On 09.04.2016 21:39, Richard W.M. Jones wrote:
> In a few places in libvirt we busy-wait for events, for example qemu
> creating a monitor socket. This is problematic because:
>
> - We need to choose a sufficiently small polling period so that
> libvirt doesn't add unnecessary delays.
>
> - We need to choose a sufficiently large polling period so that
> the effect of busy-waiting doesn't affect the system.
>
> The solution to this conflict is to use an exponential backoff.
>
> This patch adds a macro VIR_TIME_WHILE_WITH_BACKOFF to hide the
> details, and modifies a few places where we currently busy-wait.
> ---
> src/fdstream.c | 10 +++++----
> src/libvirt_private.syms | 2 ++
> src/qemu/qemu_agent.c | 10 +++++----
> src/qemu/qemu_monitor.c | 10 +++++----
> src/util/virtime.c | 55 ++++++++++++++++++++++++++++++++++++++++++++++++
> src/util/virtime.h | 34 ++++++++++++++++++++++++++++++
> 6 files changed, 109 insertions(+), 12 deletions(-)
>
> diff --git a/src/fdstream.c b/src/fdstream.c
> index a85cf9d..94b2131 100644
> --- a/src/fdstream.c
> +++ b/src/fdstream.c
> @@ -42,6 +42,7 @@
> #include "virfile.h"
> #include "configmake.h"
> #include "virstring.h"
> +#include "virtime.h"
>
> #define VIR_FROM_THIS VIR_FROM_STREAMS
>
> @@ -516,8 +517,7 @@ int virFDStreamConnectUNIX(virStreamPtr st,
> bool abstract)
> {
> struct sockaddr_un sa;
> - size_t i = 0;
> - int timeout = 3;
> + virTimeBackOffVar timeout;
> int ret;
>
> int fd = socket(AF_UNIX, SOCK_STREAM, 0);
> @@ -537,7 +537,9 @@ int virFDStreamConnectUNIX(virStreamPtr st,
> goto error;
> }
>
> - do {
> + if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0)
> + goto error;
> + VIR_TIME_WHILE_WITH_BACKOFF(timeout) {
> ret = connect(fd, (struct sockaddr *)&sa, sizeof(sa));
> if (ret == 0)
> break;
> @@ -549,7 +551,7 @@ int virFDStreamConnectUNIX(virStreamPtr st,
> }
>
> goto error;
> - } while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
> + }
>
> if (virFDStreamOpenInternal(st, fd, NULL, -1, 0) < 0)
> goto error;
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index 068bc00..60c191d 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -2360,6 +2360,8 @@ virThreadPoolSendJob;
>
>
> # util/virtime.h
> +virTimeBackOffCondition;
> +virTimeBackOffStart;
> virTimeFieldsNow;
> virTimeFieldsNowRaw;
> virTimeFieldsThen;
> diff --git a/src/qemu/qemu_agent.c b/src/qemu/qemu_agent.c
> index bee8d4c..527fd16 100644
> --- a/src/qemu/qemu_agent.c
> +++ b/src/qemu/qemu_agent.c
> @@ -42,6 +42,7 @@
> #include "virtime.h"
> #include "virobject.h"
> #include "virstring.h"
> +#include "virtime.h"
This file is already included as can be seen even from the context.
