Re: [libvirt] [PATCH v2] Add macro for handling exponential backoff loops.

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 | 9 +++++---- src/libvirt_private.syms | 2 ++ src/qemu/qemu_agent.c | 9 +++++---- src/qemu/qemu_monitor.c | 9 +++++---- src/util/virtime.c | 47 +++++++++++++++++++++++++++++++++++++++++++++++ src/util/virtime.h | 41 +++++++++++++++++++++++++++++++++++++++++ 6 files changed, 105 insertions(+), 12 deletions(-)

@@ -363,3 +367,46 @@ virTimeLocalOffsetFromUTC(long *offset) *offset = current - utc; return 0; } + +void +virTimeBackOffInit(virTimeBackOffVar *var, + unsigned long long first, unsigned long long timeout) +{ + var->start_t = 0; + var->first = first; + var->timeout = timeout; +} +

+int

+virTimeBackOffCondition(virTimeBackOffVar *var) +{ + unsigned long long t, next; + + ignore_value(virTimeMillisNowRaw(&t));

+ if (var->start_t == 0) { + var->start_t = t; + var->next = var->first; + var->limit = t + var->timeout;

+ } + + VIR_DEBUG("t=%llu, limit=%llu", t, var->limit); + + if (t > var->limit) + 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) { + next = var->limit - t - 2;

+ } + + 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..bc178e4 100644 --- a/src/util/virtime.h +++ b/src/util/virtime.h @@ -64,4 +64,45 @@ 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: + * + * virTimeBackOffInit(&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)))