From fa70eaa852aab38857b3c8663386e8759f121bbe Mon Sep 17 00:00:00 2001
Message-Id: <fa70eaa852aab38857b3c8663386e8759f121bbe.1566904946.git.pmatilai@redhat.com>
From: Panu Matilainen <pmatilai@redhat.com>
Date: Fri, 23 Aug 2019 11:17:21 +0300
Subject: [PATCH] Cap number of threads on 32bit platforms, add tunables
 (RhBug:1729382)

On 32bit plaforms, address space is easily exhausted with multiple
threads, causing arbitrary builds failure regressions (RhBug:1729382).
Simply cap the number of threads to maximum of four on any 32bit
platform to play it safe. It's still three more than we were able to
use on older releases...

In addition, introduce tunables for controlling the number of threads
used similarly to what is available for number of CPUs: forced number
and a separate ceiling.

The max logic gets surprisingly tricky with the mixed OMP and rpm
tunables and different semantics. Notably, OMP does not have a nice
way to set a maximum from inside a program (this is only possible
from an environment variable so we can't use it here), so we need
to jump through a lot of hoops to figure out whether an *unlimited*
setting would go above the platform limitation. I'm not entirely
convinced this is 100% bulletproof, but then it's all going to change
and become even more complicated when we start taking memory limitations
into account...
---
 build/parseSpec.c | 20 +++++++++++++++++---
 platform.in       |  5 +++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/build/parseSpec.c b/build/parseSpec.c
index 737a1233c..ed5c3ef63 100644
--- a/build/parseSpec.c
+++ b/build/parseSpec.c
@@ -1035,9 +1035,23 @@ static rpmSpec parseSpec(const char *specFile, rpmSpecFlags flags,
 
 #ifdef ENABLE_OPENMP
     /* Set number of OMP threads centrally */
-    int ncpus = rpmExpandNumeric("%{?_smp_build_ncpus}");
-    if (ncpus > 0)
-	omp_set_num_threads(ncpus);
+    int nthreads = rpmExpandNumeric("%{?_smp_build_nthreads}");
+    int nthreads_max = rpmExpandNumeric("%{?_smp_nthreads_max}");
+    if (nthreads <= 0)
+        nthreads = omp_get_max_threads();
+    if (nthreads_max > 0 && nthreads > nthreads_max)
+	nthreads = nthreads_max;
+#if __WORDSIZE == 32
+    /* On 32bit platforms, address space shortage is an issue. Play safe. */
+    int platlimit = 4;
+    if (nthreads > platlimit) {
+	nthreads = platlimit;
+	rpmlog(RPMLOG_DEBUG,
+	    "limiting number of threads to %d due to platform\n", platlimit);
+    }
+#endif
+    if (nthreads > 0)
+	omp_set_num_threads(nthreads);
 #endif
 
     if (spec->clean == NULL) {
diff --git a/platform.in b/platform.in
index db6d2382f..61f5e18a0 100644
--- a/platform.in
+++ b/platform.in
@@ -59,6 +59,11 @@
 
 %_smp_mflags -j%{_smp_build_ncpus}
 
+# Maximum number of threads to use when building, 0 for unlimited
+#%_smp_nthreads_max 0
+
+%_smp_build_nthreads %{_smp_build_ncpus}
+
 #==============================================================================
 # ---- Build policy macros.
 #
-- 
2.21.0