Re: [PATCH xserver] xwayland: Make it possible to force CLOCK_MONOTONIC

[Daniel Stone]

Hi,

On 9 March 2016 at 04:27, Jonas Ådahl  wrote:
> By default the X server will try CLOCK_MONOTONIC_COARSE before
> CLOCK_MONOTONIC. This causes various issues for Wayland display
> servers who may get their internal timestamps from the CLOCK_MONOTONIC,
> since it may very well happen that a timestamp from CLOCK_MONOTONIC
> retrieved before a sending an X request will still be "later" than the
> timestamp the X server than gets after receiving the request, due to
> the fact that CLOCK_MONOTONIC_COARSE has a lower resolution.

For reference, this isn't anything to do with Wayland core protocol or
similar, but that evdev devices can provide events with
CLOCK_MONOTONIC timestamps, which we try to match.

The original reason for using CLOCK_MONOTONIC_COARSE over
CLOCK_MONOTONIC is that the former had a runtime power impact by
battering HPET. I don't know if that still holds up on modern systems.

> FWIW, I would be just as fine with always forcing CLOCK_MONOTONIC for Xwayland
> if the command line argument seems unnecessary.

This one is :
Reviewed-by: Daniel Stone 

and just forcing Xwayland to do it unconditionally is:
Acked-by: Daniel Stone 

> @@ -86,6 +88,10 @@ ddxProcessArgument(int argc, char *argv[], int i)
>  else if (strcmp(argv[i], "-shm") == 0) {
>  return 1;
>  }
> +else if (strcmp(argv[i], "-forceMonotonic") == 0) {
> +ForceClockId(CLOCK_MONOTONIC);
> +return 1;
> +}

Needs #ifdef MONOTONIC_CLOCK.

> +#ifdef MONOTONIC_CLOCK
> +static clockid_t clockid;
> +#endif
> +
>  OsSigHandlerPtr
>  OsSignal(int sig, OsSigHandlerPtr handler)
>  {
> @@ -427,6 +431,26 @@ GiveUp(int sig)
>  errno = olderrno;
>  }
>
> +#ifdef MONOTONIC_CLOCK
> +void
> +ForceClockId(clockid_t forced_clockid)
> +{
> +struct timespec tp;
> +
> +if (clockid) {
> +ErrorF("Warning: clock id was forced after it was initialized.\n");

Turn this into a BUG(), especially as the one below is a FatalError ...

Cheers,
Daniel
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[PATCH xserver] xwayland: Make it possible to force CLOCK_MONOTONIC

[Jonas Ådahl]

By default the X server will try CLOCK_MONOTONIC_COARSE before
CLOCK_MONOTONIC. This causes various issues for Wayland display
servers who may get their internal timestamps from the CLOCK_MONOTONIC,
since it may very well happen that a timestamp from CLOCK_MONOTONIC
retrieved before a sending an X request will still be "later" than the
timestamp the X server than gets after receiving the request, due to
the fact that CLOCK_MONOTONIC_COARSE has a lower resolution.

To make it possible for Wayland display servers to have more control
over the clock the X server uses, add a '-forceMonotonic' command line
argument to Xwayland, that forces the X server to always use the
CLOCK_MONOTONIC clock as its source for timestamps.

This commit also makes it a build requirement to have clock_gettime()
and CLOCK_MONOTONIC support, and a runtime requirement to have
CLOCK_MONOTONIC support if the clock was forced to be monotonic.

Signed-off-by: Jonas Ådahl 
---

Hi,

This patch fixes various issues[0] related to the race condition described in
the commit message. An alternative solution discussed has been to allow a
"grace period" as long as the clock resolution difference for future
timestamps, or rounding down timestamps from CLOCK_MONOTONIC in the display
server. I think both these solutions have their own problems, while just
changing the clock here only has a (most likely negliable) performance loss.

FWIW, I would be just as fine with always forcing CLOCK_MONOTONIC for Xwayland
if the command line argument seems unnecessary.


