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@xxxxxxx> 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
