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From: Pasha Tatashin <pasha.tatashin@soleen.com>
Date: Mon, 11 Mar 2024 15:55:53 -0400
Message-ID: <CA+CK2bByuY2ADbar9s_kph_meaDdVSDh5YtXFZJKaoH506Y8hA@mail.gmail.com>
Subject: Re: [RFC 10/14] fork: Dynamic Kernel Stacks
To: Randy Dunlap <rdunlap@infradead.org>
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On Mon, Mar 11, 2024 at 3:32=E2=80=AFPM Randy Dunlap <rdunlap@infradead.org=
> 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@soleen.com>
> > ---
> >  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_VMA=
LLOC
> >         must be enabled.
> >
> > +config HAVE_ARCH_DYNAMIC_STACK
> > +     def_bool n
> > +     help
> > +       An arch should select this symbol if it can support kernel stac=
ks
> > +       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 s=
ame
>
>                                                             handled in th=
e same
>
> > +         in the consitent and predictable way. I.e. the exception shou=
ld be
>
>             consistent
>
> > +         handled the same as when stack overflow occurs when guard pag=
es 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 l=
ot 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 on=
e page
>
>                                                      does
>
> > +       the memory saving can be subsentantial. The feature requires vi=
rtually
>
>                                    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