[XFS] On machines with more than 8 cpus, when running parallel I/O

threads, the incore superblock lock becomes the limiting factor for
buffered write throughput. Make the contended fields in the incore
superblock use per-cpu counters so that there is no global lock to limit
scalability.

SGI-PV: 946630
SGI-Modid: xfs-linux-melb:xfs-kern:25106a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>

1 files changed, 544 insertions, 16 deletions

diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
index 62188ea392c7..9b43b7b3d760 100644
--- a/fs/xfs/xfs_mount.c
+++ b/fs/xfs/xfs_mount.c
@@ -51,11 +51,31 @@ STATIC int	xfs_uuid_mount(xfs_mount_t *);
 STATIC void     xfs_uuid_unmount(xfs_mount_t *mp);
 STATIC void     xfs_unmountfs_wait(xfs_mount_t *);
 
+
+#ifdef HAVE_PERCPU_SB
+STATIC void     xfs_icsb_destroy_counters(xfs_mount_t *);
+STATIC void     xfs_icsb_balance_counter(xfs_mount_t *, xfs_sb_field_t, int);
+STATIC void     xfs_icsb_sync_counters(xfs_mount_t *);
+STATIC int      xfs_icsb_modify_counters(xfs_mount_t *, xfs_sb_field_t,
+                                                int, int);
+STATIC int      xfs_icsb_modify_counters_locked(xfs_mount_t *, xfs_sb_field_t,
+                                                int, int);
+
+#else
+
+#define xfs_icsb_destroy_counters(mp)                   do { } while (0)
+#define xfs_icsb_balance_counter(mp, a, b)              do { } while (0)
+#define xfs_icsb_sync_counters(mp)                      do { } while (0)
+#define xfs_icsb_modify_counters(mp, a, b, c)           do { } while (0)
+#define xfs_icsb_modify_counters_locked(mp, a, b, c)    do { } while (0)
+
+#endif
+
 static const struct {
-    short offset;
-    short type;     /* 0 = integer
-                * 1 = binary / string (no translation)
-                */
+        short offset;
+        short type;     /* 0 = integer
+                         * 1 = binary / string (no translation)
+                         */
 } xfs_sb_info[] = {
     { offsetof(xfs_sb_t, sb_magicnum),   0 },
     { offsetof(xfs_sb_t, sb_blocksize),  0 },
@@ -113,7 +133,11 @@ xfs_mount_init(void)
 {
         xfs_mount_t *mp;
 
-        mp = kmem_zalloc(sizeof(*mp), KM_SLEEP);
+        mp = kmem_zalloc(sizeof(xfs_mount_t), KM_SLEEP);
+
+        if (xfs_icsb_init_counters(mp)) {
+                mp->m_flags |= XFS_MOUNT_NO_PERCPU_SB;
+        }
 
         AIL_LOCKINIT(&mp->m_ail_lock, "xfs_ail");
         spinlock_init(&mp->m_sb_lock, "xfs_sb");
@@ -136,8 +160,8 @@ xfs_mount_init(void)
  */
 void
 xfs_mount_free(
-        xfs_mount_t *mp,
-        int         remove_bhv)
+        xfs_mount_t     *mp,
+        int             remove_bhv)
 {
         if (mp->m_ihash)
                 xfs_ihash_free(mp);
@@ -177,6 +201,7 @@ xfs_mount_free(
                 VFS_REMOVEBHV(vfsp, &mp->m_bhv);
         }
 
+        xfs_icsb_destroy_counters(mp);
         kmem_free(mp, sizeof(xfs_mount_t));
 }
 
@@ -527,6 +552,10 @@ xfs_readsb(xfs_mount_t *mp)
                 ASSERT(XFS_BUF_VALUSEMA(bp) <= 0);
         }
 
