[PATCH BUGFIX V2 0/3] null_blk: fix throughput losses and hangs

[Paolo Valente]

Hi,
here is an updated version of the patchset, differing from the
previous version only in that it reinstates the missing extra check
pointed out in [2]. For your convenience, the content of the cover
letter for the previous version follows.

While doing some tests with the null_blk device driver, we bumped into
two problems: first, unjustified and in some cases high throughput
losses; second, actual hangs. These problems seem to be the
consequence of the combination of three causes, and this patchset
introduces a fix for each of these causes. In particular, changes
address:
. an apparent flaw in the logic with which delayed completions are
  implemented: this flaw causes, with unlucky but non-pathological
  workloads, actual request-completion delays to become arbitrarily
  larger than the configured delay;
. the missing restart of the device queue on the completion of a request in
  single-queue non-delayed mode;
. the overflow of the request-delay parameter, when extremely high values
  are used (e.g., to spot bugs).

To avoid possible confusion, we stress that these fixes *do not* have
anything to do with the problems highlighted in [1] (tests of the
multiqueue xen-blkfront and xen-blkback modules with null_blk).

You can find more details in the patch descriptions.

Thanks,
Paolo and Arianna

[1] https://lkml.org/lkml/2015/8/19/181
[2] https://lkml.org/lkml/2015/11/2/433

Arianna Avanzini (2):
  null_blk: guarantee device restart in all irq modes
  null_blk: change type of completion_nsec to unsigned long

Paolo Valente (1):
  null_blk: set a separate timer for each command

 drivers/block/null_blk.c | 94 +++++++++++++++++-------------------------------
 1 file changed, 33 insertions(+), 61 deletions(-)

-- 
1.9.1

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