From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Subject: relay: Use irq_work instead of plain timer for deferred wakeup Relay avoids calling wake_up_interruptible() for doing the wakeup of readers/consumers, waiting for the generation of new data, from the context of a process which produced the data. This is apparently done to prevent the possibility of a deadlock in case Scheduler itself is is generating data for the relay, after acquiring rq->lock. The following patch used a timer (to be scheduled at next jiffy), for delegating the wakeup to another context. commit 7c9cb38302e78d24e37f7d8a2ea7eed4ae5f2fa7 Author: Tom Zanussi <zanussi@xxxxxxxxxxx> Date: Wed May 9 02:34:01 2007 -0700 relay: use plain timer instead of delayed work relay doesn't need to use schedule_delayed_work() for waking readers when a simple timer will do. Scheduling a plain timer, at next jiffies boundary, to do the wakeup causes a significant wakeup latency for the Userspace client, which makes relay less suitable for the high-frequency low-payload use cases where the data gets generated at a very high rate, like multiple sub buffers getting filled within a milli second. Moreover the timer is re-scheduled on every newly produced sub buffer so the timer keeps getting pushed out if sub buffers are filled in a very quick succession (less than a jiffy gap between filling of 2 sub buffers). As a result relay runs out of sub buffers to store the new data. By using irq_work it is ensured that wakeup of userspace client, blocked in the poll call, is done at earliest (through self IPI or next timer tick) enabling it to always consume the data in time. Also this makes relay consistent with printk & ring buffers (trace), as they too use irq_work for deferred wake up of readers. [arnd@xxxxxxxx: select CONFIG_IRQ_WORK] Link: http://lkml.kernel.org/r/20160912154035.3222156-1-arnd@xxxxxxxx [akpm@xxxxxxxxxxxxxxxxxxxx: coding-style fixes] Link: http://lkml.kernel.org/r/1472906487-1559-1-git-send-email-akash.goel@xxxxxxxxx Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Signed-off-by: Akash Goel <akash.goel@xxxxxxxxx> Cc: Tom Zanussi <tzanussi@xxxxxxxxx> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/relay.h | 3 ++- init/Kconfig | 1 + kernel/relay.c | 24 ++++++++++++++---------- 3 files changed, 17 insertions(+), 11 deletions(-) diff -puN include/linux/relay.h~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup include/linux/relay.h --- a/include/linux/relay.h~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup +++ a/include/linux/relay.h @@ -15,6 +15,7 @@ #include <linux/timer.h> #include <linux/wait.h> #include <linux/list.h> +#include <linux/irq_work.h> #include <linux/bug.h> #include <linux/fs.h> #include <linux/poll.h> @@ -38,7 +39,7 @@ struct rchan_buf size_t subbufs_consumed; /* count of sub-buffers consumed */ struct rchan *chan; /* associated channel */ wait_queue_head_t read_wait; /* reader wait queue */ - struct timer_list timer; /* reader wake-up timer */ + struct irq_work wakeup_work; /* reader wakeup */ struct dentry *dentry; /* channel file dentry */ struct kref kref; /* channel buffer refcount */ struct page **page_array; /* array of current buffer pages */ diff -puN init/Kconfig~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup init/Kconfig --- a/init/Kconfig~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup +++ a/init/Kconfig @@ -1288,6 +1288,7 @@ config SYSFS_DEPRECATED_V2 config RELAY bool "Kernel->user space relay support (formerly relayfs)" + select IRQ_WORK help This option enables support for relay interface support in certain file systems (such as debugfs). diff -puN kernel/relay.c~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup kernel/relay.c --- a/kernel/relay.c~relay-use-irq_work-instead-of-plain-timer-for-deferred-wakeup +++ a/kernel/relay.c @@ -328,13 +328,15 @@ static struct rchan_callbacks default_ch /** * wakeup_readers - wake up readers waiting on a channel - * @data: contains the channel buffer + * @work: contains the channel buffer * - * This is the timer function used to defer reader waking. + * This is the function used to defer reader waking */ -static void wakeup_readers(unsigned long data) +static void wakeup_readers(struct irq_work *work) { - struct rchan_buf *buf = (struct rchan_buf *)data; + struct rchan_buf *buf; + + buf = container_of(work, struct rchan_buf, wakeup_work); wake_up_interruptible(&buf->read_wait); } @@ -352,9 +354,10 @@ static void __relay_reset(struct rchan_b if (init) { init_waitqueue_head(&buf->read_wait); kref_init(&buf->kref); - setup_timer(&buf->timer, wakeup_readers, (unsigned long)buf); - } else - del_timer_sync(&buf->timer); + init_irq_work(&buf->wakeup_work, wakeup_readers); + } else { + irq_work_sync(&buf->wakeup_work); + } buf->subbufs_produced = 0; buf->subbufs_consumed = 0; @@ -487,7 +490,7 @@ free_buf: static void relay_close_buf(struct rchan_buf *buf) { buf->finalized = 1; - del_timer_sync(&buf->timer); + irq_work_sync(&buf->wakeup_work); buf->chan->cb->remove_buf_file(buf->dentry); kref_put(&buf->kref, relay_remove_buf); } @@ -754,14 +757,15 @@ size_t relay_switch_subbuf(struct rchan_ buf->early_bytes += buf->chan->subbuf_size - buf->padding[old_subbuf]; smp_mb(); - if (waitqueue_active(&buf->read_wait)) + if (waitqueue_active(&buf->read_wait)) { /* * Calling wake_up_interruptible() from here * will deadlock if we happen to be logging * from the scheduler (trying to re-grab * rq->lock), so defer it. */ - mod_timer(&buf->timer, jiffies + 1); + irq_work_queue(&buf->wakeup_work); + } } old = buf->data; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
