Re: [PATCH v4] vmstat: Kernel stack usage histogram

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Jul 18, 2024 at 10:55:17PM GMT, Pasha Tatashin wrote:
> On Thu, Jul 18, 2024 at 7:36 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Thu, Jul 18, 2024 at 08:26:11PM GMT, Pasha Tatashin wrote:
> > [...]
> > > diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
> > > index ccd72b978e1f..65e8c9fb7f9b 100644
> > > --- a/include/linux/sched/task_stack.h
> > > +++ b/include/linux/sched/task_stack.h
> > > @@ -95,9 +95,51 @@ static inline int object_is_on_stack(const void *obj)
> > >  extern void thread_stack_cache_init(void);
> > >
> > >  #ifdef CONFIG_DEBUG_STACK_USAGE
> > > +#ifdef CONFIG_VM_EVENT_COUNTERS
> > > +#include <linux/vm_event_item.h>
> > > +
> > > +/* Count the maximum pages reached in kernel stacks */
> > > +static inline void kstack_histogram(unsigned long used_stack)
> >
> > Any specific reason to add this function in header?
> 
> For performance reasons to keep it inlined into stack_not_used() which
> is also defined as inline function in this header.
> 

Is this really that performance critical?

> >
> > > +{
> > > +     if (used_stack <= 1024)
> > > +             this_cpu_inc(vm_event_states.event[KSTACK_1K]);
> >
> > Why not count_vm_event(KSTACK_1K)? Avoiding header include recursion?
> 
> I could not include "linux/vmstat.h" into "linux/sched/task_stack.h"
> because it introduces some dependencies such linux/mm.h and
> linux/fs.h, uapi/linux/stat.h, and when all of those are added it
> still fails to compile on some architectures, so it was just simpler
> to stop resolving the conflicts and use this_cpu_inc() directly.
> 

The above makes me think it is better to move stack_not_used() and the
new function to C file unless we can show the negative performance
impact.

I have another question. At the moment, the metrics are exposed
conditionally through procfs based on stack size. So, based on the
kernel config someone may not see kstack_16k. Why not just output all of
these metrics irrespective of the config?

Shakeel