Re: [PATCH] workqueue: Add WQ_SCHED_FIFO

Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> · Sat, 14 Jan 2023 10:19:50 +0800

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;
  	}






