Re: [PATCH v2 0/4] Tidy up symbol end fixup

[Ian Rogers <irogers@google.com>]

On Tue, Apr 12, 2022 at 3:12 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> Hi Ian,
>
> On Tue, Apr 12, 2022 at 08:48:13AM -0700, Ian Rogers wrote:
> > Fixing up more symbol ends as introduced in:
> > https://lore.kernel.org/lkml/20220317135536.805-1-mpetlan@redhat.com/
> > caused perf annotate to run into memory limits - every symbol holds
> > all the disassembled code in the annotation, and so making symbols
> > ends further away dramatically increased memory usage (40MB to
> >  >1GB). Modify the symbol end logic so that special kernel cases aren't
> > applied in the common case.
> >
> > v2. Drops a merged patch. Fixes a build issue with libbfd enabled.
>
> How about just like this?  We can get rid of arch functions as they
> mostly do the same thing (kernel vs module boundary check).
>
> Not tested. ;-)
>
> Thanks,
> Namhyung
>
> --------------8<-------------
>
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index dea0fc495185..df41d7266d91 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -35,6 +35,7 @@
>  #include "path.h"
>  #include <linux/ctype.h>
>  #include <linux/zalloc.h>
> +#include <internal/lib.h>  // page_size
>
>  #include <elf.h>
>  #include <limits.h>
> @@ -231,8 +226,16 @@ void symbols__fixup_end(struct rb_root_cached *symbols)
>                 prev = curr;
>                 curr = rb_entry(nd, struct symbol, rb_node);
>
> -               if (prev->end == prev->start || prev->end != curr->start)
> -                       arch__symbols__fixup_end(prev, curr);
> +               if (prev->end == prev->start) {
> +                       /* Last kernel symbol mapped to end of page */

I like the simpler logic but don't like applying this in symbol-elf
given the comment says it is kernel specific - so we could keep the
is_kernel change.

> +                       if (!strchr(prev->name, '[') != !strchr(curr->name, '['))

I find this condition not to be intention revealing. On ARM there is
also an || for the condition reversed. When this is in an is_kernel
block then I think it is clear this is kernel hack, so I think it is
good to comment on what the condition is for.

> +                               prev->end = roundup(prev->end + 1, page_size);

Currently the roundup varies per architecture, but it is not clear to
me that it matters.

> +                       else

I think we should comment here that we're extending zero sized symbols
to the start of the next symbol.

> +                               prev->end = curr->start;
> +
> +                       pr_debug4("%s sym:%s end:%#" PRIx64 "\n",
> +                                 __func__, prev->name, prev->end);
> +               }

Thanks,
Ian

>         }
>
>         /* Last entry */