Add a WQ flag that allows workqueues to use SCHED_FIFO with the least imporant RT priority. This can reduce scheduler latency for IO post-processing when the CPU is under load without impacting other RT workloads. This has been shown to improve app startup time on Android [1]. Scheduler latency affects several drivers as evidenced by [1], [2], [3], [4]. Some of these drivers have moved post-processing into IRQ context. However, this can cause latency spikes for real-time threads and jitter related jank on Android. Using a workqueue with SCHED_FIFO improves scheduler latency without causing latency problems for RT threads. [1]: https://lore.kernel.org/linux-erofs/20230106073502.4017276-1-dhavale@xxxxxxxxxx/ [2]: https://lore.kernel.org/linux-f2fs-devel/20220802192437.1895492-1-daeho43@xxxxxxxxx/ [3]: https://lore.kernel.org/dm-devel/20220722093823.4158756-4-nhuck@xxxxxxxxxx/ [4]: https://lore.kernel.org/dm-crypt/20200706173731.3734-1-ignat@xxxxxxxxxxxxxx/ This change has been tested on dm-verity with the following fio config: [global] time_based runtime=120 [do-verify] ioengine=sync filename=/dev/testing rw=randread direct=1 [burn_8x90%_qsort] ioengine=cpuio cpuload=90 numjobs=8 cpumode=qsort Before: clat (usec): min=13, max=23882, avg=29.56, stdev=113.29 READ: bw=122MiB/s (128MB/s), 122MiB/s-122MiB/s (128MB/s-128MB/s), io=14.3GiB (15.3GB), run=120001-120001msec After: clat (usec): min=13, max=23137, avg=19.96, stdev=105.71 READ: bw=180MiB/s (189MB/s), 180MiB/s-180MiB/s (189MB/s-189MB/s), io=21.1GiB (22.7GB), run=120012-120012msec Cc: Sandeep Dhavale <dhavale@xxxxxxxxxx> Cc: Daeho Jeong <daehojeong@xxxxxxxxxx> Cc: Eric Biggers <ebiggers@xxxxxxxxxx> Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx> Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx> --- Documentation/core-api/workqueue.rst | 12 ++++++++++ include/linux/workqueue.h | 9 +++++++ kernel/workqueue.c | 36 +++++++++++++++++++++------- 3 files changed, 48 insertions(+), 9 deletions(-) diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst index 3b22ed137662..26faf2806c66 100644 --- a/Documentation/core-api/workqueue.rst +++ b/Documentation/core-api/workqueue.rst @@ -216,6 +216,18 @@ resources, scheduled and executed. This flag is meaningless for unbound wq. +``WQ_SCHED_FIFO`` + Work items of a fifo wq are queued to the fifo + worker-pool of the target cpu. Fifo worker-pools are + served by worker threads with scheduler policy SCHED_FIFO and + the least important real-time priority. This can be useful + for workloads where low latency is imporant. + + A workqueue cannot be both high-priority and fifo. + + Note that normal and fifo worker-pools don't interact with + each other. Each maintains its separate pool of workers and + implements concurrency management among its workers. ``max_active`` -------------- diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h index ac551b8ee7d9..43a4eeaf8ff4 100644 --- a/include/linux/workqueue.h +++ b/include/linux/workqueue.h @@ -134,6 +134,10 @@ struct workqueue_attrs { * @nice: nice level */ int nice; + /** + * @sched_fifo: is using SCHED_FIFO + */ + bool sched_fifo; /** * @cpumask: allowed CPUs @@ -334,6 +338,11 @@ enum { * http://thread.gmane.org/gmane.linux.kernel/1480396 */ WQ_POWER_EFFICIENT = 1 << 7, + /* + * Low real-time priority workqueues can reduce scheduler latency + * for latency sensitive workloads like IO post-processing. + */ + WQ_SCHED_FIFO = 1 << 8, __WQ_DESTROYING = 1 << 15, /* internal: workqueue is destroying */ __WQ_DRAINING = 1 << 16, /* internal: workqueue is draining */ diff --git a/kernel/workqueue.c b/kernel/workqueue.c index 5dc67aa9d696..99c5e0a3dc28 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -85,7 +85,7 @@ enum { WORKER_NOT_RUNNING = WORKER_PREP | WORKER_CPU_INTENSIVE | WORKER_UNBOUND | WORKER_REBOUND, - NR_STD_WORKER_POOLS = 2, /* # standard pools per cpu */ + NR_STD_WORKER_POOLS = 3, /* # standard pools per cpu */ UNBOUND_POOL_HASH_ORDER = 6, /* hashed by pool->attrs */ BUSY_WORKER_HASH_ORDER = 6, /* 64 pointers */ @@ -1949,7 +1949,8 @@ static struct worker *create_worker(struct worker_pool *pool) if (pool->cpu >= 0) snprintf(id_buf, sizeof(id_buf), "%d:%d%s", pool->cpu, id, - pool->attrs->nice < 0 ? "H" : ""); + pool->attrs->sched_fifo ? "F" : + (pool->attrs->nice < 0 ? "H" : "")); else snprintf(id_buf, sizeof(id_buf), "u%d:%d", pool->id, id); @@ -1958,7 +1959,11 @@ static struct worker *create_worker(struct worker_pool *pool) if (IS_ERR(worker->task)) goto fail; - set_user_nice(worker->task, pool->attrs->nice); + if (pool->attrs->sched_fifo) + sched_set_fifo_low(worker->task); + else + set_user_nice(worker->task, pool->attrs->nice); + kthread_bind_mask(worker->task, pool->attrs->cpumask); /* successful, attach the worker to the pool */ @@ -4323,9 +4328,17 @@ static void wq_update_unbound_numa(struct workqueue_struct *wq, int cpu, static int alloc_and_link_pwqs(struct workqueue_struct *wq) { - bool highpri = wq->flags & WQ_HIGHPRI; + int pool_index = 0; int cpu, ret; + if (wq->flags & WQ_HIGHPRI && wq->flags & WQ_SCHED_FIFO) + return -EINVAL; + + if (wq->flags & WQ_HIGHPRI) + pool_index = 1; + if (wq->flags & WQ_SCHED_FIFO) + pool_index = 2; + if (!(wq->flags & WQ_UNBOUND)) { wq->cpu_pwqs = alloc_percpu(struct pool_workqueue); if (!wq->cpu_pwqs) @@ -4337,7 +4350,7 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq) struct worker_pool *cpu_pools = per_cpu(cpu_worker_pools, cpu); - init_pwq(pwq, wq, &cpu_pools[highpri]); + init_pwq(pwq, wq, &cpu_pools[pool_index]); mutex_lock(&wq->mutex); link_pwq(pwq); @@ -4348,13 +4361,13 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq) cpus_read_lock(); if (wq->flags & __WQ_ORDERED) { - ret = apply_workqueue_attrs(wq, ordered_wq_attrs[highpri]); + ret = apply_workqueue_attrs(wq, ordered_wq_attrs[pool_index]); /* there should only be single pwq for ordering guarantee */ WARN(!ret && (wq->pwqs.next != &wq->dfl_pwq->pwqs_node || wq->pwqs.prev != &wq->dfl_pwq->pwqs_node), "ordering guarantee broken for workqueue %s\n", wq->name); } else { - ret = apply_workqueue_attrs(wq, unbound_std_wq_attrs[highpri]); + ret = apply_workqueue_attrs(wq, unbound_std_wq_attrs[pool_index]); } cpus_read_unlock(); @@ -6138,7 +6151,8 @@ static void __init wq_numa_init(void) */ void __init workqueue_init_early(void) { - int std_nice[NR_STD_WORKER_POOLS] = { 0, HIGHPRI_NICE_LEVEL }; + int std_nice[NR_STD_WORKER_POOLS] = { 0, HIGHPRI_NICE_LEVEL, 0 }; + bool std_sched_fifo[NR_STD_WORKER_POOLS] = { false, false, true }; int i, cpu; BUILD_BUG_ON(__alignof__(struct pool_workqueue) < __alignof__(long long)); @@ -6158,8 +6172,10 @@ void __init workqueue_init_early(void) BUG_ON(init_worker_pool(pool)); pool->cpu = cpu; cpumask_copy(pool->attrs->cpumask, cpumask_of(cpu)); - pool->attrs->nice = std_nice[i++]; + pool->attrs->nice = std_nice[i]; + pool->attrs->sched_fifo = std_sched_fifo[i]; pool->node = cpu_to_node(cpu); + i++; /* alloc pool ID */ mutex_lock(&wq_pool_mutex); @@ -6174,6 +6190,7 @@ void __init workqueue_init_early(void) BUG_ON(!(attrs = alloc_workqueue_attrs())); attrs->nice = std_nice[i]; + attrs->sched_fifo = std_sched_fifo[i]; unbound_std_wq_attrs[i] = attrs; /* @@ -6183,6 +6200,7 @@ void __init workqueue_init_early(void) */ BUG_ON(!(attrs = alloc_workqueue_attrs())); attrs->nice = std_nice[i]; + attrs->sched_fifo = std_sched_fifo[i]; attrs->no_numa = true; ordered_wq_attrs[i] = attrs; } -- 2.39.0.314.g84b9a713c41-goog
