Re: Deprecating and removing SLOB

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

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@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@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...
> 
> 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).
> 
> 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)
* 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.

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.

-- 
Damien Le Moal
Western Digital Research