commit cd758dd0acaaf1f76f9727d4409915f3293db07a
author Alex Bligh <alex@alex.org.uk> Wed Aug 21 16:02:44 2013 +0100
committer Stefan Hajnoczi <stefanha@redhat.com> Thu Aug 22 19:10:27 2013 +0200
tree a7d393d3cb44bc46c4a28acc491a5a53fba1bd59
parent 4e0c6529fcb5ccbed5eb2c4f094264eb447d49ea

aio / timers: Add prctl(PR_SET_TIMERSLACK, 1, ...) to reduce timer slack

Where supported, called prctl(PR_SET_TIMERSLACK, 1, ...) to
set one nanosecond timer slack to increase precision of timer
calls.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

configure

diff --git a/configure b/configure
index 5659412..0a55c20 100755
--- a/configure
+++ b/configure
@@ -2834,6 +2834,21 @@
   ppoll=yes
 fi
 
+# check for prctl(PR_SET_TIMERSLACK , ... ) support
+prctl_pr_set_timerslack=no
+cat > $TMPC << EOF
+#include <sys/prctl.h>
+
+int main(void)
+{
+    prctl(PR_SET_TIMERSLACK, 1, 0, 0, 0);
+    return 0;
+}
+EOF
+if compile_prog "" "" ; then
+  prctl_pr_set_timerslack=yes
+fi
+
 # check for epoll support
 epoll=no
 cat > $TMPC << EOF
@@ -3833,6 +3848,9 @@
 if test "$ppoll" = "yes" ; then
   echo "CONFIG_PPOLL=y" >> $config_host_mak
 fi
+if test "$prctl_pr_set_timerslack" = "yes" ; then
+  echo "CONFIG_PRCTL_PR_SET_TIMERSLACK=y" >> $config_host_mak
+fi
 if test "$epoll" = "yes" ; then
   echo "CONFIG_EPOLL=y" >> $config_host_mak
 fi

qemu-timer.c

diff --git a/qemu-timer.c b/qemu-timer.c
index 120d58f..74f904b 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -41,6 +41,10 @@
 #include <poll.h>
 #endif
 
+#ifdef CONFIG_PRCTL_PR_SET_TIMERSLACK
+#include <sys/prctl.h>
+#endif
+
 /***********************************************************/
 /* timers */
 
@@ -507,6 +511,9 @@
         vm_clock = qemu_clock_new(QEMU_CLOCK_VIRTUAL);
         host_clock = qemu_clock_new(QEMU_CLOCK_HOST);
     }
+#ifdef CONFIG_PRCTL_PR_SET_TIMERSLACK
+    prctl(PR_SET_TIMERSLACK, 1, 0, 0, 0);
+#endif
 }
 
 uint64_t timer_expire_time_ns(QEMUTimer *ts)