Re: [libvirt] [PATCH v3] 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 | 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/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)))