Currently, WQ_HIGHPRI workqueues share the same worker pool as the normal priority ones. The only difference is that work items from highpri wq are queued at the head instead of tail of the worklist. On pathological cases, this simplistics highpri implementation doesn't seem to be sufficient.
For example, block layer request_queue delayed processing uses high priority delayed_work to restart request processing after a short delay. Unfortunately, it doesn't seem to take too much to push the latency between the delay timer expiring and the work item execution to few second range leading to unintended long idling of the underlying device. There seem to be real-world cases where this latency shows up[1]. A simplistic test case is measuring queue-to-execution latencies with a lot of threads saturating CPU cycles. Measuring over 300sec period with 3000 0-nice threads performing 1ms sleeps continuously and a highpri work item being repeatedly queued with 1 jiffy interval on a single CPU machine, the top latency was 1624ms and the average of top 20 was 1268ms with stdev 927ms. This patchset reimplements high priority workqueues so that it uses a separate worklist and worker pool. Now each global_cwq contains two worker_pools - one for normal priority work items and the other for high priority. Each has its own worklist and worker pool and the highpri worker pool is populated with worker threads w/ -20 nice value. This reimplementation brings down the top latency to 16ms with top 20 average of 3.8ms w/ stdev 5.6ms. The original block layer bug hasn't been verfieid to be fixed yet (Josh?). The addition of separate worker pools doesn't add much to the complexity but does add more threads per cpu. Highpri worker pool is expected to remain small, but the effect is noticeable especially in idle states. I'm cc'ing all WQ_HIGHPRI users - block, bio-integrity, crypto, gfs2, xfs and bluetooth. Now you guys get proper high priority scheduling for highpri work items; however, with more power comes more responsibility. Especially, the ones with both WQ_HIGHPRI and WQ_CPU_INTENSIVE - bio-integrity and crypto - may end up dominating CPU usage. I think it should be mostly okay for bio-integrity considering it sits right in the block request completion path. I don't know enough about tegra-aes tho. aes_workqueue_handler() seems to mostly interact with the hardware crypto. Is it actually cpu cycle intensive? This patchset contains the following six patches. 0001-workqueue-don-t-use-WQ_HIGHPRI-for-unbound-workqueue.patch 0002-workqueue-factor-out-worker_pool-from-global_cwq.patch 0003-workqueue-use-pool-instead-of-gcwq-or-cpu-where-appl.patch 0004-workqueue-separate-out-worker_pool-flags.patch 0005-workqueue-introduce-NR_WORKER_POOLS-and-for_each_wor.patch 0006-workqueue-reimplement-WQ_HIGHPRI-using-a-separate-wo.patch 0001 makes unbound wq not use WQ_HIGHPRI as its meaning will be changing and won't suit the purpose unbound wq is using it for. 0002-0005 gradually pulls out worker_pool from global_cwq and update code paths to be able to deal with multiple worker_pools per global_cwq. 0006 replaces the head-queueing WQ_HIGHPRI implementation with the one with separate worker_pool using the multiple worker_pool mechanism previously implemented. The patchset is available in the following git branch. git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git review-wq-highpri diffstat follows. Documentation/workqueue.txt | 103 ++---- include/trace/events/workqueue.h | 2 kernel/workqueue.c | 624 +++++++++++++++++++++------------------ 3 files changed, 385 insertions(+), 344 deletions(-) Thanks. -- tejun [1] https://lkml.org/lkml/2012/3/6/475 -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