+        xfs_icsb_balance_counter(mp, XFS_SBS_ICOUNT, 0);
+        xfs_icsb_balance_counter(mp, XFS_SBS_IFREE, 0);
+        xfs_icsb_balance_counter(mp, XFS_SBS_FDBLOCKS, 0);
+
         mp->m_sb_bp = bp;
         xfs_buf_relse(bp);
         ASSERT(XFS_BUF_VALUSEMA(bp) > 0);
@@ -1154,6 +1183,9 @@ xfs_unmountfs_writesb(xfs_mount_t *mp)
         sbp = xfs_getsb(mp, 0);
         if (!(XFS_MTOVFS(mp)->vfs_flag & VFS_RDONLY ||
                 XFS_FORCED_SHUTDOWN(mp))) {
+
+                xfs_icsb_sync_counters(mp);
+
                 /*
                  * mark shared-readonly if desired
                  */
@@ -1227,7 +1259,6 @@ xfs_mod_sb(xfs_trans_t *tp, __int64_t fields)
 
         xfs_trans_log_buf(tp, bp, first, last);
 }
-
 /*
  * xfs_mod_incore_sb_unlocked() is a utility routine common used to apply
  * a delta to a specified field in the in-core superblock.  Simply
@@ -1237,7 +1268,7 @@ xfs_mod_sb(xfs_trans_t *tp, __int64_t fields)
  *
  * The SB_LOCK must be held when this routine is called.
  */
-STATIC int
+int
 xfs_mod_incore_sb_unlocked(xfs_mount_t *mp, xfs_sb_field_t field,
                         int delta, int rsvd)
 {
@@ -1406,9 +1437,26 @@ xfs_mod_incore_sb(xfs_mount_t *mp, xfs_sb_field_t field, int delta, int rsvd)
         unsigned long   s;
         int     status;
 
-        s = XFS_SB_LOCK(mp);
-        status = xfs_mod_incore_sb_unlocked(mp, field, delta, rsvd);
-        XFS_SB_UNLOCK(mp, s);
+        /* check for per-cpu counters */
+        switch (field) {
+#ifdef HAVE_PERCPU_SB
+        case XFS_SBS_ICOUNT:
+        case XFS_SBS_IFREE:
+        case XFS_SBS_FDBLOCKS:
+                if (!(mp->m_flags & XFS_MOUNT_NO_PERCPU_SB)) {
+                        status = xfs_icsb_modify_counters(mp, field,
+                                                        delta, rsvd);
+                        break;
+                }
+                /* FALLTHROUGH */
+#endif
+        default:
+                s = XFS_SB_LOCK(mp);
+                status = xfs_mod_incore_sb_unlocked(mp, field, delta, rsvd);
+                XFS_SB_UNLOCK(mp, s);
+                break;
+        }
+
         return status;
 }
 
