Messages in this thread | | | Date | Wed, 13 Apr 2022 14:50:39 -0700 | Subject | Re: [PATCH RESEND 1/1] lib/Kconfig: remove DEBUG_PER_CPU_MAPS dependency for CPUMASK_OFFSTACK | From | Libo Chen <> |
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On 4/13/22 13:52, Arnd Bergmann wrote: > On Wed, Apr 13, 2022 at 9:28 PM Libo Chen <libo.chen@oracle.com> wrote: >> On 4/13/22 08:41, Randy Dunlap wrote: >>> On 4/12/22 23:56, Libo Chen wrote: >>>>> --- a/lib/Kconfig >>>>> +++ b/lib/Kconfig >>>>> @@ -511,7 +511,8 @@ config CHECK_SIGNATURE >>>>> bool >>>>> config CPUMASK_OFFSTACK >>>>> - bool "Force CPU masks off stack" if DEBUG_PER_CPU_MAPS >>>>> + bool "Force CPU masks off stack" >>>>> + depends on DEBUG_PER_CPU_MAPS >>>> This forces every arch to enable DEBUG_PER_CPU_MAPS if they want to enable CPUMASK_OFFSTACK, it's even stronger than "if". My whole argument is CPUMASK_OFFSTACK should be enable/disabled independent of DEBUG_PER_CPU_MASK >>>>> help >>>>> Use dynamic allocation for cpumask_var_t, instead of putting >>>>> them on the stack. This is a bit more expensive, but avoids >>>>> >>>>> >>>>> As I said earlier, the "if" on the "bool" line just controls the prompt message. >>>>> This patch make CPUMASK_OFFSTACK require DEBUG_PER_CPU_MAPS -- which might be overkill. >>>>> >>>> Okay I understand now "if" on the "boot" is not a dependency and it only controls the prompt message, then the question is why we cannot enable CPUMASK_OFFSTACK without DEBUG_PER_CPU_MAPS if it only controls prompt message? Is it not the behavior we expect? >>> Yes, it is. I don't know that the problem is... >> Masahiro explained that CPUMASK_OFFSTACK can only be configured by >> options not users if DEBUG_PER_CPU_MASK is not enabled. This doesn't >> seem to be what we want. > I think the correct way to do it is to follow x86 and powerpc, and tying > CPUMASK_OFFSTACK to "large" values of CONFIG_NR_CPUS. > For smaller values of NR_CPUS, the onstack masks are obviously > cheaper, we just need to decide what the cut-off point is. I agree. It appears enabling CPUMASK_OFFSTACK breaks kernel builds on some architectures such as parisc and nios2 as reported by kernel test robot. Maybe it makes sense to use DEBUG_PER_CPU_MAPS as some kind of guard on CPUMASK_OFFSTACK. > In x86, the onstack masks can be selected for normal SMP builds with > up to 512 CPUs, while CONFIG_MAXSMP=y raises the limit to 8192 > CPUs while selecting CPUMASK_OFFSTACK. > PowerPC does it the other way round, selecting CPUMASK_OFFSTACK > implicitly whenever NR_CPUS is set to 8192 or more. > > I think we can easily do the same as powerpc on arm64. With the I am leaning more towards x86's way because even NR_CPUS=160 is too expensive for 4-core arm64 VMs according to apachebench. I highly doubt that there is a good cut-off point to make everybody happy (or not unhappy). > ApacheBench test you cite in the patch description, what is the > value of NR_CPUS at which you start seeing a noticeable > benefit for offstack masks? Can you do the same test for > NR_CPUS=1024 or 2048? As mentioned above, a good cut-off point moves depends on the actual number of CPUs. But yeah I can do the same test for 1024 or even smaller NR_CPUs values on the same 64-core arm64 VM setup.
Libo
> > Arnd
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