Re: [PATCH v2 2/2] fork: make number of cached stacks (vmapped) configurable using Kbuild
From: Hoeun Ryu
Date: Thu Feb 09 2017 - 08:37:35 EST
On Thu, Feb 9, 2017 at 5:40 PM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> On Thu 09-02-17 13:03:47, Hoeun Ryu wrote:
>> Introducing NR_VMAP_STACK_CACHE, the number of cached stacks for virtually
>> mapped kernel stack can be configurable using Kbuild system.
>> default value is 2.
> This should really explain _why_ we want/need this. The config space is
> really large already and there should better be a good reason to make it
> even larger. Which workload will benefit from a different than default
> cache size and why?
> Without a really good reasons NACK from me.
OK, I understand your NACK. Thank you.
>> Signed-off-by: Hoeun Ryu <hoeun.ryu@xxxxxxxxx>
>> arch/Kconfig | 8 ++++++++
>> kernel/fork.c | 2 +-
>> 2 files changed, 9 insertions(+), 1 deletion(-)
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index d49a8e6..066d111 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -849,6 +849,14 @@ config VMAP_STACK
>> the stack to map directly to the KASAN shadow map using a formula
>> that is incorrect if the stack is in vmalloc space.
>> +config NR_VMAP_STACK_CACHE
>> + int "Number of cached stacks"
>> + default "2"
>> + depends on VMAP_STACK
>> + help
>> + This determines how many stacks can be cached for virtually
>> + mapped kernel stacks.
>> config ARCH_WANT_RELAX_ORDER
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 7911ed2..73ba1da 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -166,7 +166,7 @@ void __weak arch_release_thread_stack(unsigned long *stack)
>> * vmalloc() is a bit slow, and calling vfree() enough times will force a TLB
>> * flush. Try to minimize the number of calls by caching stacks.
>> -#define NR_CACHED_STACKS 2
>> +#define NR_CACHED_STACKS CONFIG_NR_VMAP_STACK_CACHE
>> static DEFINE_PER_CPU(struct vm_struct *, cached_stacks[NR_CACHED_STACKS]);
> Michal Hocko
> SUSE Labs