[Xen-devel] [PATCH 1/2] blkback: Implement VBD QoS mechanics.

[William Pitcock <nenolod@xxxxxxxxxxxxxxxx>]

Impact: add ability to define I/O usage policies

This enables a nieve token-based QoS system which allows for guests to
get an equal share of I/O requests. Without this patch, a misbehaving or
misconfigured guest can use up any available I/O resources, degrading I/O
performance for other guests on the machine.

The method for specifying these limitations is not implemented in this patch,
because that is an operational detail.

Signed-off-by: William Pitcock <nenolod@xxxxxxxxxxxxxxxx>
---
 drivers/xen/blkback/blkback.c |   22 ++++++++++++++++++++++
 drivers/xen/blkback/common.h  |    6 ++++++
 2 files changed, 28 insertions(+), 0 deletions(-)

diff --git a/drivers/xen/blkback/blkback.c b/drivers/xen/blkback/blkback.c
index 8d988f4..996869e 100644
--- a/drivers/xen/blkback/blkback.c
+++ b/drivers/xen/blkback/blkback.c
@@ -203,6 +203,14 @@ static void print_stats(blkif_t *blkif)
        blkif->st_oo_req = 0;
 }
 
+static void refill_reqcount(blkif_t *blkif)
+{
+       blkif->reqtime = jiffies + msecs_to_jiffies(1000);
+       blkif->reqcount += blkif->reqrate;
+       if (blkif->reqcount > blkif->reqmax)
+               blkif->reqcount = blkif->reqmax;
+}
+
 int blkif_schedule(void *arg)
 {
        blkif_t *blkif = arg;
@@ -232,6 +240,9 @@ int blkif_schedule(void *arg)
 
                if (log_stats && time_after(jiffies, blkif->st_print))
                        print_stats(blkif);
+
+               if (time_after(jiffies, blkif->reqtime))
+                       refill_reqcount(blkif);
        }
 
        if (log_stats)
@@ -313,11 +324,22 @@ static int do_block_io_op(blkif_t *blkif)
        rp = blk_rings->common.sring->req_prod;
        rmb(); /* Ensure we see queued requests up to 'rp'. */
 
+       /* if there's no available request tokens right now, and limiting
+        * is requested, then don't bother with going any further. */
+       if (blkif->reqcount <= 0 && blkif->reqmax != 0)
+               return (rc != rp) ? 1 : 0;
+
        while (rc != rp) {
 
                if (RING_REQUEST_CONS_OVERFLOW(&blk_rings->common, rc))
                        break;
 
+               /* FIXME: Should we report QoS overages as VBD_OO, or not? */
+               if (--blkif->reqcount <= 0) {
+                       more_to_do = 1;
+                       break;
+               }
+
                if (kthread_should_stop()) {
                        more_to_do = 1;
                        break;
diff --git a/drivers/xen/blkback/common.h b/drivers/xen/blkback/common.h
index 57b7825..45af42f 100644
--- a/drivers/xen/blkback/common.h
+++ b/drivers/xen/blkback/common.h
@@ -92,6 +92,12 @@ typedef struct blkif_st {
 
        grant_handle_t shmem_handle;
        grant_ref_t    shmem_ref;
+
+       /* qos information */
+       long                reqtime;
+       int                 reqcount;
+       int                 reqmax;
+       int                 reqfill;
 } blkif_t;
 
 blkif_t *blkif_alloc(domid_t domid);
-- 
1.6.1.3


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