Implement concurrent execution of print_debug_* functions during handling
of -w, --debug-dump using libthread.a.
A new `-C, --concurrency=NUM` command line option controls the maximum
number of threads that may be used. This value defaults to the number of CPUs.
Job output is buffered and printed in the order that jobs were added to
the queue. This helps preserve the existing order of stdout output.
* src/readelf.c (default_concurrency): Function estimating the
maximum number of threads.
(parse_opt): Handle -C, --concurrency=NUM.
(do_job): Entry point function for threads.
(schedule_job): If thread safety is enabled, add job to the
job queue. Otherwise just run the job from the main thread.
(print_debug): Pass print_debug_* function pointers and
args to schedule_job. Also call run_jobs if thread safety
is enabled.
Signed-off-by: Aaron Merey <ame...@redhat.com>
---
v4: allow num_thread jobs instead of num_thread - 1
heap alloc job_data
Use max_threads instead of max_threads - 1 for determining the maximum
number of jobs running concurrently.
Improve error output when the argument to -C is invalid.
Allocate the job_data on the heap instead of the stack to avoid stack overflow
if there are a very large number of debug sections.
> On Tue, May 27, 2025 at 10:04:20AM -0400, Aaron Merey wrote:
> > Implement concurrent execution of print_debug_* functions during handling
> > of -w, --debug-dump using libthread.a.
>
> Do you intend to also enable this for other sections?
Yes I will extend parallelization to non-debug sections in future
patches.
> > +/* Estimate the maximum number of threads. This is normally
> > + #CPU. Return value is guaranteed to be at least 1. */
> > +static int
> > +default_concurrency (void)
> > +{
> > + unsigned aff = 0;
> > +#ifdef HAVE_SCHED_GETAFFINITY
> > + {
> > + int ret;
> > + cpu_set_t mask;
> > + CPU_ZERO (&mask);
> > + ret = sched_getaffinity (0, sizeof(mask), &mask);
> > + if (ret == 0)
> > + aff = CPU_COUNT (&mask);
> > + }
> > +#endif
>
> Is this better/worse than using get_nprocs or sysconf
> (_SC_NPROCESSORS_ONLN)?
> https://www.gnu.org/software/libc/manual/html_node/Processor-Resources.html
>
> Also from the sched_getaffinity man page it looks like cpu_set_t might
> be too small and the call might fail on systems with > 1024
> cores. Maybe this is a non-issue though?
>
> > + unsigned fn = 0;
> > +#ifdef HAVE_GETRLIMIT
> > + {
> > + struct rlimit rlim;
> > + int rc = getrlimit (RLIMIT_NOFILE, &rlim);
> > + if (rc == 0)
> > + fn = MAX ((rlim_t) 1, (rlim.rlim_cur - 100) / 2);
> > + /* Conservatively estimate that at least 2 fds are used
> > + by each thread. */
> > + }
> > +#endif
>
> Interesting. I wouldn't have thought about that. But makes sense. If
> you check available file descriptors, would it also make sense to
> check for available memory? And limit the number of jobs to mem/1024M?
This function is copied from debuginfod.cxx where we haven't run into
issues with >1024 cores or the number of jobs exceeding mem/1024M.
> > /* To store the name used in compare_listptr */
> > -static const char *sort_listptr_name;
> > +_Thread_local const char *sort_listptr_name;
>
> O, that is interesting and not in your ChangeLog.
> Is this the simplest way to make this thread-safe?
> Could there be a lock around sort_listptr ?
> What about the static listptr_table known_xxxx[ptr|bases]?
> Should building up/accessing those be made thread-safe?
In the current implementation print_debug_info_section, which
populates the listptr_tables, runs on its own before add_jobs
and run_jobs are called. After that each listptr_table is used
only within one print_debug_* function, so there isn't a risk
of race conditions at the moment.
