Re: [RFC 10/14] fork: Dynamic Kernel Stacks

[Pasha Tatashin]

On Mon, Mar 11, 2024 at 3:32 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote:
>
> Hi,
>
> just typos etc.
>
> On 3/11/24 09:46, Pasha Tatashin wrote:
> > The core implementation of dynamic kernel stacks.
> >
>
> ...
>
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > ---
> >  arch/Kconfig                     |  34 +++++
> >  include/linux/sched.h            |   2 +-
> >  include/linux/sched/task_stack.h |  41 +++++-
> >  kernel/fork.c                    | 239 +++++++++++++++++++++++++++++++
> >  kernel/sched/core.c              |   1 +
> >  5 files changed, 315 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index a5af0edd3eb8..da3df347b069 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -1241,6 +1241,40 @@ config VMAP_STACK
> >         backing virtual mappings with real shadow memory, and KASAN_VMALLOC
> >         must be enabled.
> >
> > +config HAVE_ARCH_DYNAMIC_STACK
> > +     def_bool n
> > +     help
> > +       An arch should select this symbol if it can support kernel stacks
> > +       dynamic growth.
> > +
> > +       - Arch must have support for HAVE_ARCH_VMAP_STACK, in order to handle
> > +         stack related page faults
>
>             stack-related
>
> > +
> > +       - Arch must be able to faults from interrupt context.
>
>                                  fault
>
> > +       - Arch must allows the kernel to handle stack faults gracefully, even
>
>                       allow
>
> > +         during interrupt handling.
> > +
> > +       - Exceptions such as no pages available should be handled the same
>
>                                                             handled in the same
>
> > +         in the consitent and predictable way. I.e. the exception should be
>
>             consistent
>
> > +         handled the same as when stack overflow occurs when guard pages are
> > +         touched with extra information about the allocation error.
> > +
> > +config DYNAMIC_STACK
> > +     default y
> > +     bool "Dynamically grow kernel stacks"
> > +     depends on THREAD_INFO_IN_TASK
> > +     depends on HAVE_ARCH_DYNAMIC_STACK
> > +     depends on VMAP_STACK
> > +     depends on !KASAN
> > +     depends on !DEBUG_STACK_USAGE
> > +     depends on !STACK_GROWSUP
> > +     help
> > +       Dynamic kernel stacks allow to save memory on machines with a lot of
> > +       threads by starting with small stacks, and grow them only when needed.
> > +       On workloads where most of the stack depth do not reach over one page
>
>                                                      does
>
> > +       the memory saving can be subsentantial. The feature requires virtually
>
>                                    substantial.
>
> > +       mapped kernel stacks in order to handle page faults.
> > +
> >  config HAVE_ARCH_RANDOMIZE_KSTACK_OFFSET
> >       def_bool n
> >       help
>
>
>
> > +/*
> > + * This flag is used to pass information from fault handler to refill about
> > + * which pages were allocated, and should be charged to memcg.
> > + */
> > +#define DYNAMIC_STACK_PAGE_AQUIRED_FLAG      0x1
>
>                               ACQUIRED
> please

Thank you Randy, I will address your comments in my next revision.

Pasha

>
>
>
> --
> #Randy