[ 086/108] vhost: dont forget to schedule()

Ben Hutchings
2012-07-23T02:04:22Z

From: Nadav Har'El <nyh@math.technion.ac.il>

commit d550dda192c1bd039afb774b99485e88b70d7cb8 upstream.

This is a tiny, but important, patch to vhost.

Vhost's worker thread only called schedule() when it had no work to do, and
it wanted to go to sleep. But if there's always work to do, e.g., the guest
is running a network-intensive program like netperf with small message sizes,
schedule() was *never* called. This had several negative implications (on
non-preemptive kernels):

1. Passing time was not properly accounted to the "vhost" process (ps and
top would wrongly show it using zero CPU time).

2. Sometimes error messages about RCU timeouts would be printed, if the
core running the vhost thread didn't schedule() for a very long time.

3. Worst of all, a vhost thread would "hog" the core. If several vhost
threads need to share the same core, typically one would get most of the
CPU time (and its associated guest most of the performance), while the
others hardly get any work done.

The trivial solution is to add

if (need_resched())
schedule();

After doing every piece of work. This will not do the heavy schedule() all
the time, just when the timer interrupt decided a reschedule is warranted
(so need_resched returns true).

Thanks to Abel Gordon for this patch.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/vhost/vhost.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index c14c42b..ae66278 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -222,6 +222,8 @@ static int vhost_worker(void *data)
if (work) {
__set_current_state(TASK_RUNNING);
work->fn(work);
+ if (need_resched())
+ schedule();
} else
schedule();