When we extend eu-readelf paralellization in future patches
print_debug_info_section will run in paralell with other
print_debug_* functions and the listptr_tables will be protected
by locks.
src/readelf.c | 164 ++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 160 insertions(+), 4 deletions(-)
diff --git a/src/readelf.c b/src/readelf.c
index efb445ed..11082830 100644
--- a/src/readelf.c
+++ b/src/readelf.c
@@ -57,6 +57,18 @@
#include "../libdw/known-dwarf.h"
+#ifdef USE_LOCKS
+#include "threadlib.h"
+#endif
+
+#ifdef HAVE_SCHED_H
+#include <sched.h>
+#endif
+
+#ifdef HAVE_SYS_RESOURCE_H
+#include <sys/resource.h>
+#endif
+
#ifdef __linux__
#define CORE_SIGILL SIGILL
#define CORE_SIGBUS SIGBUS
@@ -150,6 +162,10 @@ static const struct argp_option options[] =
N_("Ignored for compatibility (lines always wide)"), 0 },
{ "decompress", 'z', NULL, 0,
N_("Show compression information for compressed sections (when used with
-S); decompress section before dumping data (when used with -p or -x)"), 0 },
+#ifdef USE_LOCKS
+ { "concurrency", 'C', "NUM", 0,
+ N_("Set maximum number of threads. Defaults to the number of CPUs."), 0 },
+#endif
{ NULL, 0, NULL, 0, NULL, 0 }
};
@@ -249,6 +265,11 @@ static bool print_decompress = false;
/* True if we want to show split compile units for debug_info skeletons. */
static bool show_split_units = false;
+#if USE_LOCKS
+/* Maximum number of threads. */
+static int max_threads = 0;
+#endif
+
/* Select printing of debugging sections. */
static enum section_e
{
@@ -380,6 +401,43 @@ cleanup_list (struct section_argument *list)
}
}
+#ifdef USE_LOCKS
+/* Estimate the maximum number of threads. This is normally
+ #CPU. Return value is guaranteed to be at least 1. */
+static int
+default_concurrency (void)
+{
+ unsigned aff = 0;
+#ifdef HAVE_SCHED_GETAFFINITY
+ {
+ int ret;
+ cpu_set_t mask;
+ CPU_ZERO (&mask);
+ ret = sched_getaffinity (0, sizeof(mask), &mask);
+ if (ret == 0)
+ aff = CPU_COUNT (&mask);
+ }
+#endif
+
+ unsigned fn = 0;
+#ifdef HAVE_GETRLIMIT
+ {
+ struct rlimit rlim;
+ int rc = getrlimit (RLIMIT_NOFILE, &rlim);
+ if (rc == 0)
+ fn = MAX ((rlim_t) 1, (rlim.rlim_cur - 100) / 2);
+ /* Conservatively estimate that at least 2 fds are used
+ by each thread. */
+ }
+#endif
+
+ unsigned d = MIN (MAX (aff, 2U),
+ MAX (fn, 2U));
+
+ return d;
+}
+#endif
+
int
main (int argc, char *argv[])
{
@@ -403,6 +461,12 @@ main (int argc, char *argv[])
/* Before we start tell the ELF library which version we are using. */
elf_version (EV_CURRENT);
+#ifdef USE_LOCKS
+ /* If concurrency wasn't set by argp_parse, then set a default value. */
+ if (max_threads == 0)
+ max_threads = default_concurrency ();
+#endif
+
/* Now process all the files given at the command line. */
bool only_one = remaining + 1 == argc;
do
@@ -527,6 +591,19 @@ parse_opt (int key, char *arg,
case 'c':
print_archive_index = true;
break;
+#if USE_LOCKS
+ case 'C':
+ if (arg != NULL)
+ {
+ max_threads = atoi (arg);
+ if (max_threads < 1)
+ {
+ argp_error (state, _("-C NUM minimum 1"));
+ return EINVAL;
+ }
+ }
+ break;
+#endif
case 'w':
if (arg == NULL)
{
@@ -5496,7 +5573,7 @@ listptr_base (struct listptr *p)
}
/* To store the name used in compare_listptr */
-static const char *sort_listptr_name;
+_Thread_local const char *sort_listptr_name;
static int
compare_listptr (const void *a, const void *b)
@@ -11997,6 +12074,65 @@ getone_dwflmod (Dwfl_Module *dwflmod,
return DWARF_CB_OK;
}
+typedef struct {
+ Dwfl_Module *dwflmod;
+ Ebl *ebl;
+ GElf_Ehdr *ehdr;
+ Elf_Scn scn;
+ GElf_Shdr shdr;
+ Dwarf *dbg;
+ FILE *out;
+ void (*fp) (Dwfl_Module *, Ebl *, GElf_Ehdr *,
+ Elf_Scn *, GElf_Shdr *, Dwarf *, FILE *);
+} job_data;
+
+#ifdef USE_LOCKS
+
+/* Thread entry point. */
+static void *
+do_job (void *data, FILE *out)
+{
+ job_data *d = (job_data *) data;
+ d->fp (d->dwflmod, d->ebl, d->ehdr, &d->scn, &d->shdr, d->dbg, out);
+ return NULL;
+}
+#endif
+
+/* If readelf is built with thread safety, then set up JDATA at index IDX
+ and add it to the job queue.
