util/oslib-posix: Don't create too many threads with small memory or little pages

Let's limit the number of threads to something sane, especially that
- We don't have more threads than the number of pages we have
- We don't have threads that initialize small (< 64 MiB) memory

Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20211217134611.31172-5-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

util/oslib-posix.c

@@ -40,6 +40,7 @@
 #include <libgen.h>
 #include "qemu/cutils.h"
 #include "qemu/compiler.h"
+#include "qemu/units.h"
 
 #ifdef CONFIG_LINUX
 #include <sys/syscall.h>
@@ -525,7 +526,8 @@ static void *do_madv_populate_write_pages(void *arg)
     return (void *)(uintptr_t)ret;
 }
 
-static inline int get_memset_num_threads(int smp_cpus)
+static inline int get_memset_num_threads(size_t hpagesize, size_t numpages,
+                                         int smp_cpus)
 {
     long host_procs = sysconf(_SC_NPROCESSORS_ONLN);
     int ret = 1;
@@ -533,6 +535,12 @@ static inline int get_memset_num_threads(int smp_cpus)
     if (host_procs > 0) {
         ret = MIN(MIN(host_procs, MAX_MEM_PREALLOC_THREAD_COUNT), smp_cpus);
     }
+
+    /* Especially with gigantic pages, don't create more threads than pages. */
+    ret = MIN(ret, numpages);
+    /* Don't start threads to prealloc comparatively little memory. */
+    ret = MIN(ret, MAX(1, hpagesize * numpages / (64 * MiB)));
+
     /* In case sysconf() fails, we fall back to single threaded */
     return ret;
 }
@@ -542,7 +550,7 @@ static int touch_all_pages(char *area, size_t hpagesize, size_t numpages,
 {
     static gsize initialized = 0;
     MemsetContext context = {
-        .num_threads = get_memset_num_threads(smp_cpus),
+        .num_threads = get_memset_num_threads(hpagesize, numpages, smp_cpus),
     };
     size_t numpages_per_thread, leftover;
     void *(*touch_fn)(void *);