[PATCH] workqueue: Add WQ_SCHED_FIFO
Gao Xiang
hsiangkao at linux.alibaba.com
Sat Jan 14 13:19:50 AEDT 2023
Hi Nathan!
On 2023/1/14 05:07, Nathan Huckleberry wrote:
> 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].
Thank you all for your effort on this. Unfortunately I have no time to
setup the test [1] until now. If it can be addressed as a new workqueue
feature, that would be much helpful to me. Otherwise, I still need to
find a way to resolve the latest Android + EROFS latency problem.
>
> 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.
softirq context is actually mainly for post-interrupt handling I think.
but considering decompression/verification/decryption all workload are much
complex than that and less important than real post-interrupt handling.
I don't think softirq context is the best place to handle these
CPU-intensive jobs. Beside, it could cause some important work moving to
softirqd unexpectedly in the extreme cases. Also such many post-processing
jobs are as complex as they could sleep so that softirq context is
unsuitable as well.
Anyway, I second this proposal if possible:
Acked-by: Gao Xiang <hsiangkao at linux.alibaba.com>
Thanks,
Gao Xiang
>
> [1]:
> https://lore.kernel.org/linux-erofs/20230106073502.4017276-1-dhavale@google.com/
> [2]:
> https://lore.kernel.org/linux-f2fs-devel/20220802192437.1895492-1-daeho43@gmail.com/
> [3]:
> https://lore.kernel.org/dm-devel/20220722093823.4158756-4-nhuck@google.com/
> [4]:
> https://lore.kernel.org/dm-crypt/20200706173731.3734-1-ignat@cloudflare.com/
>
> 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 at google.com>
> Cc: Daeho Jeong <daehojeong at google.com>
> Cc: Eric Biggers <ebiggers at kernel.org>
> Cc: Sami Tolvanen <samitolvanen at google.com>
> Signed-off-by: Nathan Huckleberry <nhuck at google.com>
> ---
> 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;
> }
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