On 2022/11/17 2:51, Vlastimil Babka wrote: > On 11/15/22 05:24, 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@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... >>>>> >>>>> 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. >> >> Hope this helps. Let me know if you want to test something else. > > Could you please try this branch with CONFIG_SLUB_TINY=y? > https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=slub-tiny-v1r0 > > Seeing your results I didn't modify default slub_max_order by this new > CONFIG (yet?) so maybe after trying the default, trying then also with > manual slub_max_order=0 and slub_max_order=1 would be useful too. Otherwise > it should be all changes to lower SLUB memory footprint. Hopefully it will > be visible in the number of free pages. But if fragmentation is an issue, it > might not be enough. BTW, during boot there should be a line "Built X > zonelists, mobility grouping ..." can you grep for it and provide please, I > wonder if mobility grouping ends up being off or on on that system. OK. Will try as soon as I can (before end of week hopefully). My pipeline is a little crowded right now :) > > Thanks! > >> Cheers. >> > -- Damien Le Moal Western Digital Research
