s390/cio: More efficient handling of CHPID availability events

The CIO layer processes hardware events that indicate that a channel
path has become available by performing a scan of available subchannels
using the Store Subchannel (STSCH) instruction. Performing too many
STSCH instructions in a tight loop can cause high Hypervisor overhead
which can negatively impact the performance of the virtual machine as
a whole.

This patch reduces the number of STSCH instructions performed while
processing a resource accessibility event and while varying a CHPID
online.

In both cases, Linux first performs a STSCH instruction on each unused
subchannel to see if the subchannel has become available. If the STSCH
instruction indicates that the subchannel is available, a full
evaluation of this subchannel is scheduled. Since the full evaluation
includes performing a STSCH instruction, the initial STSCH is
unnecessary and can be removed.

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, 4 insertions, 36 deletions

diff --git a/drivers/s390/cio/chsc.c b/drivers/s390/cio/chsc.c
index 13299f902676..eee70cb8730b 100644
--- a/drivers/s390/cio/chsc.c
+++ b/drivers/s390/cio/chsc.c
@@ -237,26 +237,6 @@ void chsc_chp_offline(struct chp_id chpid)
         for_each_subchannel_staged(s390_subchannel_remove_chpid, NULL, &link);
 }
 
-static int s390_process_res_acc_new_sch(struct subchannel_id schid, void *data)
-{
-        struct schib schib;
-        /*
-         * We don't know the device yet, but since a path
-         * may be available now to the device we'll have
-         * to do recognition again.
-         * Since we don't have any idea about which chpid
-         * that beast may be on we'll have to do a stsch
-         * on all devices, grr...
-         */
-        if (stsch_err(schid, &schib))
-                /* We're through */
-                return -ENXIO;
-
-        /* Put it on the slow path. */
-        css_schedule_eval(schid);
-        return 0;
-}
-
 static int __s390_process_res_acc(struct subchannel *sch, void *data)
 {
         spin_lock_irq(sch->lock);
@@ -287,8 +267,8 @@ static void s390_process_res_acc(struct chp_link *link)
          * The more information we have (info), the less scanning
          * will we have to do.
          */
-        for_each_subchannel_staged(__s390_process_res_acc,
-                                   s390_process_res_acc_new_sch, link);
+        for_each_subchannel_staged(__s390_process_res_acc, NULL, link);
+        css_schedule_reprobe();
 }
 
 static int
@@ -663,19 +643,6 @@ static int s390_subchannel_vary_chpid_on(struct subchannel *sch, void *data)
         return 0;
 }
 
-static int
-__s390_vary_chpid_on(struct subchannel_id schid, void *data)
-{
-        struct schib schib;
-
-        if (stsch_err(schid, &schib))
-                /* We're through */
-                return -ENXIO;
-        /* Put it on the slow path. */
-        css_schedule_eval(schid);
-        return 0;
-}
-
 /**
  * chsc_chp_vary - propagate channel-path vary operation to subchannels
  * @chpid: channl-path ID
@@ -694,7 +661,8 @@ int chsc_chp_vary(struct chp_id chpid, int on)
                 /* Try to update the channel path description. */
                 chp_update_desc(chp);
                 for_each_subchannel_staged(s390_subchannel_vary_chpid_on,
-                                           __s390_vary_chpid_on, &chpid);
+                                           NULL, &chpid);
+                css_schedule_reprobe();
         } else
                 for_each_subchannel_staged(s390_subchannel_vary_chpid_off,
                                            NULL, &chpid);