xfs: only run xfs_error_test if error injection is active

Recent tests writing lots of small files showed the flusher thread
being CPU bound and taking a long time to do allocations on a debug
kernel. perf showed this as the prime reason:

             samples  pcnt function                    DSO
             _______ _____ ___________________________ _________________

           224648.00 36.8% xfs_error_test              [kernel.kallsyms]
            86045.00 14.1% xfs_btree_check_sblock      [kernel.kallsyms]
            39778.00  6.5% prandom32                   [kernel.kallsyms]
            37436.00  6.1% xfs_btree_increment         [kernel.kallsyms]
            29278.00  4.8% xfs_btree_get_rec           [kernel.kallsyms]
            27717.00  4.5% random32                    [kernel.kallsyms]

Walking btree blocks during allocation checking them requires each
block (a cache hit, so no I/O) call xfs_error_test(), which then
does a random32() call as the first operation.  IOWs, ~50% of the
CPU is being consumed just testing whether we need to inject an
error, even though error injection is not active.

Kill this overhead when error injection is not active by adding a
global counter of active error traps and only calling into
xfs_error_test when fault injection is active.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/xfs/xfs_error.c b/fs/xfs/xfs_error.c
index ed9990267661..c78cc6a3d87c 100644
--- a/fs/xfs/xfs_error.c
+++ b/fs/xfs/xfs_error.c
@@ -58,6 +58,7 @@ xfs_error_trap(int e)
 int     xfs_etest[XFS_NUM_INJECT_ERROR];
 int64_t xfs_etest_fsid[XFS_NUM_INJECT_ERROR];
 char *  xfs_etest_fsname[XFS_NUM_INJECT_ERROR];
+int     xfs_error_test_active;
 
 int
 xfs_error_test(int error_tag, int *fsidp, char *expression,
@@ -108,6 +109,7 @@ xfs_errortag_add(int error_tag, xfs_mount_t *mp)
                         len = strlen(mp->m_fsname);
                         xfs_etest_fsname[i] = kmem_alloc(len + 1, KM_SLEEP);
                         strcpy(xfs_etest_fsname[i], mp->m_fsname);
+                        xfs_error_test_active++;
                         return 0;
                 }
         }
@@ -137,6 +139,7 @@ xfs_errortag_clearall(xfs_mount_t *mp, int loud)
                         xfs_etest_fsid[i] = 0LL;
                         kmem_free(xfs_etest_fsname[i]);
                         xfs_etest_fsname[i] = NULL;
+                        xfs_error_test_active--;
                 }
         }