Deprecating and removing SLOB
Damien Le Moal
damien.lemoal at opensource.wdc.com
Mon Nov 14 20:28:14 PST 2022
On 11/15/22 13:24, Damien Le Moal wrote:
> On 11/14/22 23:47, Hyeonggon Yoo wrote:
>> On Mon, Nov 14, 2022 at 08:35:31PM +0900, Damien Le Moal wrote:
>>> On 11/14/22 18:36, Vlastimil Babka wrote:
>>>> On 11/14/22 06:48, Damien Le Moal wrote:
>>>>> On 11/14/22 10:55, Damien Le Moal wrote:
>>>>>> On 11/12/22 05:46, Conor Dooley wrote:
>>>>>>> On Fri, Nov 11, 2022 at 11:33:30AM +0100, Vlastimil Babka wrote:
>>>>>>>> On 11/8/22 22:44, Pasha Tatashin wrote:
>>>>>>>>> On Tue, Nov 8, 2022 at 10:55 AM Vlastimil Babka <vbabka at suse.cz> wrote:
>>>>>>>>>>
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>> as we all know, we currently have three slab allocators. As we discussed
>>>>>>>>>> at LPC [1], it is my hope that one of these allocators has a future, and
>>>>>>>>>> two of them do not.
>>>>>>>>>>
>>>>>>>>>> The unsurprising reasons include code maintenance burden, other features
>>>>>>>>>> compatible with only a subset of allocators (or more effort spent on the
>>>>>>>>>> features), blocking API improvements (more on that below), and my
>>>>>>>>>> inability to pronounce SLAB and SLUB in a properly distinguishable way,
>>>>>>>>>> without resorting to spelling out the letters.
>>>>>>>>>>
>>>>>>>>>> I think (but may be proven wrong) that SLOB is the easier target of the
>>>>>>>>>> two to be removed, so I'd like to focus on it first.
>>>>>>>>>>
>>>>>>>>>> I believe SLOB can be removed because:
>>>>>>>>>>
>>>>>>>>>> - AFAIK nobody really uses it? It strives for minimal memory footprint
>>>>>>>>>> by putting all objects together, which has its CPU performance costs
>>>>>>>>>> (locking, lack of percpu caching, searching for free space...). I'm not
>>>>>>>>>> aware of any "tiny linux" deployment that opts for this. For example,
>>>>>>>>>> OpenWRT seems to use SLUB and the devices these days have e.g. 128MB
>>>>>>>>>> RAM, not up to 16 MB anymore. I've heard anecdotes that the performance
>>>>>>>>>> SLOB impact is too much for those who tried. Googling for
>>>>>>>>>> "CONFIG_SLOB=y" yielded nothing useful.
>>>>>>>>>
>>>>>>>>> I am all for removing SLOB.
>>>>>>>>>
>>>>>>>>> There are some devices with configs where SLOB is enabled by default.
>>>>>>>>> Perhaps, the owners/maintainers of those devices/configs should be
>>>>>>>>> included into this thread:
>>>>>>>>>
>>>>>>>>> tatashin at soleen:~/x/linux$ git grep SLOB=y
>>>>>>>
>>>>>>>>> arch/riscv/configs/nommu_k210_defconfig:CONFIG_SLOB=y
>>>>>>>>> arch/riscv/configs/nommu_k210_sdcard_defconfig:CONFIG_SLOB=y
>>>>>>>>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_SLOB=y
>>>>>>>
>>>>>>>>
>>>>>>>> Turns out that since SLOB depends on EXPERT, many of those lack it so
>>>>>>>> running make defconfig ends up with SLUB anyway, unless I miss something.
>>>>>>>> Only a subset has both SLOB and EXPERT:
>>>>>>>>
>>>>>>>>> git grep CONFIG_EXPERT `git grep -l "CONFIG_SLOB=y"`
>>>>>>>
>>>>>>>> arch/riscv/configs/nommu_virt_defconfig:CONFIG_EXPERT=y
>>>>>>>
>>>>>>> I suppose there's not really a concern with the virt defconfig, but I
>>>>>>> did check the output of `make nommu_k210_defconfig" and despite not
>>>>>>> having expert it seems to end up CONFIG_SLOB=y in the generated .config.
