On 4/13/22 13:52, Arnd Bergmann wrote:
On Wed, Apr 13, 2022 at 9:28 PM Libo Chen <libo.chen@xxxxxxxxxx> 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
