Hello, Over the past several months, percpu_ref grew use cases where it's used as a persistent on/off switch which may be cycled multiple times using percpu_ref_reinit(). One of such use cases is blk-mq's mq_usage_counter which tracks the number of in-flight commands and is used to drain them. Unfortunately, SCSI device probing involves synchronously creating and destroying request_queues for non-existent devices and the sched RCU grace period involved in percpu_ref killing adds upto a significant amount of latency.
Block layer already experienced the same issue in other areas and works around it by starting the queue in a degraded mode which is faster to shut down and making it fully functional only after it's known that the queue isn't a temporary one for probing. This patchset implements percpu_ref mechanisms to manually switch between atomic and percpu operation modes so that blk-mq can implement a similar degraded operation mode. This will also allow implementing debug mode for percpu_ref so that underflow can be detected sooner. This patchset contains the following nine patches. 0001-percpu_ref-relocate-percpu_ref_reinit.patch 0002-percpu_ref-minor-code-and-comment-updates.patch 0003-percpu_ref-replace-pcpu_-prefix-with-percpu_.patch 0004-percpu_ref-rename-things-to-prepare-for-decoupling-p.patch 0005-percpu_ref-add-PCPU_REF_DEAD.patch 0006-percpu_ref-decouple-switching-to-atomic-mode-and-kil.patch 0007-percpu_ref-decouple-switching-to-percpu-mode-and-rei.patch 0008-percpu_ref-add-PERCPU_REF_INIT_-flags.patch 0009-percpu_ref-make-INIT_ATOMIC-and-switch_to_atomic-sti.patch 0001-0005 are prep patches. 0006-0007 implement percpu_ref_switch_to_atomic/percpu(). 0008 extends percpu_ref_init() so that a percpu_ref can be initialized in different states including atomic mode. 0009 makes atomic mode sticky so that it survives through reinits. This patchset is on top of percpu/for-3.18 and available in the following git branch. git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu.git review-percpu_ref-switch diffstat follows. block/blk-mq.c | 2 fs/aio.c | 4 include/linux/percpu-refcount.h | 108 +++++++++----- kernel/cgroup.c | 7 lib/percpu-refcount.c | 291 +++++++++++++++++++++++++++++----------- 5 files changed, 295 insertions(+), 117 deletions(-) Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