@@ -1445,8 +1493,26 @@ xfs_mod_incore_sb_batch(xfs_mount_t *mp, xfs_mod_sb_t *msb, uint nmsb, int rsvd)
                  * from the loop so we'll fall into the undo loop
                  * below.
                  */
-                status = xfs_mod_incore_sb_unlocked(mp, msbp->msb_field,
-                                                    msbp->msb_delta, rsvd);
+                switch (msbp->msb_field) {
+#ifdef HAVE_PERCPU_SB
+                case XFS_SBS_ICOUNT:
+                case XFS_SBS_IFREE:
+                case XFS_SBS_FDBLOCKS:
+                        if (!(mp->m_flags & XFS_MOUNT_NO_PERCPU_SB)) {
+                                status = xfs_icsb_modify_counters_locked(mp,
+                                                        msbp->msb_field,
+                                                        msbp->msb_delta, rsvd);
+                                break;
+                        }
+                        /* FALLTHROUGH */
+#endif
+                default:
+                        status = xfs_mod_incore_sb_unlocked(mp,
+                                                msbp->msb_field,
+                                                msbp->msb_delta, rsvd);
+                        break;
+                }
+
                 if (status != 0) {
                         break;
                 }
@@ -1463,8 +1529,28 @@ xfs_mod_incore_sb_batch(xfs_mount_t *mp, xfs_mod_sb_t *msb, uint nmsb, int rsvd)
         if (status != 0) {
                 msbp--;
                 while (msbp >= msb) {
-                        status = xfs_mod_incore_sb_unlocked(mp,
-                                    msbp->msb_field, -(msbp->msb_delta), rsvd);
+                        switch (msbp->msb_field) {
+#ifdef HAVE_PERCPU_SB
+                        case XFS_SBS_ICOUNT:
+                        case XFS_SBS_IFREE:
+                        case XFS_SBS_FDBLOCKS:
+                                if (!(mp->m_flags & XFS_MOUNT_NO_PERCPU_SB)) {
+                                        status =
+                                            xfs_icsb_modify_counters_locked(mp,
+                                                        msbp->msb_field,
+                                                        -(msbp->msb_delta),
+                                                        rsvd);
+                                        break;
+                                }
+                                /* FALLTHROUGH */
+#endif
+                        default:
+                                status = xfs_mod_incore_sb_unlocked(mp,
+                                                        msbp->msb_field,
+                                                        -(msbp->msb_delta),
+                                                        rsvd);
+                                break;
+                        }
                         ASSERT(status == 0);
                         msbp--;
                 }
@@ -1577,3 +1663,445 @@ xfs_mount_log_sbunit(
         xfs_mod_sb(tp, fields);
         xfs_trans_commit(tp, 0, NULL);
 }
+
+
+#ifdef HAVE_PERCPU_SB
+/*
+ * Per-cpu incore superblock counters
+ *
+ * Simple concept, difficult implementation
+ *
+ * Basically, replace the incore superblock counters with a distributed per cpu
+ * counter for contended fields (e.g.  free block count).
+ *
+ * Difficulties arise in that the incore sb is used for ENOSPC checking, and
+ * hence needs to be accurately read when we are running low on space. Hence
+ * there is a method to enable and disable the per-cpu counters based on how
+ * much "stuff" is available in them.
+ *
+ * Basically, a counter is enabled if there is enough free resource to justify
+ * running a per-cpu fast-path. If the per-cpu counter runs out (i.e. a local
+ * ENOSPC), then we disable the counters to synchronise all callers and
+ * re-distribute the available resources.
+ *
+ * If, once we redistributed the available resources, we still get a failure,
+ * we disable the per-cpu counter and go through the slow path.
+ *
+ * The slow path is the current xfs_mod_incore_sb() function.  This means that
+ * when we disable a per-cpu counter, we need to drain it's resources back to
+ * the global superblock. We do this after disabling the counter to prevent
