s390/css: Prevent unnecessary allocation in subchannel loop

Subchannel looping function for_each_subchannel_staged() allocates a
subchannel-ID-bitmap to efficiently iterate over the list of known
and unknown subchannels. Since this function is also used to iterate
over known-subchannels only, optimize that case by not requiring the
ID-bitmap allocation and falling back to simple bus_for_each_dev()
looping.

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Reviewed-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/s390/cio/css.c b/drivers/s390/cio/css.c
index 604333ebf01e..0268e5fd59b5 100644
--- a/drivers/s390/cio/css.c
+++ b/drivers/s390/cio/css.c
@@ -69,7 +69,8 @@ static int call_fn_known_sch(struct device *dev, void *data)
         struct cb_data *cb = data;
         int rc = 0;
 
-        idset_sch_del(cb->set, sch->schid);
+        if (cb->set)
+                idset_sch_del(cb->set, sch->schid);
         if (cb->fn_known_sch)
                 rc = cb->fn_known_sch(sch, cb->data);
         return rc;
@@ -115,6 +116,13 @@ int for_each_subchannel_staged(int (*fn_known)(struct subchannel *, void *),
         cb.fn_known_sch = fn_known;
         cb.fn_unknown_sch = fn_unknown;
 
+        if (fn_known && !fn_unknown) {
+                /* Skip idset allocation in case of known-only loop. */
+                cb.set = NULL;
+                return bus_for_each_dev(&css_bus_type, NULL, &cb,
+                                        call_fn_known_sch);
+        }
+
         cb.set = idset_sch_new();
         if (!cb.set)
                 /* fall back to brute force scanning in case of oom */