OpenAFS Master Repository branch, openafs-stable-1_6_x, updated. openafs-stable-1_6_20_2-5-g5757a0d

[Gerrit Code Review]

The following commit has been merged in the openafs-stable-1_6_x branch:
commit 5757a0dc3debb6b1d7e47faee18373a2866e4b03
Author: Michael Meffie <mmeffie@sinenomine.net>
Date:   Thu Aug 27 13:06:05 2015 -0400

    afs: shake harder in shake-loose-vcaches
    
    Linux based cache managers will allocate vcaches on demand and
    deallocate batches of vcaches in the background. This feature is called
    dynamic vcaches.
    
    Vcaches to be deallocated are found by traversing the vcache LRU list
    (VLRU) from the oldest vcache to the newest. Up to a target number of
    vcaches are attempted to be evicted.  The afs_xvcache lock protecting
    the VLRU may be dropped and re-acquired while attempting to evict a
    vcache. When this happens, it is possible the VLRU may have changed, so
    the traversal of the VLRU is restarted.  This restarting of the VLRU
    transversal is limited to 100 iterations to avoid looping indefinitely.
    
    Vcaches which are busy cannot be evicted and remain in the VLRU. When a
    busy cache was not evicted and the afs_xvache lock was dropped, the VLRU
    traversal is restarted from the end of the VLRU. When the busy vcache is
    encountered on the retry, it will trigger additional retries until the
    loop limit is reached, at which point the target number of vcaches will
    not be deallocated.
    
    This can leave a very large number of unbusy vcaches which are never
    deallocated.  On a busy machine, tens of millions of unused vcaches can
    remain in memory. When the busy vcache at the end of the VLRU is finally
    evicted, the log jam is broken, and the background deamon will hold the
    afs_xvcache lock for an excessively long time, hanging the system.
    
    Fix this by moving busy vcaches to the head of the VLRU before
    restarting the VLRU traversal. These busy vcaches will be skipped when
    retrying the VLRU traversal, allowing the cache manager to make progress
    deallocating vcaches down to the target level.
    
    This was already done on the mac osx platform while attempting to evict
    vcaches. Move the code to move busy vcaches to the head of the VLRU up
    the the platform agnostic caller.
    
    Thanks to Andrew Deason for the initial version of this patch.
    
    Reviewed-on: https://gerrit.openafs.org/11654
    Tested-by: BuildBot <buildbot@rampaginggeek.com>
    Reviewed-by: Andrew Deason <adeason@dson.org>
    Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
    (cherry picked from commit 5c136c7d93ed97166f39bf716cc7f5d579b70677)
    
    Change-Id: If60b1889d012a739aa5b43e842abb80a6ebfdb6a
    Reviewed-on: https://gerrit.openafs.org/12451
    Tested-by: BuildBot <buildbot@rampaginggeek.com>
    Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
    Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
    Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>

 src/afs/DARWIN/osi_vcache.c |    5 +----
 src/afs/afs_vcache.c        |   14 +++++++++++++-
 2 files changed, 14 insertions(+), 5 deletions(-)

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