+ * more threads from queueing up on the counter.
+ *
+ * Essentially, this means that we still need a lock in the fast path to enable
+ * synchronisation between the global counters and the per-cpu counters. This
+ * is not a problem because the lock will be local to a CPU almost all the time
+ * and have little contention except when we get to ENOSPC conditions.
+ *
+ * Basically, this lock becomes a barrier that enables us to lock out the fast
+ * path while we do things like enabling and disabling counters and
+ * synchronising the counters.
+ *
+ * Locking rules:
+ *
+ *      1. XFS_SB_LOCK() before picking up per-cpu locks
+ *      2. per-cpu locks always picked up via for_each_online_cpu() order
+ *      3. accurate counter sync requires XFS_SB_LOCK + per cpu locks
+ *      4. modifying per-cpu counters requires holding per-cpu lock
+ *      5. modifying global counters requires holding XFS_SB_LOCK
+ *      6. enabling or disabling a counter requires holding the XFS_SB_LOCK
+ *         and _none_ of the per-cpu locks.
+ *
+ * Disabled counters are only ever re-enabled by a balance operation
+ * that results in more free resources per CPU than a given threshold.
+ * To ensure counters don't remain disabled, they are rebalanced when
+ * the global resource goes above a higher threshold (i.e. some hysteresis
+ * is present to prevent thrashing).
+ *
+ * Note: hotplug CPUs not yet supported
+ */
+int
+xfs_icsb_init_counters(
+        xfs_mount_t     *mp)
+{
+        xfs_icsb_cnts_t *cntp;
+        int             i;
+
+        mp->m_sb_cnts = alloc_percpu(xfs_icsb_cnts_t);
+        if (mp->m_sb_cnts == NULL)
+                return -ENOMEM;
+
+        for_each_online_cpu(i) {
+                cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
+                spin_lock_init(&cntp->icsb_lock);
+        }
+        /*
+         * start with all counters disabled so that the
+         * initial balance kicks us off correctly
+         */
+        mp->m_icsb_counters = -1;
+        return 0;
+}
+
+STATIC void
+xfs_icsb_destroy_counters(
+        xfs_mount_t     *mp)
+{
+        if (mp->m_sb_cnts)
+                free_percpu(mp->m_sb_cnts);
+}
+
+
+STATIC inline void
+xfs_icsb_lock_all_counters(
+        xfs_mount_t     *mp)
+{
+        xfs_icsb_cnts_t *cntp;
+        int             i;
+
+        for_each_online_cpu(i) {
+                cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
+                spin_lock(&cntp->icsb_lock);
+        }
+}
+
+STATIC inline void
+xfs_icsb_unlock_all_counters(
+        xfs_mount_t     *mp)
+{
+        xfs_icsb_cnts_t *cntp;
+        int             i;
+
+        for_each_online_cpu(i) {
+                cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
+                spin_unlock(&cntp->icsb_lock);
+        }
+}
+
+STATIC void
+xfs_icsb_count(
+        xfs_mount_t     *mp,
+        xfs_icsb_cnts_t *cnt,
+        int             flags)
+{
+        xfs_icsb_cnts_t *cntp;
+        int             i;
+
+        memset(cnt, 0, sizeof(xfs_icsb_cnts_t));
+
+        if (!(flags & XFS_ICSB_LAZY_COUNT))
+                xfs_icsb_lock_all_counters(mp);
+
+        for_each_online_cpu(i) {
+                cntp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, i);
+                cnt->icsb_icount += cntp->icsb_icount;
+                cnt->icsb_ifree += cntp->icsb_ifree;
+                cnt->icsb_fdblocks += cntp->icsb_fdblocks;
+        }
+
+        if (!(flags & XFS_ICSB_LAZY_COUNT))
+                xfs_icsb_unlock_all_counters(mp);
+}
+
+STATIC int
+xfs_icsb_counter_disabled(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field)
+{
+        ASSERT((field >= XFS_SBS_ICOUNT) && (field <= XFS_SBS_FDBLOCKS));
+        return test_bit(field, &mp->m_icsb_counters);
+}
+
+STATIC int
+xfs_icsb_disable_counter(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field)
+{
+        xfs_icsb_cnts_t cnt;
+
+        ASSERT((field >= XFS_SBS_ICOUNT) && (field <= XFS_SBS_FDBLOCKS));
+
+        xfs_icsb_lock_all_counters(mp);
+        if (!test_and_set_bit(field, &mp->m_icsb_counters)) {
+                /* drain back to superblock */
+
+                xfs_icsb_count(mp, &cnt, XFS_ICSB_SB_LOCKED|XFS_ICSB_LAZY_COUNT);
+                switch(field) {
+                case XFS_SBS_ICOUNT:
+                        mp->m_sb.sb_icount = cnt.icsb_icount;
+                        break;
+                case XFS_SBS_IFREE:
+                        mp->m_sb.sb_ifree = cnt.icsb_ifree;
+                        break;
+                case XFS_SBS_FDBLOCKS:
+                        mp->m_sb.sb_fdblocks = cnt.icsb_fdblocks;
+                        break;
+                default:
+                        BUG();
+                }
+        }
+
+        xfs_icsb_unlock_all_counters(mp);
+
+        return 0;
+}
+
+STATIC void
+xfs_icsb_enable_counter(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field,
+        uint64_t        count,
+        uint64_t        resid)
+{
+        xfs_icsb_cnts_t *cntp;
+        int             i;
+
+        ASSERT((field >= XFS_SBS_ICOUNT) && (field <= XFS_SBS_FDBLOCKS));
+
+        xfs_icsb_lock_all_counters(mp);
+        for_each_online_cpu(i) {
+                cntp = per_cpu_ptr(mp->m_sb_cnts, i);
+                switch (field) {
+                case XFS_SBS_ICOUNT:
+                        cntp->icsb_icount = count + resid;
+                        break;
+                case XFS_SBS_IFREE:
+                        cntp->icsb_ifree = count + resid;
+                        break;
+                case XFS_SBS_FDBLOCKS:
+                        cntp->icsb_fdblocks = count + resid;
+                        break;
+                default:
+                        BUG();
+                        break;
+                }
+                resid = 0;
+        }
+        clear_bit(field, &mp->m_icsb_counters);
+        xfs_icsb_unlock_all_counters(mp);
+}
+
+STATIC void
+xfs_icsb_sync_counters_int(
+        xfs_mount_t     *mp,
+        int             flags)
+{
+        xfs_icsb_cnts_t cnt;
+        int             s;
+
+        /* Pass 1: lock all counters */
+        if ((flags & XFS_ICSB_SB_LOCKED) == 0)
+                s = XFS_SB_LOCK(mp);
+
+        xfs_icsb_count(mp, &cnt, flags);
+
+        /* Step 3: update mp->m_sb fields */
+        if (!xfs_icsb_counter_disabled(mp, XFS_SBS_ICOUNT))
+                mp->m_sb.sb_icount = cnt.icsb_icount;
+        if (!xfs_icsb_counter_disabled(mp, XFS_SBS_IFREE))
+                mp->m_sb.sb_ifree = cnt.icsb_ifree;
+        if (!xfs_icsb_counter_disabled(mp, XFS_SBS_FDBLOCKS))
+                mp->m_sb.sb_fdblocks = cnt.icsb_fdblocks;
+
+        if ((flags & XFS_ICSB_SB_LOCKED) == 0)
+                XFS_SB_UNLOCK(mp, s);
+}
+
+/*
+ * Accurate update of per-cpu counters to incore superblock
+ */
+STATIC void
+xfs_icsb_sync_counters(
+        xfs_mount_t     *mp)
+{
+        xfs_icsb_sync_counters_int(mp, 0);
+}
+
+/*
+ * lazy addition used for things like df, background sb syncs, etc
+ */
+void
+xfs_icsb_sync_counters_lazy(
+        xfs_mount_t     *mp)
+{
+        xfs_icsb_sync_counters_int(mp, XFS_ICSB_LAZY_COUNT);
+}
+
+/*
+ * Balance and enable/disable counters as necessary.
+ *
+ * Thresholds for re-enabling counters are somewhat magic.
+ * inode counts are chosen to be the same number as single
+ * on disk allocation chunk per CPU, and free blocks is
+ * something far enough zero that we aren't going thrash
+ * when we get near ENOSPC.
+ */
+#define XFS_ICSB_INO_CNTR_REENABLE      64
+#define XFS_ICSB_FDBLK_CNTR_REENABLE    512
+STATIC void
+xfs_icsb_balance_counter(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field,
+        int             flags)
+{
+        uint64_t        count, resid = 0;
+        int             weight = num_online_cpus();
+        int             s;
+
+        if (!(flags & XFS_ICSB_SB_LOCKED))
+                s = XFS_SB_LOCK(mp);
+
+        /* disable counter and sync counter */
+        xfs_icsb_disable_counter(mp, field);
+
+        /* update counters  - first CPU gets residual*/
+        switch (field) {
+        case XFS_SBS_ICOUNT:
+                count = mp->m_sb.sb_icount;
+                resid = do_div(count, weight);
+                if (count < XFS_ICSB_INO_CNTR_REENABLE)
+                        goto out;
+                break;
+        case XFS_SBS_IFREE:
+                count = mp->m_sb.sb_ifree;
+                resid = do_div(count, weight);
+                if (count < XFS_ICSB_INO_CNTR_REENABLE)
+                        goto out;
+                break;