>>>>>>>
>>>>>>> I do have a board with a k210 so I checked with s/SLOB/SLUB and it still
>>>>>>> boots etc, but I have no workloads or w/e to run on it.
>>>>>>
>>>>>> I sent a patch to change the k210 defconfig to using SLUB. However...
>>>>
>>>> Thanks!
>>>>
>>>>>> The current default config using SLOB gives about 630 free memory pages
>>>>>> after boot (cat /proc/vmstat). Switching to SLUB, this is down to about
>>>>>> 400 free memory pages (CONFIG_SLUB_CPU_PARTIAL is off).
>>>>
>>>> Thanks for the testing! How much RAM does the system have btw? I found 8MB
>>>> somewhere, is that correct?
>>>
>>> Yep, 8MB, that's it.
>>>
>>>> So 230 pages that's a ~920 kB difference. Last time we saw less dramatic
>>>> difference [1]. But that was looking at Slab pages, not free pages. The
>>>> extra overhead could be also in percpu allocations, code etc.
>>>>
>>>>>> This is with a buildroot kernel 5.19 build including a shell and sd-card
>>>>>> boot. With SLUB, I get clean boots and a shell prompt as expected. But I
>>>>>> definitely see more errors with shell commands failing due to allocation
>>>>>> failures for the shell process fork. So as far as the K210 is concerned,
>>>>>> switching to SLUB is not ideal.
>>>>>>
>>>>>> I would not want to hold on kernel mm improvements because of this toy
>>>>>> k210 though, so I am not going to prevent SLOB deprecation. I just wish
>>>>>> SLUB itself used less memory :)
>>>>>
>>>>> Did further tests with kernel 6.0.1:
>>>>> * SLOB: 630 free pages after boot, shell working (occasional shell fork
>>>>> failure happen though)
>>>>> * SLAB: getting memory allocation for order 7 failures on boot already
>>>>> (init process). Shell barely working (high frequency of shell command fork
>>>>> failures)
>>>
>>> I forgot to add here that the system was down to about 500 free pages
>>> after boot (again from the shell with "cat /proc/vmstat").
>>>
>>>>> * SLUB: getting memory allocation for order 7 failures on boot. I do get a
>>>>> shell prompt but cannot run any shell command that involves forking a new
>>>>> process.
>>>
>>> For both slab and slub, I had cpu partial off, debug off and slab merge
>>> on, as I suspected that would lead to less memory overhead.
>>> I suspected memory fragmentation may be an issue but doing
>>>
>>> echo 3 > /proc/sys/vm/drop_caches
>>>
>>> before trying a shell command did not help much at all (it usually does on
>>> that board with SLOB). Note that this is all with buildroot, so this echo
>>> & redirect always works as it does not cause a shell fork.
>>>
>>>>>
>>>>> So if we want to keep the k210 support functional with a shell, we need
>>>>> slob. If we reduce that board support to only one application started as
>>>>> the init process, then I guess anything is OK.
>>>>
>>>> In [1] it was possible to save some more memory with more tuning. Some of
>>>> that required boot parameters and other code changes. In another reply [2] I
>>>> considered adding something like SLUB_TINY to take care of all that, so
>>>> looks like it would make sense to proceed with that.
>>>
>>> If you want me to test something, let me know.
>>
>> Would you try this please?
>>
>> diff --git a/mm/slub.c b/mm/slub.c
>> index a24b71041b26..1c36c4b9aaa0 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -4367,9 +4367,7 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
>> * The larger the object size is, the more slabs we want on the partial
>> * list to avoid pounding the page allocator excessively.
>> */
>> - s->min_partial = min_t(unsigned long, MAX_PARTIAL, ilog2(s->size) / 2);
>> - s->min_partial = max_t(unsigned long, MIN_PARTIAL, s->min_partial);
>> -
>> + s->min_partial = 0;
>> set_cpu_partial(s);
>>
>> #ifdef CONFIG_NUMA
>>
>>
>> and booting with and without boot parameter slub_max_order=0?
