alsa: disable timer-based scheduling inside a VM

In virtual machines sound card clocks and OS scheduling tend to become
unreliable, adding various 'uneven' latencies. The adaptive algorithm
that handles drop-outs does not handle it this well: in contrast to
drop-outs on real machines that are evenly distributed, small and can
easily be encountered via the adpative algorithms, drop-outs in VMs tend
to happen abruptly, and massively, which is not easy to counter.

This patch simply disables timer based scheduling in VMs reverting to
classic IO based scheduling. This should help make PA perform better in
VMs.

https://bugzilla.redhat.com/show_bug.cgi?id=532775

src/modules/alsa/alsa-sink.c

@@ -1698,10 +1698,7 @@ pa_sink *pa_alsa_sink_new(pa_module *m, pa_modargs *ma, const char*driver, pa_ca
         goto fail;
     }
 
-    if (use_tsched && !pa_rtclock_hrtimer()) {
-        pa_log_notice("Disabling timer-based scheduling because high-resolution timers are not available from the kernel.");
-        use_tsched = FALSE;
-    }
+    use_tsched = pa_alsa_may_tsched(use_tsched);
 
     u = pa_xnew0(struct userdata, 1);
     u->core = m->core;

src/modules/alsa/alsa-source.c

@@ -1541,10 +1541,7 @@ pa_source *pa_alsa_source_new(pa_module *m, pa_modargs *ma, const char*driver, p
         goto fail;
     }
 
-    if (use_tsched && !pa_rtclock_hrtimer()) {
-        pa_log_notice("Disabling timer-based scheduling because high-resolution timers are not available from the kernel.");
-        use_tsched = FALSE;
-    }
+    use_tsched = pa_alsa_may_tsched(use_tsched);
 
     u = pa_xnew0(struct userdata, 1);
     u->core = m->core;

src/modules/alsa/alsa-util.c

@@ -43,6 +43,7 @@
 #include <pulsecore/once.h>
 #include <pulsecore/thread.h>
 #include <pulsecore/conf-parser.h>
+#include <pulsecore/core-rtclock.h>
 
 #include "alsa-util.h"
 #include "alsa-mixer.h"
@@ -1308,3 +1309,26 @@ const char* pa_alsa_strerror(int errnum) {
 
     return translated;
 }
+
+pa_bool_t pa_alsa_may_tsched(pa_bool_t want) {
+
+    if (!want)
+        return FALSE;
+
+    if (!pa_rtclock_hrtimer()) {
+        /* We cannot depend on being woken up in time when the timers
+        are inaccurate, so let's fallback to classic IO based playback
+        then. */
+        pa_log_notice("Disabling timer-based scheduling because high-resolution timers are not available from the kernel.");
+        return FALSE; }
+
+    if (pa_running_in_vm()) {
+        /* We cannot depend on being woken up when we ask for in a VM,
+         * so let's fallback to classic IO based playback then. */
+        pa_log_notice("Disabling timer-based scheduling because running inside a VM.");
+        return FALSE;
+    }
+
+
+    return TRUE;
+}

src/modules/alsa/alsa-util.h

@@ -142,4 +142,6 @@ pa_bool_t pa_alsa_pcm_is_modem(snd_pcm_t *pcm);
 
 const char* pa_alsa_strerror(int errnum);
 
+pa_bool_t pa_alsa_may_tsched(pa_bool_t want);
+
 #endif