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[PATCH 5/5] xfs: only run xfs_error_test if error injection is active

To: xfs@xxxxxxxxxxx
Subject: [PATCH 5/5] xfs: only run xfs_error_test if error injection is active
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 29 Nov 2010 11:20:24 +1100
In-reply-to: <1290990024-15898-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1290990024-15898-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 fs/xfs/xfs_error.c |    3 +++
 fs/xfs/xfs_error.h |    5 +++--
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/xfs/xfs_error.c b/fs/xfs/xfs_error.c
index ed99902..c78cc6a 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--;
                }
        }
 
diff --git a/fs/xfs/xfs_error.h b/fs/xfs/xfs_error.h
index c2c1a07..f338847 100644
--- a/fs/xfs/xfs_error.h
+++ b/fs/xfs/xfs_error.h
@@ -127,13 +127,14 @@ extern void xfs_corruption_error(const char *tag, int 
level,
 #define        XFS_RANDOM_BMAPIFORMAT                          
XFS_RANDOM_DEFAULT
 
 #ifdef DEBUG
+extern int xfs_error_test_active;
 extern int xfs_error_test(int, int *, char *, int, char *, unsigned long);
 
 #define        XFS_NUM_INJECT_ERROR                            10
 #define XFS_TEST_ERROR(expr, mp, tag, rf)              \
-       ((expr) || \
+       ((expr) || (xfs_error_test_active && \
         xfs_error_test((tag), (mp)->m_fixedfsid, "expr", __LINE__, __FILE__, \
-                       (rf)))
+                       (rf))))
 
 extern int xfs_errortag_add(int error_tag, xfs_mount_t *mp);
 extern int xfs_errortag_clearall(xfs_mount_t *mp, int loud);
-- 
1.7.2.3

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