Messages in this thread | | | Date | Mon, 7 Feb 2022 23:27:46 +0800 | From | Jisheng Zhang <> | Subject | Re: [RFC PATCH] riscv: reduce THREAD_SIZE from 16KB to 8KB for RV64 |
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On Mon, Feb 07, 2022 at 08:35:54AM +0100, Arnd Bergmann wrote: > On Sun, Feb 6, 2022 at 6:43 PM Jisheng Zhang <jszhang@kernel.org> wrote: > > > > After irq stack is supported, it's possible to use small THREAD_SIZE. > > In fact, I tested this patch on a Lichee RV board, looks good so far. > > > > Signed-off-by: Jisheng Zhang <jszhang@kernel.org> > > I can definitely see the value in this, in particular when you get hardware with > small RAM configurations that would run a 32-bit kernel on other architectures. > > However, it's worth pointing out that all other 64-bit architectures use 16KB or > more, so it is rather dangerous to be the first architecture to try this out in > a long time. Some on-stack structures have a lot of pointers and 'unsigned long' > members, so they need twice the space, while other structures are the > same size either way. > > IRQ stacks obviously help here, but I don't think the 8KB size can be made > the default without a lot of testing of some of the more rarely used code paths. > > Here are a few things that would be worth doing on the way to a smaller > kernel stack: > > - do more compile-time testing with a lower CONFIG_FRAME_WARN value. > Historically, this defaults to 2048 bytes on 64-bit architectures. This is > much higher than we really want, but it takes work to find and eliminate > the outliers. I previously had a series to reduce the limit to 1280 bytes on > arm64 and still build all 'randconfig' configurations. > > - Use a much lower limit during regression testing. There is already a config > option to randomize the start of the thread stack, but you can also try > adding a configurable offset to see how far you can push it for a given > workload before you run into the guard page. > > - With vmap stacks, using 12KB may be an option as well, giving you > three pages for the stack plus one guard page, instead of 4 pages > stack plus four guard pages. > > - If you can make a convincing case for using a lower THREAD_SIZE, > make this a compile-time option across all 64-bit architectures that > support both IRQ stacks and VMAP stacks. The actual frame size > does depend on the ISA a bit, and we know that parisc and ia64 are > particularly, possibly s390 as well, but I would expect risc-v to be > not much different from arm64 and x86 here. >
Hi Arnd
Thanks so much for all the suggestions. Item3 and Item4 look more interesting to me.
Thanks a lot
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