Jonas

[0] https://bugzilla.gnome.org/show_bug.cgi?id=756272


 configure.ac   |  4 
 hw/xwayland/xwayland.c |  6 ++
 include/os.h   |  7 +++
 os/utils.c | 26 --
 4 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index f4de90f..8cc508c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2471,6 +2471,10 @@ if test "x$XWAYLAND" = xyes; then
AC_SUBST([XWAYLAND_LIBS])
AC_SUBST([XWAYLAND_SYS_LIBS])
 
+   if test "x$MONOTONIC_CLOCK" != xyes; then
+   AC_MSG_ERROR([Xwayland requires CLOCK_MONOTONIC support.])
+   fi
+
WAYLAND_PREFIX=`$PKG_CONFIG --variable=prefix wayland-client`
AC_PATH_PROG([WAYLAND_SCANNER], [wayland-scanner],,
 [${WAYLAND_PREFIX}/bin$PATH_SEPARATOR$PATH])
diff --git a/hw/xwayland/xwayland.c b/hw/xwayland/xwayland.c
index 3d36205..fa0143e 100644
--- a/hw/xwayland/xwayland.c
+++ b/hw/xwayland/xwayland.c
@@ -32,6 +32,7 @@
 #include 
 #include 
 #include 
+#include 
 
 void
 ddxGiveUp(enum ExitCode error)
@@ -68,6 +69,7 @@ ddxUseMsg(void)
 ErrorF("-rootless  run rootless, requires wm support\n");
 ErrorF("-wm fd create X client for wm on given fd\n");
 ErrorF("-listen fd add give fd as a listen socket\n");
+ErrorF("-forceMonotonicforce usage of CLOCK_MONOTONIC\n");
 }
 
 int
@@ -86,6 +88,10 @@ ddxProcessArgument(int argc, char *argv[], int i)
 else if (strcmp(argv[i], "-shm") == 0) {
 return 1;
 }
+else if (strcmp(argv[i], "-forceMonotonic") == 0) {
+ForceClockId(CLOCK_MONOTONIC);
+return 1;
+}
 
 return 0;
 }
diff --git a/include/os.h b/include/os.h
index 461d5d6..f37ad5d 100644
--- a/include/os.h
+++ b/include/os.h
@@ -51,6 +51,9 @@ SOFTWARE.
 #include 
 #include 
 #include 
+#ifdef MONOTONIC_CLOCK
+#include 
+#endif
 
 #define SCREEN_SAVER_ON   0
 #define SCREEN_SAVER_OFF  1
@@ -180,6 +183,10 @@ extern _X_EXPORT void ListenOnOpenFD(int /* fd */ , int /* 
noxauth */ );
 
 extern _X_EXPORT Bool AddClientOnOpenFD(int /* fd */ );
 
+#ifdef MONOTONIC_CLOCK
+extern _X_EXPORT void ForceClockId(clockid_t /* forced_clockid */);
+#endif
+
 extern _X_EXPORT CARD32 GetTimeInMillis(void);
 extern _X_EXPORT CARD64 GetTimeInMicros(void);
 
diff --git a/os/utils.c b/os/utils.c
index e48d9f8..85e7b07 100644
--- a/os/utils.c
+++ b/os/utils.c
@@ -210,6 +210,10 @@ sig_atomic_t inSignalContext = FALSE;
 #define HAS_SAVED_IDS_AND_SETEUID
 #endif
 
+#ifdef MONOTONIC_CLOCK
+static clockid_t clockid;
+#endif
+
 OsSigHandlerPtr
 OsSignal(int sig, OsSigHandlerPtr handler)
 {
@@ -427,6 +431,26 @@ GiveUp(int sig)
 errno = olderrno;
 }
 
+#ifdef MONOTONIC_CLOCK
+void
+ForceClockId(clockid_t forced_clockid)
+{
+struct timespec tp;
+
+if (clockid) {
+ErrorF("Warning: clock id was forced after it was initialized.\n");
+return;
+}
+clockid = forced_clockid;
+
+if (clock_gettime(clockid, ) != 0) {
+FatalError("Forced clock id failed to retrieve current time: %s\n",
+   strerror(errno));
+return;
+}
+}
+#endif
+
 #if (defined WIN32 && defined __MINGW32__) || defined(__CYGWIN__)
 CARD32
 GetTimeInMillis(void)
@@ -446,7 +470,6 @@ GetTimeInMillis(void)
 
 #ifdef MONOTONIC_CLOCK
 struct timespec tp;
-static clockid_t clockid;
 
 if