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SubjectRe: [RFC PATCH 0/6] Convert all tasklets to workqueues
> so how about the following, different approach: anyone who has a tasklet
> in any performance-sensitive codepath, please yell now. We'll also do a
> proactive search for such places. We can convert those places to
> softirqs, or move them back into hardirq context. Once this is done -
> and i doubt it will go beyond 1-2 places - we can just mass-convert the
> other 110 places to the lame but compatible solution of doing them in a
> global thread context.
>

I have a driver / testcase that reacts negatively to a workqueue
conversion. This is with the iop-adma driver on an ARM based platform
re-syncing a degraded raid5 array. The driver is currently in -mm and
it uses tasklets to run a short callback routine upon completion of an
offloaded memcpy or xor operation. Quick tests show that
write-throughput does not go down too much, but resync speed, as
reported by /proc/mdstat, drops from ~50MB/s to ~30MB/s.

Context switches on this platform flush the L1 cache so bouncing
between a workqueue and the MD thread is painful.

The conversion patch is attached.

--
Dan
diff --git a/drivers/dma/iop-adma.c b/drivers/dma/iop-adma.c
index 5d8a6cf..7e89003 100644
--- a/drivers/dma/iop-adma.c
+++ b/drivers/dma/iop-adma.c
@@ -41,6 +41,8 @@
#define tx_to_iop_adma_slot(tx) \
container_of(tx, struct iop_adma_desc_slot, async_tx)

+static struct workqueue_struct *iop_adma_workqueue;
+
/**
* iop_adma_free_slots - flags descriptor slots for reuse
* @slot: Slot to free
@@ -273,9 +275,11 @@ iop_adma_slot_cleanup(struct iop_adma_chan *iop_chan)
spin_unlock_bh(&iop_chan->lock);
}

-static void iop_adma_tasklet(unsigned long data)
+static void iop_adma_work_routine(struct work_struct *work)
{
- struct iop_adma_chan *chan = (struct iop_adma_chan *) data;
+ struct iop_adma_chan *chan =
+ container_of(work, struct iop_adma_chan, work);
+
__iop_adma_slot_cleanup(chan);
}

@@ -370,7 +374,7 @@ retry:
goto retry;

/* try to free some slots if the allocation fails */
- tasklet_schedule(&iop_chan->irq_tasklet);
+ queue_work(iop_adma_workqueue, &iop_chan->work);

return NULL;
}
@@ -704,7 +708,7 @@ iop_adma_prep_dma_zero_sum(struct dma_chan *chan, unsigned int src_cnt,
static void iop_adma_dependency_added(struct dma_chan *chan)
{
struct iop_adma_chan *iop_chan = to_iop_adma_chan(chan);
- tasklet_schedule(&iop_chan->irq_tasklet);
+ queue_work(iop_adma_workqueue, &iop_chan->work);
}

static void iop_adma_free_chan_resources(struct dma_chan *chan)
@@ -785,7 +789,7 @@ static irqreturn_t iop_adma_eot_handler(int irq, void *data)

dev_dbg(chan->device->common.dev, "%s\n", __FUNCTION__);

- tasklet_schedule(&chan->irq_tasklet);
+ queue_work(iop_adma_workqueue, &chan->work);

iop_adma_device_clear_eot_status(chan);

@@ -798,7 +802,7 @@ static irqreturn_t iop_adma_eoc_handler(int irq, void *data)

dev_dbg(chan->device->common.dev, "%s\n", __FUNCTION__);

- tasklet_schedule(&chan->irq_tasklet);
+ queue_work(iop_adma_workqueue, &chan->work);

iop_adma_device_clear_eoc_status(chan);

@@ -1244,8 +1248,6 @@ static int __devinit iop_adma_probe(struct platform_device *pdev)
ret = -ENOMEM;
goto err_free_iop_chan;
}
- tasklet_init(&iop_chan->irq_tasklet, iop_adma_tasklet, (unsigned long)
- iop_chan);

/* clear errors before enabling interrupts */
iop_adma_device_clear_err_status(iop_chan);
@@ -1268,11 +1270,13 @@ static int __devinit iop_adma_probe(struct platform_device *pdev)

spin_lock_init(&iop_chan->lock);
init_timer(&iop_chan->cleanup_watchdog);
- iop_chan->cleanup_watchdog.data = (unsigned long) iop_chan;
- iop_chan->cleanup_watchdog.function = iop_adma_tasklet;
+ iop_chan->cleanup_watchdog.data = (unsigned long) &iop_chan->work;
+ iop_chan->cleanup_watchdog.function = iop_adma_work_routine;
INIT_LIST_HEAD(&iop_chan->chain);
INIT_LIST_HEAD(&iop_chan->all_slots);
INIT_RCU_HEAD(&iop_chan->common.rcu);
+ INIT_WORK(&iop_chan->work, iop_adma_work_routine);
+
iop_chan->common.device = dma_dev;
list_add_tail(&iop_chan->common.device_node, &dma_dev->channels);

@@ -1443,6 +1447,10 @@ static struct platform_driver iop_adma_driver = {

static int __init iop_adma_init (void)
{
+ iop_adma_workqueue = create_workqueue("iop-adma");
+ if (!iop_adma_workqueue)
+ return -ENODEV;
+
/* it's currently unsafe to unload this module */
/* if forced, worst case is that rmmod hangs */
__unsafe(THIS_MODULE);
diff --git a/include/asm-arm/hardware/iop_adma.h b/include/asm-arm/hardware/iop_adma.h
index 8eb5990..7d8742b 100644
--- a/include/asm-arm/hardware/iop_adma.h
+++ b/include/asm-arm/hardware/iop_adma.h
@@ -67,7 +67,7 @@ struct iop_adma_chan {
struct list_head all_slots;
struct timer_list cleanup_watchdog;
int slots_allocated;
- struct tasklet_struct irq_tasklet;
+ struct work_struct work;
};

/**
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