+        case XFS_SBS_FDBLOCKS:
+                count = mp->m_sb.sb_fdblocks;
+                resid = do_div(count, weight);
+                if (count < XFS_ICSB_FDBLK_CNTR_REENABLE)
+                        goto out;
+                break;
+        default:
+                BUG();
+                break;
+        }
+
+        xfs_icsb_enable_counter(mp, field, count, resid);
+out:
+        if (!(flags & XFS_ICSB_SB_LOCKED))
+                XFS_SB_UNLOCK(mp, s);
+}
+
+STATIC int
+xfs_icsb_modify_counters_int(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field,
+        int             delta,
+        int             rsvd,
+        int             flags)
+{
+        xfs_icsb_cnts_t *icsbp;
+        long long       lcounter;       /* long counter for 64 bit fields */
+        int             cpu, s, locked = 0;
+        int             ret = 0, balance_done = 0;
+
+again:
+        cpu = get_cpu();
+        icsbp = (xfs_icsb_cnts_t *)per_cpu_ptr(mp->m_sb_cnts, cpu),
+        spin_lock(&icsbp->icsb_lock);
+        if (unlikely(xfs_icsb_counter_disabled(mp, field)))
+                goto slow_path;
+
+        switch (field) {
+        case XFS_SBS_ICOUNT:
+                lcounter = icsbp->icsb_icount;
+                lcounter += delta;
+                if (unlikely(lcounter < 0))
+                        goto slow_path;
+                icsbp->icsb_icount = lcounter;
+                break;
+
+        case XFS_SBS_IFREE:
+                lcounter = icsbp->icsb_ifree;
+                lcounter += delta;
+                if (unlikely(lcounter < 0))
+                        goto slow_path;
+                icsbp->icsb_ifree = lcounter;
+                break;
+
+        case XFS_SBS_FDBLOCKS:
+                BUG_ON((mp->m_resblks - mp->m_resblks_avail) != 0);
+
+                lcounter = icsbp->icsb_fdblocks;
+                lcounter += delta;
+                if (unlikely(lcounter < 0))
+                        goto slow_path;
+                icsbp->icsb_fdblocks = lcounter;
+                break;
+        default:
+                BUG();
+                break;
+        }
+        spin_unlock(&icsbp->icsb_lock);
+        put_cpu();
+        if (locked)
+                XFS_SB_UNLOCK(mp, s);
+        return 0;
+
+        /*
+         * The slow path needs to be run with the SBLOCK
+         * held so that we prevent other threads from
+         * attempting to run this path at the same time.
+         * this provides exclusion for the balancing code,
+         * and exclusive fallback if the balance does not
+         * provide enough resources to continue in an unlocked
+         * manner.
+         */
+slow_path:
+        spin_unlock(&icsbp->icsb_lock);
+        put_cpu();
+
+        /* need to hold superblock incase we need
+         * to disable a counter */
+        if (!(flags & XFS_ICSB_SB_LOCKED)) {
+                s = XFS_SB_LOCK(mp);
+                locked = 1;
+                flags |= XFS_ICSB_SB_LOCKED;
+        }
+        if (!balance_done) {
+                xfs_icsb_balance_counter(mp, field, flags);
+                balance_done = 1;
+                goto again;
+        } else {
+                /*
+                 * we might not have enough on this local
+                 * cpu to allocate for a bulk request.
+                 * We need to drain this field from all CPUs
+                 * and disable the counter fastpath
+                 */
+                xfs_icsb_disable_counter(mp, field);
+        }
+
+        ret = xfs_mod_incore_sb_unlocked(mp, field, delta, rsvd);
+
+        if (locked)
+                XFS_SB_UNLOCK(mp, s);
+        return ret;
+}
+
+STATIC int
+xfs_icsb_modify_counters(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field,
+        int             delta,
+        int             rsvd)
+{
+        return xfs_icsb_modify_counters_int(mp, field, delta, rsvd, 0);
+}
+
+/*
+ * Called when superblock is already locked
+ */
+STATIC int
+xfs_icsb_modify_counters_locked(
+        xfs_mount_t     *mp,
+        xfs_sb_field_t  field,
+        int             delta,
+        int             rsvd)
+{
+        return xfs_icsb_modify_counters_int(mp, field, delta,
+                                                rsvd, XFS_ICSB_SB_LOCKED);
+}
+#endif