> #include "base64.h"
>
> #define VIR_FROM_THIS VIR_FROM_QEMU
> @@ -173,9 +174,8 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
> {
> struct sockaddr_un addr;
> int monfd;
> - int timeout = 3; /* In seconds */
> + virTimeBackOffVar timeout;
> int ret;
> - size_t i = 0;
>
> *inProgress = false;
>
> @@ -207,7 +207,9 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
> goto error;
> }
>
> - do {
> + if (virTimeBackOffStart(&timeout, 1, 3*1000 /* ms */) < 0)
> + goto error;
> + VIR_TIME_WHILE_WITH_BACKOFF(timeout) {
> ret = connect(monfd, (struct sockaddr *) &addr, sizeof(addr));
>
> if (ret == 0)
> @@ -232,7 +234,7 @@ qemuAgentOpenUnix(const char *monitor, pid_t cpid, bool *inProgress)
> _("failed to connect to monitor socket"));
> goto error;
>
> - } while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
> + }
>
> if (ret != 0) {
> virReportSystemError(errno, "%s",
> diff --git a/src/qemu/qemu_monitor.c b/src/qemu/qemu_monitor.c
> index 10a6713..fda3cd9 100644
> --- a/src/qemu/qemu_monitor.c
> +++ b/src/qemu/qemu_monitor.c
> @@ -42,6 +42,7 @@
> #include "virobject.h"
> #include "virprobe.h"
> #include "virstring.h"
> +#include "virtime.h"
>
> #ifdef WITH_DTRACE_PROBES
> # include "libvirt_qemu_probes.h"
> @@ -327,9 +328,8 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
> {
> struct sockaddr_un addr;
> int monfd;
> - int timeout = 30; /* In seconds */
> + virTimeBackOffVar timeout;
> int ret;
> - size_t i = 0;
>
> if ((monfd = socket(AF_UNIX, SOCK_STREAM, 0)) < 0) {
> virReportSystemError(errno,
> @@ -345,7 +345,9 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
> goto error;
> }
>
> - do {
> + if (virTimeBackOffStart(&timeout, 1, 30*1000 /* ms */) < 0)
> + goto error;
> + VIR_TIME_WHILE_WITH_BACKOFF(timeout) {
> ret = connect(monfd, (struct sockaddr *) &addr, sizeof(addr));
>
> if (ret == 0)
> @@ -362,7 +364,7 @@ qemuMonitorOpenUnix(const char *monitor, pid_t cpid)
> _("failed to connect to monitor socket"));
> goto error;
>
> - } while ((++i <= timeout*5) && (usleep(.2 * 1000000) <= 0));
> + }
>
> if (ret != 0) {
> virReportSystemError(errno, "%s",
> diff --git a/src/util/virtime.c b/src/util/virtime.c
> index 9d365d5..9c4c520 100644
> --- a/src/util/virtime.c
> +++ b/src/util/virtime.c
> @@ -34,14 +34,18 @@
> #include <config.h>
>
> #include <stdio.h>
> +#include <unistd.h>
> #include <sys/time.h>
>
> #include "virtime.h"
> #include "viralloc.h"
> #include "virerror.h"
> +#include "virlog.h"
>
> #define VIR_FROM_THIS VIR_FROM_NONE
>
> +VIR_LOG_INIT("util.time");
> +
> /* We prefer clock_gettime if available because that is officially
> * async signal safe according to POSIX. Many platforms lack it
> * though, so fallback to gettimeofday everywhere else
> @@ -363,3 +367,54 @@ virTimeLocalOffsetFromUTC(long *offset)
> *offset = current - utc;
> return 0;
> }
> +
> +/**
> + * virTimeBackOffStart:
> + * @var: Timeout variable (with type virTimeBackOffVar).
> + * @first: Initial time to wait (milliseconds).
> + * @timeout: Timeout (milliseconds).
> + *
> + * Initialize the timeout variable @var and start the timer running.
> + *
> + * Returns 0 on success, -1 on error and raises a libvirt error.
> + */
> +int
> +virTimeBackOffStart(virTimeBackOffVar *var,
> + unsigned long long first, unsigned long long timeout)
> +{
> + if (virTimeMillisNow(&var->start_t) < 0)
> + return -1;
> +
> + var->next = first;
> + var->limit_t = var->start_t + timeout;
> + return 0;
> +}
> +
> +bool
> +virTimeBackOffCondition(virTimeBackOffVar *var)
> +{
> + unsigned long long t, next;
> +
> + ignore_value(virTimeMillisNowRaw(&t));
> +
> + VIR_DEBUG("t=%llu, limit=%llu", t, var->limit_t);
> +
> + if (t > var->limit_t)
> + return 0; /* ends the while loop */
> +
> + next = var->next;
> + var->next *= 2;
> +
> + /* If sleeping would take us beyond the limit, then shorten the
> + * sleep. This is so we always run the body just before the final
> + * timeout.
