On Fri, Jan 13, 2023 at 6:20 PM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
>
> 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.
>
The above patch and following diff should have equivalent performance
to [1], but I have not tested it.
diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
index ccf7c55d477f..a9c3893ad1d4 100644
--- a/fs/erofs/zdata.c
+++ b/fs/erofs/zdata.c
@@ -201,7 +201,7 @@ static inline int z_erofs_init_workqueue(void)
* scheduling overhead, perhaps per-CPU threads should be better?
*/
z_erofs_workqueue = alloc_workqueue("erofs_unzipd",
- WQ_UNBOUND | WQ_HIGHPRI,
+ WQ_SCHED_FIFO,
onlinecpus + onlinecpus / 4);
return z_erofs_workqueue ? 0 : -ENOMEM;
Thanks,
Huck
}
> >
> > 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;
> > }
