Re: [PATCH v2 1/1] pgo: Fix allocate_node() v2

[Nathan Chancellor]

On 6/3/2021 1:50 PM, Nick Desaulniers wrote:
> On Thu, Jun 3, 2021 at 6:41 AM Jarmo Tiitto <jarmo.tiitto@xxxxxxxxx> wrote:
> > Based on Kees and others feedback here is v2 patch
> > that clarifies why the current checks in allocate_node()
> > are flawed. I did fair amount of KGDB time on it.
>
> Kees can probably cut it when merging, but the above paragraph might
> be better "below the fold" below next time (doesn't necessitate a v3).
>
> > When clang instrumentation eventually calls allocate_node()
> > the struct llvm_prf_data *p argument tells us from what section
> > we should reserve the vnode: It either points into vmlinux's
> > core __llvm_prf_data section or some loaded module's
> > __llvm_prf_data section.
> >
> > But since we don't have access to corresponding
> > __llvm_prf_vnds section(s) for any module, the function
> > should return just NULL and ignore any profiling attempts
> > from modules for now.
> >
> > Signed-off-by: Jarmo Tiitto <jarmo.tiitto@xxxxxxxxx>
> > ---
>
> ^ ie. here. If you put text between the `---` and the diffstat, git
> just discards it when applying. It's a good way to hide commentary
> just meant for reviewers when sending a patch.
>
>
> >   kernel/pgo/instrument.c | 21 ++++++++++++---------
> >   1 file changed, 12 insertions(+), 9 deletions(-)
> >
> > diff --git a/kernel/pgo/instrument.c b/kernel/pgo/instrument.c
> > index 0e07ee1b17d9..afe9982b07a3 100644
> > --- a/kernel/pgo/instrument.c
> > +++ b/kernel/pgo/instrument.c
> > @@ -23,6 +23,7 @@
> >   #include <linux/export.h>
> >   #include <linux/spinlock.h>
> >   #include <linux/types.h>
> > +#include <asm-generic/sections.h>
> >   #include "pgo.h"
> >
> >   /*
> > @@ -55,17 +56,19 @@ void prf_unlock(unsigned long flags)
> >   static struct llvm_prf_value_node *allocate_node(struct llvm_prf_data *p,
> >                                                   u32 index, u64 value)
> >   {
> > -       if (&__llvm_prf_vnds_start[current_node + 1] >= __llvm_prf_vnds_end)
> > -               return NULL; /* Out of nodes */
> > -
> > -       current_node++;
> > -
> > -       /* Make sure the node is entirely within the section */
> > -       if (&__llvm_prf_vnds_start[current_node] >= __llvm_prf_vnds_end ||
> > -           &__llvm_prf_vnds_start[current_node + 1] > __llvm_prf_vnds_end)
> > +       const int max_vnds = prf_vnds_count();
>
> Sorry, where was prf_vnds_count() defined? I don't see it in
> linux-next, but I'm also not sure which tree has
> 5d0cda65918279ada060417c5fecb7e86ccb3def.

It is generated via the __DEFINE_PRF_SIZE macro in kernel/pgo/pgo.h.

> > +       /* Check that p is within vmlinux __llvm_prf_data section.
> > +        * If not, don't allocate since we can't handle modules yet.
> > +        */
> > +       if (!memory_contains(__llvm_prf_data_start,
> > +               __llvm_prf_data_end, p, sizeof(*p)))
> >                  return NULL;
> >
> > -       return &__llvm_prf_vnds_start[current_node];
> > +       if (WARN_ON_ONCE(current_node >= max_vnds))
> > +               return NULL; /* Out of nodes */
> > +
> > +       /* reserve vnode for vmlinux */
> > +       return &__llvm_prf_vnds_start[current_node++];
> >   }
> >
> >   /*
> >
> > base-commit: 5d0cda65918279ada060417c5fecb7e86ccb3def
> > --
> > 2.31.1
>
>
> --
> Thanks,
> ~Nick Desaulniers