Re: [PATCH] workqueue: Add WQ_SCHED_FIFO
From: Gao Xiang
Date: Fri Jan 13 2023 - 21:20:09 EST
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@xxxxxxxxxxxxxxxxx>
Thanks,
Gao Xiang
[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;
}