> + */
> + if (t + next > var->limit_t) {
> + next = var->limit_t - t;
> + }
We tend to not put curly braces around single lines.
> +
> + VIR_DEBUG("sleeping for %llu ms", next);
> +
> + usleep(next * 1000);
> + return 1;
> +}
> diff --git a/src/util/virtime.h b/src/util/virtime.h
> index 8ebad38..b0d9f89 100644
> --- a/src/util/virtime.h
> +++ b/src/util/virtime.h
> @@ -64,4 +64,38 @@ char *virTimeStringThen(unsigned long long when);
> int virTimeLocalOffsetFromUTC(long *offset)
> ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK;
>
> +/**
> + * VIR_TIME_WHILE_WITH_BACKOFF:
> + * @var: Timeout variable (with type virTimeBackOffVar).
> + *
> + * You must initialize @var first by calling this function, which
> + * also starts the timer:
> + *
> + * virTimeBackOffStart(&var, first, timeout);
> + *
> + * This macro is a while loop that runs the body of the code
> + * repeatedly, with an exponential backoff. It first waits for first
> + * milliseconds, then runs the body, then waits for 2*first ms, then
> + * runs the body again. Then 4*first ms, and so on.
> + *
> + * When timeout milliseconds is reached, the while loop ends.
> + *
> + * The body should use "break" or "goto" when whatever condition it is
> + * testing for succeeds (or there is an unrecoverable error).
> + */
> +#define VIR_TIME_WHILE_WITH_BACKOFF(var) \
> + while (virTimeBackOffCondition(&(var)))
And here there should be a space between '#' and 'define'.
> +
> +typedef struct {
> + unsigned long long start_t;
> + unsigned long long next;
> + unsigned long long limit_t;
> +} virTimeBackOffVar;
> +
> +int virTimeBackOffStart(virTimeBackOffVar *var,
> + unsigned long long first, unsigned long long timeout);
> +
> +/* Internal function, don't call this directly. */
> +bool virTimeBackOffCondition(virTimeBackOffVar *var);
> +
> #endif
>
Therefore ACK with this squashed in:
diff --git a/src/qemu/qemu_agent.c b/src/qemu/qemu_agent.c
index 6f6e8a2..e3882a2 100644
--- a/src/qemu/qemu_agent.c
+++ b/src/qemu/qemu_agent.c
@@ -42,7 +42,6 @@
#include "virtime.h"
#include "virobject.h"
#include "virstring.h"
-#include "virtime.h"
#include "base64.h"
#define VIR_FROM_THIS VIR_FROM_QEMU
diff --git a/src/util/virtime.c b/src/util/virtime.c
index 9c4c520..c8fd94c 100644
--- a/src/util/virtime.c
+++ b/src/util/virtime.c
@@ -409,9 +409,8 @@ virTimeBackOffCondition(virTimeBackOffVar *var)
* sleep. This is so we always run the body just before the final
* timeout.
*/
- if (t + next > var->limit_t) {
+ if (t + next > var->limit_t)
next = var->limit_t - t;
- }
VIR_DEBUG("sleeping for %llu ms", next);
diff --git a/src/util/virtime.h b/src/util/virtime.h
index b0d9f89..2b9bc81 100644
--- a/src/util/virtime.h
+++ b/src/util/virtime.h
@@ -83,7 +83,7 @@ int virTimeLocalOffsetFromUTC(long *offset)
* The body should use "break" or "goto" when whatever condition it is
* testing for succeeds (or there is an unrecoverable error).
*/
-#define VIR_TIME_WHILE_WITH_BACKOFF(var) \
+# define VIR_TIME_WHILE_WITH_BACKOFF(var) \
while (virTimeBackOffCondition(&(var)))
typedef struct {
Michal
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