+
+ If thread safety is not supported or the maximum number of threads is set
+ to 1, then immediately call START_ROUTINE with the given arguments. */
+static void
+schedule_job (job_data jdata[], size_t idx,
+ void (*start_routine) (Dwfl_Module *, Ebl *, GElf_Ehdr *,
+ Elf_Scn *, GElf_Shdr *, Dwarf *, FILE *),
+ Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr, Elf_Scn *scn,
+ GElf_Shdr *shdr, Dwarf *dbg)
+{
+#ifdef USE_LOCKS
+ if (max_threads > 1)
+ {
+ /* Add to the job queue. */
+ jdata[idx].dwflmod = dwflmod;
+ jdata[idx].ebl = ebl;
+ jdata[idx].ehdr = ehdr;
+ jdata[idx].scn = *scn;
+ jdata[idx].shdr = *shdr;
+ jdata[idx].dbg = dbg;
+ jdata[idx].fp = start_routine;
+
+ add_job (do_job, (void *) &jdata[idx]);
+ }
+ else
+ start_routine (dwflmod, ebl, ehdr, scn, shdr, dbg, stdout);
+#else
+ (void) jdata; (void) idx;
+
+ start_routine (dwflmod, ebl, ehdr, scn, shdr, dbg, stdout);
+#endif
+}
+
static void
print_debug (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr *ehdr)
{
@@ -12169,6 +12305,9 @@ print_debug (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr
*ehdr)
if (unlikely (elf_getshdrstrndx (ebl->elf, &shstrndx) < 0))
error_exit (0, _("cannot get section header string table index"));
+ size_t num_jobs = 0;
+ job_data *jdata = NULL;
+
/* If the .debug_info section is listed as implicitly required then
we must make sure to handle it before handling any other debug
section. Various other sections depend on the CU DIEs being
@@ -12266,6 +12405,13 @@ print_debug (Dwfl_Module *dwflmod, Ebl *ebl, GElf_Ehdr
*ehdr)
if (name == NULL)
continue;
+ if (jdata == NULL)
+ {
+ jdata = calloc (ndebug_sections, sizeof (*jdata));
+ if (jdata == NULL)
+ error_exit (0, _("failed to allocate job data"));
+ }
+
int n;
for (n = 0; n < ndebug_sections; ++n)
{
@@ -12287,17 +12433,27 @@ print_debug (Dwfl_Module *dwflmod, Ebl *ebl,
GElf_Ehdr *ehdr)
&& strcmp (&name[14], debug_sections[n].name) == 0)
)
{
- if ((print_debug_sections | implicit_debug_sections)
- & debug_sections[n].bitmask)
- debug_sections[n].fp (dwflmod, ebl, ehdr, scn, shdr, dbg,
stdout);
+ if (((print_debug_sections | implicit_debug_sections)
+ & debug_sections[n].bitmask))
+ schedule_job (jdata, num_jobs++, debug_sections[n].fp,
+ dwflmod, ebl, ehdr, scn, shdr, dbg);
+
+ assert (num_jobs <= ndebug_sections);
break;
}
}
}
}
+#ifdef USE_LOCKS
+ /* If max_threads <= 1, then jobs were immediately run in schedule_job. */
+ if (max_threads > 1)
+ run_jobs (max_threads);
+#endif
+
dwfl_end (skel_dwfl);
free (skel_name);
+ free (jdata);
/* Turn implicit and/or explicit back on in case we go over another file. */
if (implicit_info)
--
2.49.0
