[Qemu-block] [PATCH 1/3] blockjob: Introduce block_job_relax_cpu

[Fam Zheng]

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.

Introduce block_job_relax_cpu which will at least for
BLOCK_JOB_RELAX_CPU_NS. Existing block_job_sleep_ns(job, 0) callers can
be replaced by this later.

Reported-by: Alexandre DERUMIER <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
---
 include/block/blockjob.h | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/include/block/blockjob.h b/include/block/blockjob.h
index 57d8ef1..53ac4f4 100644
--- a/include/block/blockjob.h
+++ b/include/block/blockjob.h
@@ -157,6 +157,22 @@ void *block_job_create(const BlockJobDriver *driver, 
BlockDriverState *bs,
  */
 void block_job_sleep_ns(BlockJob *job, QEMUClockType type, int64_t ns);
 
+#define BLOCK_JOB_RELAX_CPU_NS 10000000L
+
+/**
+ * block_job_relax_cpu:
+ * @job: The job that calls the function.
+ *
+ * Sleep a little to avoid intensive cpu time occupation. Block jobs should
+ * call this or block_job_sleep_ns (for more precision, but note that 0 ns is
+ * usually not enought) periodically, otherwise the QMP and VCPU could starve
+ * on CPU and/or BQL.
+ */
+static inline void block_job_relax_cpu(BlockJob *job)
+{
+    return block_job_sleep_ns(job, QEMU_CLOCK_REALTIME, 
BLOCK_JOB_RELAX_CPU_NS);
+}
+
 /**
  * block_job_yield:
  * @job: The job that calls the function.
-- 
2.4.3