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

[Namhyung Kim]

On Tue, Apr 12, 2022 at 4:48 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Tue, Apr 12, 2022 at 3:12 PM Namhyung Kim <namhyung@xxxxxxxxxx> 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@xxxxxxxxxx/
> > > 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.

I'm fine with the 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.

Yeah, usually modules are loaded after the kernel image but
it seems ARM could load them before the kernel.
So I made the change not to call strchr() again.

But we might need to consider the special "[__builtin_kprobes]"
symbols.

>
> > +                               prev->end = roundup(prev->end + 1, page_size);
>
> Currently the roundup varies per architecture, but it is not clear to
> me that it matters.

Yeah, it would be the same as the logic for the last entry to be
more conservative.

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

Sounds good.

Thanks,
Namhyung


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