_page and kmalloc/kfree calls with allocation
> sizes growing from 8 to 240 bytes with CPU frequency set to max and CPU
> affinity set to a specific CPU to minimize the noise. Below is performance
> comparison between the baseline kernel, profiling when enabled, profiling
> when disabled and (for comparison purposes) baseline with
> CONFIG_MEMCG_KMEM enabled and allocations using __GFP_ACCOUNT:
> 
>                         kmalloc                 pgalloc
> (1 baseline)            12.041s                 49.190s
> (2 default disabled)    14.970s (+24.33%)       49.684s (+1.00%)
> (3 default enabled)     16.859s (+40.01%)       56.287s (+14.43%)
> (4 runtime enabled)     16.983s (+41.04%)       55.760s (+13.36%)
> (5 memcg)               33.831s (+180.96%)      51.433s (+4.56%)

some recent changes [1] to the kmem accounting should have made it quite a bit
faster. Would be great if you can provide new numbers for the comparison.
Maybe with the next revision?

And btw thank you (and Kent): your numbers inspired me to do this kmemcg
performance work. I expect it still to be ~twice more expensive than your
stuff because on the memcg side we handle separately charge and statistics,
but hopefully the difference will be lower.

Thank you!

[1]:
  patches from next tree, so no stable hashes:
    mm: kmem: reimplement get_obj_cgroup_from_current()
    percpu: scoped objcg protection
    mm: kmem: scoped objcg protection
    mm: kmem: make memcg keep a reference to the original objcg
    mm: kmem: add direct objcg pointer to task_struct
    mm: kmem: optimize get_obj_cgroup_from_current()