RFC percpu_tags: Prototype implementation

Hello Gentleman,

This is a development of "percpu_ida" library. I named it "percpu_tags" to simplify review, since most of "percpu_ida" code has gone and a diff would not be informative.

While the concept of per-cpu arrays is is preserved, the implementation is heavily reworked. The main objective is to improve the "percpu_ida" locking scheme.

Here is the list of major differrences between "percpu_ida" and "percpu_tags":

* The global freelist has gone. As result, CPUs do not compete for the global lock.

* Long-running operatons (scanning thru a cpumask) are executed with local interrupts enabled;

* percpu_ida::percpu_max_size limit is eliminated. Instead, the limit is determined dynamically. Depending from how many CPUs are requesting tags each CPU gets a fair share of the tag space;

* A tag is attempted to return to the CPU it was allocated on. As result, it is expected the locality of data associated with the tag benefits;

The code is largely raw and untested. The reason I am posting is the prototype implementation performs 2-3 times faster than "percpu_ida", so I would like to ensure if it worth going further with this approach or is there a no-go.

The performance test is not decent, though. I used "fio" random read against a "null_blk" device sitting on top of "percpu_tags", which is not exactly how "percpu_ida" is used. This is another reason I am posting - an advice on how to properly test is very appreciated.

The source code could be found at
https://github.com/a-gordeev/linux.git percpu_tags-v0

Thanks!

Alexander Gordeev (2):
percpu_tags: Prototype implementation
percpu_tags: Use percpu_tags instead of percpu_ida

 drivers/scsi/qla2xxx/qla_target.c        |    6 +-
 drivers/target/iscsi/iscsi_target_util.c |    6 +-
 drivers/target/target_core_transport.c   |    4 +-
 drivers/target/tcm_fc/tfc_cmd.c          |    8 +-
 drivers/vhost/scsi.c                     |    6 +-
 include/linux/percpu_tags.h              |   37 ++
 include/target/target_core_base.h        |    4 +-
 lib/Makefile                             |    2 +-
 lib/percpu_tags.c                        |  556 ++++++++++++++++++++++++++++++
 9 files changed, 611 insertions(+), 18 deletions(-)
 create mode 100644 include/linux/percpu_tags.h
 create mode 100644 lib/percpu_tags.c

1.7.7.6