Re: [PATCH][BZ #15533] Avoid unnecessary slowdown from profiling with audit

[OndÅej BÃlka <neleai at seznam dot cz>]

On Tue, Aug 20, 2013 at 04:47:56PM +0400, Alexander Monakov wrote:
> Ping.  URL to the original submission:
> http://sourceware.org/ml/libc-alpha/2013-05/msg00888.html
> 
> The patch still applies as posted and passes testing (on x86_64-linux).
>
Roland, could you review this one? I do not know linker enough to
quickly verify if assumptions in this patch are true.
 
> On Sat, 25 May 2013, Alexander Monakov wrote:
> 
> > Hello,
> > 
> > Presently, using the rtld-audit interfaces introduces a slowdown due to
> > enabling profiling instrumentation (as if LD_AUDIT implied LD_PROFILE).
> > However, instrumenting is only necessary if one of audit libraries provides
> > PLT hooks (la_plt{enter,exit} symbols).  Otherwise, the slowdown can be
> > avoided.
> > 
> > In the synthetic test attached to the bugzilla issue, the slowdown is 19x.  On
> > another application I tested, the slowdown from profiling was 10%.  The
> > current behavior strongly discourages using the audit interface when
> > compromising performance is not desired.
> > 
> > The following patch adjusts the logic that enables profiling to iterate over
> > all audit modules and check if any of those provides a PLT hook.
> > 
> > I have an FSF copyright assignment as a GCC contributor (but I do not have
> > commit access in glibc).
> > 
> > Thanks.
> > 
> > 
> > 2013-05-25  Alexander Monakov  <amonakov@ispras.ru>
> > 
> > 	* elf/dl-reloc.c (_dl_relocate_object): Enable profiling only if one
> > 	of audit libraries provides PLT hooks.
> > 
> > diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
> > index 73d98f8..675e15b 100644
> > --- a/elf/dl-reloc.c
> > +++ b/elf/dl-reloc.c
> > @@ -167,7 +167,16 @@ _dl_relocate_object (struct link_map *l, struct r_scope_elem *scope[],
> >  #ifdef SHARED
> >    /* If we are auditing, install the same handlers we need for profiling.  */
> >    if ((reloc_mode & __RTLD_AUDIT) == 0)
> > -    consider_profiling |= GLRO(dl_audit) != NULL;
> > +    {
> > +      struct audit_ifaces *afct = GLRO(dl_audit);
> > +      for (unsigned int cnt = 0; cnt < GLRO(dl_naudit); ++cnt)
> > +	{
> > +	  /* Profiling is needed only if PLT hooks are provided.  */
> > +	  if (afct->ARCH_LA_PLTENTER != NULL || afct->ARCH_LA_PLTEXIT != NULL)
> > +	    consider_profiling = 1;
> > +	  afct = afct->next;
> > +	}
> > +    }
> >  #elif defined PROF
> >    /* Never use dynamic linker profiling for gprof profiling code.  */
> >  # define consider_profiling 0
> >