[PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Fangrui Song]

Original author is Ambrose Feinstein while working at Google.

_dl_map_object iterates over loaded objects and calls _dl_name_match_p.
If l->l_soname_added is 0, we incur two costs.

First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
every library's string table is in a different page.  The cost will be
avoided if the string is on the heap.

Second, add_name_to_object repeats the l_libname comparison already done
by the _dl_name_match_p call.

To remove these costs, we eagerly add the SONAME to the libname_list.
l_soname_added is typically 1, so laziness doesn't provide savings.
---
 elf/dl-load.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/elf/dl-load.c b/elf/dl-load.c
index 9a0e40c0e9..1b17410ce0 100644
--- a/elf/dl-load.c
+++ b/elf/dl-load.c
@@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
 
   /* When we profile the SONAME might be needed for something else but
      loading.  Add it right away.  */
-  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
-      && l->l_info[DT_SONAME] != NULL)
+  if (l->l_info[DT_SONAME] != NULL) {
     add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
 			    + l->l_info[DT_SONAME]->d_un.d_val));
+    l->l_soname_added = 1;
+  }
 
   /* If we have newly loaded libc.so, update the namespace
      description.  */
-- 
2.40.1.495.gc816e09b53d-goog

Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Florian Weimer]

* Fangrui Song via Libc-alpha:

> Original author is Ambrose Feinstein while working at Google.
>
> _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
> If l->l_soname_added is 0, we incur two costs.
>
> First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
> every library's string table is in a different page.  The cost will be
> avoided if the string is on the heap.
>
> Second, add_name_to_object repeats the l_libname comparison already done
> by the _dl_name_match_p call.
>
> To remove these costs, we eagerly add the SONAME to the libname_list.
> l_soname_added is typically 1, so laziness doesn't provide savings.
> ---
>  elf/dl-load.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index 9a0e40c0e9..1b17410ce0 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
>  
>    /* When we profile the SONAME might be needed for something else but
>       loading.  Add it right away.  */
> -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
> -      && l->l_info[DT_SONAME] != NULL)
> +  if (l->l_info[DT_SONAME] != NULL) {
>      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
>  			    + l->l_info[DT_SONAME]->d_un.d_val));
> +    l->l_soname_added = 1;
> +  }
>  
>    /* If we have newly loaded libc.so, update the namespace
>       description.  */

The comment is now outdated.

Not sure if this kind of micro-optimization makes sense.  Maybe until we
add a hash table here …

Thanks,
Florian

Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Fangrui Song]

On Fri, Apr 28, 2023 at 1:42 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Fangrui Song via Libc-alpha:
>
> > Original author is Ambrose Feinstein while working at Google.
> >
> > _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
> > If l->l_soname_added is 0, we incur two costs.
> >
> > First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
> > every library's string table is in a different page.  The cost will be
> > avoided if the string is on the heap.
> >
> > Second, add_name_to_object repeats the l_libname comparison already done
> > by the _dl_name_match_p call.
> >
> > To remove these costs, we eagerly add the SONAME to the libname_list.
> > l_soname_added is typically 1, so laziness doesn't provide savings.
> > ---
> >  elf/dl-load.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/elf/dl-load.c b/elf/dl-load.c
> > index 9a0e40c0e9..1b17410ce0 100644
> > --- a/elf/dl-load.c
> > +++ b/elf/dl-load.c
> > @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
> >
> >    /* When we profile the SONAME might be needed for something else but
> >       loading.  Add it right away.  */
> > -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
> > -      && l->l_info[DT_SONAME] != NULL)
> > +  if (l->l_info[DT_SONAME] != NULL) {
> >      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
> >                           + l->l_info[DT_SONAME]->d_un.d_val));
> > +    l->l_soname_added = 1;
> > +  }
> >
> >    /* If we have newly loaded libc.so, update the namespace
> >       description.  */
>
> The comment is now outdated.
>
> Not sure if this kind of micro-optimization makes sense.  Maybe until we
> add a hash table here …
>
> Thanks,
> Florian
>

Perhaps just remove the comment?

I think there is some merit replacing a tricky condition
__glibc_unlikely (GLRO(dl_profile) != NULL  to a simple one `
l->l_soname_added = 1;`.

This optimization matters for an executable with O(1000) shared object
dependencies.
The quadratic time DSO comparison multiplied by the strcmp dominates
the load time. This patch removes one bottleneck.

Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Florian Weimer]

* Florian Weimer:

> * Fangrui Song via Libc-alpha:
>
>> Original author is Ambrose Feinstein while working at Google.
>>
>> _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
>> If l->l_soname_added is 0, we incur two costs.
>>
>> First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
>> every library's string table is in a different page.  The cost will be
>> avoided if the string is on the heap.
>>
>> Second, add_name_to_object repeats the l_libname comparison already done
>> by the _dl_name_match_p call.
>>
>> To remove these costs, we eagerly add the SONAME to the libname_list.
>> l_soname_added is typically 1, so laziness doesn't provide savings.
>> ---
>>  elf/dl-load.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/elf/dl-load.c b/elf/dl-load.c
>> index 9a0e40c0e9..1b17410ce0 100644
>> --- a/elf/dl-load.c
>> +++ b/elf/dl-load.c
>> @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
>>  
>>    /* When we profile the SONAME might be needed for something else but
>>       loading.  Add it right away.  */
>> -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
>> -      && l->l_info[DT_SONAME] != NULL)
>> +  if (l->l_info[DT_SONAME] != NULL) {
>>      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
>>  			    + l->l_info[DT_SONAME]->d_un.d_val));
>> +    l->l_soname_added = 1;
>> +  }
>>  
>>    /* If we have newly loaded libc.so, update the namespace
>>       description.  */
>
> The comment is now outdated.
>
> Not sure if this kind of micro-optimization makes sense.  Maybe until we
> add a hash table here …

The hash table is implemented here:

  [PATCH 32/33] elf: Add hash tables to speed up DT_NEEDED, dlopen lookups
  <https://sourceware.org/pipermail/libc-alpha/2023-July/149669.html>

Thanks,
Florian

Re: [PATCH] elf: Add the soname to the libname_list eagerly on loading a library

[Fangrui Song]

On Tue, Jul 4, 2023 at 11:17 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Florian Weimer:
>
> > * Fangrui Song via Libc-alpha:
> >
> >> Original author is Ambrose Feinstein while working at Google.
> >>
> >> _dl_map_object iterates over loaded objects and calls _dl_name_match_p.
> >> If l->l_soname_added is 0, we incur two costs.
> >>
> >> First, loading l->l_info[DT_SONAME]->d_un.d_val has TLB pressure as
> >> every library's string table is in a different page.  The cost will be
> >> avoided if the string is on the heap.
> >>
> >> Second, add_name_to_object repeats the l_libname comparison already done
> >> by the _dl_name_match_p call.
> >>
> >> To remove these costs, we eagerly add the SONAME to the libname_list.
> >> l_soname_added is typically 1, so laziness doesn't provide savings.
> >> ---
> >>  elf/dl-load.c | 5 +++--
> >>  1 file changed, 3 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/elf/dl-load.c b/elf/dl-load.c
> >> index 9a0e40c0e9..1b17410ce0 100644
> >> --- a/elf/dl-load.c
> >> +++ b/elf/dl-load.c
> >> @@ -1451,10 +1451,11 @@ cannot enable executable stack as shared object requires");
> >>
> >>    /* When we profile the SONAME might be needed for something else but
> >>       loading.  Add it right away.  */
> >> -  if (__glibc_unlikely (GLRO(dl_profile) != NULL)
> >> -      && l->l_info[DT_SONAME] != NULL)
> >> +  if (l->l_info[DT_SONAME] != NULL) {
> >>      add_name_to_object (l, ((const char *) D_PTR (l, l_info[DT_STRTAB])
> >>                          + l->l_info[DT_SONAME]->d_un.d_val));
> >> +    l->l_soname_added = 1;
> >> +  }
> >>
> >>    /* If we have newly loaded libc.so, update the namespace
> >>       description.  */
> >
> > The comment is now outdated.
> >
> > Not sure if this kind of micro-optimization makes sense.  Maybe until we
> > add a hash table here …
>
> The hash table is implemented here:
>
>   [PATCH 32/33] elf: Add hash tables to speed up DT_NEEDED, dlopen lookups
>   <https://sourceware.org/pipermail/libc-alpha/2023-July/149669.html>
>
> Thanks,
> Florian
>

Thank you for improving the dynamic loader performance and obsoleting
this patch! :)


-- 
宋方睿