[Qemu-devel] [PATCH] blockjob: Don't sleep too short

[Fam Zheng <famz@redhat.com>]

block_job_sleep_ns is called by block job coroutines to yield the
execution to VCPU threads and monitor etc. It is pointless to sleep for
0 or a few nanoseconds, because that equals to a "yield + enter" with no
intermission in between (the timer fires immediately in the same
iteration of event loop), which means other code still doesn't get a
fair share of main loop / BQL.

Trim the sleep duration with a minimum value.

Reported-by: Alexandre DERUMIER <aderumier@odiso.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
---
 blockjob.c               | 2 ++
 include/block/blockjob.h | 4 +++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/blockjob.c b/blockjob.c
index ec46fad..b17ed1f 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -238,6 +238,8 @@ void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns)
         return;
     }
 
+    ns = MAX(ns, BLOCK_JOB_SLEEP_NS_MIN);
+
     job->busy = false;
     if (block_job_is_paused(job)) {
         qemu_coroutine_yield();
diff --git a/include/block/blockjob.h b/include/block/blockjob.h
index 57d8ef1..3deb731 100644
--- a/include/block/blockjob.h
+++ b/include/block/blockjob.h
@@ -146,11 +146,13 @@ void *block_job_create(const BlockJobDriver *driver, BlockDriverState *bs,
                        int64_t speed, BlockCompletionFunc *cb,
                        void *opaque, Error **errp);
 
+#define BLOCK_JOB_SLEEP_NS_MIN 10000000L
 /**
  * block_job_sleep_ns:
  * @job: The job that calls the function.
  * @clock: The clock to sleep on.
- * @ns: How many nanoseconds to stop for.
+ * @ns: How many nanoseconds to stop for. It sleeps at least
+ * for BLOCK_JOB_SLEEP_NS_MIN ns, even if a smaller value is specified.
  *
  * Put the job to sleep (assuming that it wasn't canceled) for @ns
  * nanoseconds.  Canceling the job will interrupt the wait immediately.
-- 
2.4.3