>
> Test notes: I used Linus 6.1-rc5 as the base. That is the only thing I
> changed in buildroot default config for the sipeed maix bit card, booting
> with SD card. The test is: booting and run "cat /proc/vmstat" and register
> the nr_free_pages value. I repeated the boot + cat 3 to 4 times for each case.
>
> Here are the results:
>
> 6.1-rc5, SLOB:
> - 623 free pages
> - 629 free pages
> - 629 free pages
> 6.1-rc5, SLUB:
> - 448 free pages
> - 448 free pages
> - 429 free pages
> 6.1-rc5, SLUB + slub_max_order=0:
> - Init error, shell prompt but no shell command working
> - Init error, no shell prompt
> - 508 free pages
> - Init error, shell prompt but no shell command working
> 6.1-rc5, SLUB + patch:
> - Init error, shell prompt but no shell command working
> - 433 free pages
> - 448 free pages
> - 423 free pages
> 6.1-rc5, SLUB + slub_max_order=0 + patch:
> - Init error, no shell prompt
> - Init error, shell prompt, 499 free pages
> - Init error, shell prompt but no shell command working
> - Init error, no shell prompt
>
> No changes for SLOB results, expected.
>
> For default SLUB, I did get all clean boots this time and could run the
> cat command. But I do see shell fork failures if I keep running commands.
>
> For SLUB + slub_max_order=0, I only got one clean boot with 508 free
> pages. Remaining runs failed to give a shell prompt or allow running cat
> command. For the clean boot, I do see higher number of free pages.
>
> SLUB with the patch was nearly identical to SLUB without the patch.
>
> And SLUB+patch+slub_max_order=0 gave again a lot of errors/bad boot. I
> could run the cat command only once, giving 499 free pages, so better than
> regular SLUB. But it seems that the memory is more fragmented as
> allocations fail more often.
Note about the last case (SLUB+patch+slub_max_order=0). Here are the
messages I got when the init shell process fork failed:
[ 1.217998] nommu: Allocation of length 491520 from process 1 (sh) failed
[ 1.224098] active_anon:0 inactive_anon:0 isolated_anon:0
[ 1.224098] active_file:5 inactive_file:12 isolated_file:0
[ 1.224098] unevictable:0 dirty:0 writeback:0
[ 1.224098] slab_reclaimable:38 slab_unreclaimable:459
[ 1.224098] mapped:0 shmem:0 pagetables:0
[ 1.224098] sec_pagetables:0 bounce:0
[ 1.224098] kernel_misc_reclaimable:0
[ 1.224098] free:859 free_pcp:0 free_cma:0
[ 1.260419] Node 0 active_anon:0kB inactive_anon:0kB active_file:20kB
inactive_file:48kB unevictable:0kB isolated(anon):0kB isolated(file):0kB
mapped:0kB dirty:0kB writeback:0kB shmem:0kB writeback_tmp:0kB
kernel_stack:576kB pagetables:0kB sec_pagetables:0kB all_unreclaimable? no
[ 1.285147] DMA32 free:3436kB boost:0kB min:312kB low:388kB high:464kB
reserved_highatomic:0KB active_anon:0kB inactive_anon:0kB active_file:0kB
inactive_file:28kB unevictable:0kB writepending:0kB present:8192kB
managed:6240kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB
[ 1.310654] lowmem_reserve[]: 0 0 0
[ 1.314089] DMA32: 17*4kB (U) 10*8kB (U) 7*16kB (U) 6*32kB (U) 11*64kB
(U) 6*128kB (U) 6*256kB (U) 0*512kB 0*1024kB 0*2048kB 0*4096kB = 3460kB
[ 1.326883] 33 total pagecache pages
[ 1.330420] binfmt_flat: Unable to allocate RAM for process text/data,
errno -12
[ 1.337858] Kernel panic - not syncing: Attempted to kill init!
exitcode=0x0000000b
--
Damien Le Moal
Western Digital Research
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