aoe: use high-resolution RTTs with fallback to low-res

These changes improve the accuracy of the decision about whether it's time
to retransmit an AoE command by using the microsecond-resolution
gettimeofday instead of jiffies.

Because the system time can jump suddenly, the decision reverts to using
jiffies if the high-resolution time difference is relatively large.
Otherwise the AoE targets could be considered failed inappropriately.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 55 insertions, 11 deletions

diff --git a/drivers/block/aoe/aoe.h b/drivers/block/aoe/aoe.h
index 9e884acd75fc..9fb68fc3b280 100644
--- a/drivers/block/aoe/aoe.h
+++ b/drivers/block/aoe/aoe.h
@@ -88,8 +88,7 @@ enum {
         TIMERTICK = HZ / 10,
         RTTSCALE = 8,
         RTTDSCALE = 3,
-        MAXTIMER = HZ << 1,
-        RTTAVG_INIT = HZ / 4 << RTTSCALE,
+        RTTAVG_INIT = USEC_PER_SEC / 4 << RTTSCALE,
         RTTDEV_INIT = RTTAVG_INIT / 4,
 };
 
@@ -106,6 +105,8 @@ struct buf {
 struct frame {
         struct list_head head;
         u32 tag;
+        struct timeval sent;    /* high-res time packet was sent */
+        u32 sent_jiffs;         /* low-res jiffies-based sent time */
         ulong waited;
         struct aoetgt *t;               /* parent target I belong to */
         sector_t lba;
@@ -143,11 +144,11 @@ struct aoedev {
         struct aoedev *next;
         ulong sysminor;
         ulong aoemajor;
+        u32 rttavg;             /* scaled AoE round trip time average */
+        u32 rttdev;             /* scaled round trip time mean deviation */
         u16 aoeminor;
         u16 flags;
         u16 nopen;              /* (bd_openers isn't available without sleeping) */
-        u16 rttavg;             /* scaled AoE round trip time average */
-        u16 rttdev;             /* scaled round trip time mean deviation */
         u16 fw_ver;             /* version of blade's firmware */
         u16 lasttag;            /* last tag sent */
         u16 useme;
diff --git a/drivers/block/aoe/aoecmd.c b/drivers/block/aoe/aoecmd.c
index 9aefbe3957ca..a99220ad6262 100644
--- a/drivers/block/aoe/aoecmd.c
+++ b/drivers/block/aoe/aoecmd.c
@@ -387,6 +387,8 @@ aoecmd_ata_rw(struct aoedev *d)
         skb->dev = t->ifp->nd;
         skb = skb_clone(skb, GFP_ATOMIC);
         if (skb) {
+                do_gettimeofday(&f->sent);
+                f->sent_jiffs = (u32) jiffies;
                 __skb_queue_head_init(&queue);
                 __skb_queue_tail(&queue, skb);
                 aoenet_xmit(&queue);
@@ -475,12 +477,46 @@ resend(struct aoedev *d, struct frame *f)
         skb = skb_clone(skb, GFP_ATOMIC);
         if (skb == NULL)
                 return;
+        do_gettimeofday(&f->sent);
+        f->sent_jiffs = (u32) jiffies;
         __skb_queue_head_init(&queue);
         __skb_queue_tail(&queue, skb);
         aoenet_xmit(&queue);
 }
 
 static int
+tsince_hr(struct frame *f)
+{
+        struct timeval now;
+        int n;
+
+        do_gettimeofday(&now);
+        n = now.tv_usec - f->sent.tv_usec;
+        n += (now.tv_sec - f->sent.tv_sec) * USEC_PER_SEC;
+
+        if (n < 0)
+                n = -n;
+
+        /* For relatively long periods, use jiffies to avoid
+         * discrepancies caused by updates to the system time.
+         *
+         * On system with HZ of 1000, 32-bits is over 49 days
+         * worth of jiffies, or over 71 minutes worth of usecs.
+         *
+         * Jiffies overflow is handled by subtraction of unsigned ints:
+         * (gdb) print (unsigned) 2 - (unsigned) 0xfffffffe
+         * $3 = 4
+         * (gdb)
+         */
+        if (n > USEC_PER_SEC / 4) {
+                n = ((u32) jiffies) - f->sent_jiffs;
+                n *= USEC_PER_SEC / HZ;
+        }
+
+        return n;
+}
+
+static int
 tsince(u32 tag)
 {
         int n;
@@ -489,7 +525,7 @@ tsince(u32 tag)
         n -= tag & 0xffff;
         if (n < 0)
                 n += 1<<16;
-        return n;
+        return jiffies_to_usecs(n + 1);
 }
 
 static struct aoeif *
@@ -552,6 +588,7 @@ sthtith(struct aoedev *d)
                         nf->bv = f->bv;
                         nf->bv_off = f->bv_off;
                         nf->waited = 0;
+                        nf->sent_jiffs = f->sent_jiffs;
                         f->skb = skb;
                         aoe_freetframe(f);
                         ht->nout--;
@@ -621,7 +658,7 @@ rexmit_timer(ulong vp)
                 head = &d->factive[i];
                 list_for_each_safe(pos, nx, head) {
                         f = list_entry(pos, struct frame, head);
-                        if (tsince(f->tag) < timeout)
+                        if (tsince_hr(f) < timeout)
                                 break;  /* end of expired frames */
                         /* move to flist for later processing */
                         list_move_tail(pos, &flist);
@@ -632,8 +669,8 @@ rexmit_timer(ulong vp)
         while (!list_empty(&flist)) {
                 pos = flist.next;
                 f = list_entry(pos, struct frame, head);
-                n = f->waited += tsince(f->tag);
-                n /= HZ;
+                n = f->waited += tsince_hr(f);
+                n /= USEC_PER_SEC;
                 if (n > aoe_deadsecs) {
                         /* Waited too long.  Device failure.
                          * Hang all frames on first hash bucket for downdev
@@ -1193,12 +1230,12 @@ aoecmd_ata_rsp(struct sk_buff *skb)
         n = be32_to_cpu(get_unaligned(&h->tag));
         f = getframe(d, n);
         if (f) {
-                calc_rttavg(d, f->t, tsince(n));
+                calc_rttavg(d, f->t, tsince_hr(f));
                 f->t->nout--;
         } else {
                 f = getframe_deferred(d, n);
                 if (f) {
-                        calc_rttavg(d, NULL, tsince(n));
+                        calc_rttavg(d, NULL, tsince_hr(f));
                 } else {
                         calc_rttavg(d, NULL, tsince(n));
                         spin_unlock_irqrestore(&d->lock, flags);
@@ -1276,7 +1313,13 @@ aoecmd_ata_id(struct aoedev *d)
         d->rttdev = RTTDEV_INIT;
         d->timer.function = rexmit_timer;
 
-        return skb_clone(skb, GFP_ATOMIC);
+        skb = skb_clone(skb, GFP_ATOMIC);
+        if (skb) {
+                do_gettimeofday(&f->sent);
+                f->sent_jiffs = (u32) jiffies;
+        }
+
+        return skb;
 }
 
 static struct aoetgt *