util: Introduce qemu_thread_set_affinity() and qemu_thread_get_affinity()

Usually, we let upper layers handle CPU pinning, because
pthread_setaffinity_np() (-> sched_setaffinity()) is blocked via
seccomp when starting QEMU with
    -sandbox enable=on,resourcecontrol=deny

However, we want to configure and observe the CPU affinity of threads
from QEMU directly in some cases when the sandbox option is either not
enabled or not active yet.

So let's add a way to configure CPU pinning via
qemu_thread_set_affinity() and obtain CPU affinity via
qemu_thread_get_affinity() and implement them under POSIX using
pthread_setaffinity_np() + pthread_getaffinity_np().

Implementation under Windows is possible using SetProcessAffinityMask()
+ GetProcessAffinityMask(), however, that is left as future work.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20221014134720.168738-3-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

diff --git a/include/qemu/thread.h b/include/qemu/thread.h
index af19f2b3fc7d418b1f0322a09052cb8b4b5d147d..79e507c7f0c91cfcba57dbd83012f74bf9b01072 100644 (file)
--- a/include/qemu/thread.h
+++ b/include/qemu/thread.h
@@ -185,6 +185,10 @@ void qemu_event_destroy(QemuEvent *ev);
 void qemu_thread_create(QemuThread *thread, const char *name,
                         void *(*start_routine)(void *),
                         void *arg, int mode);
+int qemu_thread_set_affinity(QemuThread *thread, unsigned long *host_cpus,
+                             unsigned long nbits);
+int qemu_thread_get_affinity(QemuThread *thread, unsigned long **host_cpus,
+                             unsigned long *nbits);
 void *qemu_thread_join(QemuThread *thread);
 void qemu_thread_get_self(QemuThread *thread);
 bool qemu_thread_is_self(QemuThread *thread);

diff --git a/meson.build b/meson.build
index b686dfef7567b0c691d1ba6a612b00ddda1d6ced..3e0aa4925d8c0be3d6077019f1cce6b5600624fa 100644 (file)
--- a/meson.build
+++ b/meson.build
@@ -2114,7 +2114,23 @@ config_host_data.set('CONFIG_PTHREAD_CONDATTR_SETCLOCK', cc.links(gnu_source_pre
     pthread_condattr_setclock(&attr, CLOCK_MONOTONIC);
     return 0;
   }''', dependencies: threads))
+config_host_data.set('CONFIG_PTHREAD_AFFINITY_NP', cc.links(gnu_source_prefix + '''
+  #include <pthread.h>
 
+  static void *f(void *p) { return NULL; }
+  int main(void)
+  {
+    int setsize = CPU_ALLOC_SIZE(64);
+    pthread_t thread;
+    cpu_set_t *cpuset;
+    pthread_create(&thread, 0, f, 0);
+    cpuset = CPU_ALLOC(64);
+    CPU_ZERO_S(setsize, cpuset);
+    pthread_setaffinity_np(thread, setsize, cpuset);
+    pthread_getaffinity_np(thread, setsize, cpuset);
+    CPU_FREE(cpuset);
+    return 0;
+  }''', dependencies: threads))
 config_host_data.set('CONFIG_SIGNALFD', cc.links(gnu_source_prefix + '''
   #include <sys/signalfd.h>
   #include <stddef.h>

diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
index ac1d56e673f37e45cb870ec5cc6c182d68ec24ea..bae938c670845f5fa5c43edbe46f824bc335a19f 100644 (file)
--- a/util/qemu-thread-posix.c
+++ b/util/qemu-thread-posix.c
@@ -16,6 +16,7 @@
 #include "qemu/notify.h"
 #include "qemu-thread-common.h"
 #include "qemu/tsan.h"
+#include "qemu/bitmap.h"
 
 static bool name_threads;
 
@@ -552,6 +553,75 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     pthread_attr_destroy(&attr);
 }
 
+int qemu_thread_set_affinity(QemuThread *thread, unsigned long *host_cpus,
+                             unsigned long nbits)
+{
+#if defined(CONFIG_PTHREAD_AFFINITY_NP)
+    const size_t setsize = CPU_ALLOC_SIZE(nbits);
+    unsigned long value;
+    cpu_set_t *cpuset;
+    int err;
+
+    cpuset = CPU_ALLOC(nbits);
+    g_assert(cpuset);
+
+    CPU_ZERO_S(setsize, cpuset);
+    value = find_first_bit(host_cpus, nbits);
+    while (value < nbits) {
+        CPU_SET_S(value, setsize, cpuset);
+        value = find_next_bit(host_cpus, nbits, value + 1);
+    }
+
+    err = pthread_setaffinity_np(thread->thread, setsize, cpuset);
+    CPU_FREE(cpuset);
+    return err;
+#else
+    return -ENOSYS;
+#endif
+}
+
+int qemu_thread_get_affinity(QemuThread *thread, unsigned long **host_cpus,
+                             unsigned long *nbits)
+{
+#if defined(CONFIG_PTHREAD_AFFINITY_NP)
+    unsigned long tmpbits;
+    cpu_set_t *cpuset;
+    size_t setsize;
+    int i, err;
+
+    tmpbits = CPU_SETSIZE;
+    while (true) {
+        setsize = CPU_ALLOC_SIZE(tmpbits);
+        cpuset = CPU_ALLOC(tmpbits);
+        g_assert(cpuset);
+
+        err = pthread_getaffinity_np(thread->thread, setsize, cpuset);
+        if (err) {
+            CPU_FREE(cpuset);
+            if (err != -EINVAL) {
+                return err;
+            }
+            tmpbits *= 2;
+        } else {
+            break;
+        }
+    }
+
+    /* Convert the result into a proper bitmap. */
+    *nbits = tmpbits;
+    *host_cpus = bitmap_new(tmpbits);
+    for (i = 0; i < tmpbits; i++) {
+        if (CPU_ISSET(i, cpuset)) {
+            set_bit(i, *host_cpus);
+        }
+    }
+    CPU_FREE(cpuset);
+    return 0;
+#else
+    return -ENOSYS;
+#endif
+}
+
 void qemu_thread_get_self(QemuThread *thread)
 {
     thread->thread = pthread_self();

diff --git a/util/qemu-thread-win32.c b/util/qemu-thread-win32.c
index b9a467d7db331630c66788004e5cf2f2fec2f2ca..69db254ac7c1835676a095d76efd7c70f6beb104 100644 (file)
--- a/util/qemu-thread-win32.c
+++ b/util/qemu-thread-win32.c
@@ -477,6 +477,18 @@ void qemu_thread_create(QemuThread *thread, const char *name,
     thread->data = data;
 }
 
+int qemu_thread_set_affinity(QemuThread *thread, unsigned long *host_cpus,
+                             unsigned long nbits)
+{
+    return -ENOSYS;
+}
+
+int qemu_thread_get_affinity(QemuThread *thread, unsigned long **host_cpus,
+                             unsigned long *nbits)
+{
+    return -ENOSYS;
+}
+
 void qemu_thread_get_self(QemuThread *thread)
 {
     thread->data = qemu_thread_data;