767 lines
27 KiB
Plaintext
767 lines
27 KiB
Plaintext
HISTORY:
|
|
February 16/2002 -- revision 0.2.1:
|
|
COR typo corrected
|
|
February 10/2002 -- revision 0.2:
|
|
some spell checking ;->
|
|
January 12/2002 -- revision 0.1
|
|
This is still work in progress so may change.
|
|
To keep up to date please watch this space.
|
|
|
|
Introduction to NAPI
|
|
====================
|
|
|
|
NAPI is a proven (www.cyberus.ca/~hadi/usenix-paper.tgz) technique
|
|
to improve network performance on Linux. For more details please
|
|
read that paper.
|
|
NAPI provides a "inherent mitigation" which is bound by system capacity
|
|
as can be seen from the following data collected by Robert on Gigabit
|
|
ethernet (e1000):
|
|
|
|
Psize Ipps Tput Rxint Txint Done Ndone
|
|
---------------------------------------------------------------
|
|
60 890000 409362 17 27622 7 6823
|
|
128 758150 464364 21 9301 10 7738
|
|
256 445632 774646 42 15507 21 12906
|
|
512 232666 994445 241292 19147 241192 1062
|
|
1024 119061 1000003 872519 19258 872511 0
|
|
1440 85193 1000003 946576 19505 946569 0
|
|
|
|
|
|
Legend:
|
|
"Ipps" stands for input packets per second.
|
|
"Tput" == packets out of total 1M that made it out.
|
|
"txint" == transmit completion interrupts seen
|
|
"Done" == The number of times that the poll() managed to pull all
|
|
packets out of the rx ring. Note from this that the lower the
|
|
load the more we could clean up the rxring
|
|
"Ndone" == is the converse of "Done". Note again, that the higher
|
|
the load the more times we couldn't clean up the rxring.
|
|
|
|
Observe that:
|
|
when the NIC receives 890Kpackets/sec only 17 rx interrupts are generated.
|
|
The system cant handle the processing at 1 interrupt/packet at that load level.
|
|
At lower rates on the other hand, rx interrupts go up and therefore the
|
|
interrupt/packet ratio goes up (as observable from that table). So there is
|
|
possibility that under low enough input, you get one poll call for each
|
|
input packet caused by a single interrupt each time. And if the system
|
|
cant handle interrupt per packet ratio of 1, then it will just have to
|
|
chug along ....
|
|
|
|
|
|
0) Prerequisites:
|
|
==================
|
|
A driver MAY continue using the old 2.4 technique for interfacing
|
|
to the network stack and not benefit from the NAPI changes.
|
|
NAPI additions to the kernel do not break backward compatibility.
|
|
NAPI, however, requires the following features to be available:
|
|
|
|
A) DMA ring or enough RAM to store packets in software devices.
|
|
|
|
B) Ability to turn off interrupts or maybe events that send packets up
|
|
the stack.
|
|
|
|
NAPI processes packet events in what is known as dev->poll() method.
|
|
Typically, only packet receive events are processed in dev->poll().
|
|
The rest of the events MAY be processed by the regular interrupt handler
|
|
to reduce processing latency (justified also because there are not that
|
|
many of them).
|
|
Note, however, NAPI does not enforce that dev->poll() only processes
|
|
receive events.
|
|
Tests with the tulip driver indicated slightly increased latency if
|
|
all of the interrupt handler is moved to dev->poll(). Also MII handling
|
|
gets a little trickier.
|
|
The example used in this document is to move the receive processing only
|
|
to dev->poll(); this is shown with the patch for the tulip driver.
|
|
For an example of code that moves all the interrupt driver to
|
|
dev->poll() look at the ported e1000 code.
|
|
|
|
There are caveats that might force you to go with moving everything to
|
|
dev->poll(). Different NICs work differently depending on their status/event
|
|
acknowledgement setup.
|
|
There are two types of event register ACK mechanisms.
|
|
I) what is known as Clear-on-read (COR).
|
|
when you read the status/event register, it clears everything!
|
|
The natsemi and sunbmac NICs are known to do this.
|
|
In this case your only choice is to move all to dev->poll()
|
|
|
|
II) Clear-on-write (COW)
|
|
i) you clear the status by writing a 1 in the bit-location you want.
|
|
These are the majority of the NICs and work the best with NAPI.
|
|
Put only receive events in dev->poll(); leave the rest in
|
|
the old interrupt handler.
|
|
ii) whatever you write in the status register clears every thing ;->
|
|
Cant seem to find any supported by Linux which do this. If
|
|
someone knows such a chip email us please.
|
|
Move all to dev->poll()
|
|
|
|
C) Ability to detect new work correctly.
|
|
NAPI works by shutting down event interrupts when there's work and
|
|
turning them on when there's none.
|
|
New packets might show up in the small window while interrupts were being
|
|
re-enabled (refer to appendix 2). A packet might sneak in during the period
|
|
we are enabling interrupts. We only get to know about such a packet when the
|
|
next new packet arrives and generates an interrupt.
|
|
Essentially, there is a small window of opportunity for a race condition
|
|
which for clarity we'll refer to as the "rotting packet".
|
|
|
|
This is a very important topic and appendix 2 is dedicated for more
|
|
discussion.
|
|
|
|
Locking rules and environmental guarantees
|
|
==========================================
|
|
|
|
-Guarantee: Only one CPU at any time can call dev->poll(); this is because
|
|
only one CPU can pick the initial interrupt and hence the initial
|
|
netif_rx_schedule(dev);
|
|
- The core layer invokes devices to send packets in a round robin format.
|
|
This implies receive is totally lockless because of the guarantee that only
|
|
one CPU is executing it.
|
|
- contention can only be the result of some other CPU accessing the rx
|
|
ring. This happens only in close() and suspend() (when these methods
|
|
try to clean the rx ring);
|
|
****guarantee: driver authors need not worry about this; synchronization
|
|
is taken care for them by the top net layer.
|
|
-local interrupts are enabled (if you dont move all to dev->poll()). For
|
|
example link/MII and txcomplete continue functioning just same old way.
|
|
This improves the latency of processing these events. It is also assumed that
|
|
the receive interrupt is the largest cause of noise. Note this might not
|
|
always be true.
|
|
[according to Manfred Spraul, the winbond insists on sending one
|
|
txmitcomplete interrupt for each packet (although this can be mitigated)].
|
|
For these broken drivers, move all to dev->poll().
|
|
|
|
For the rest of this text, we'll assume that dev->poll() only
|
|
processes receive events.
|
|
|
|
new methods introduce by NAPI
|
|
=============================
|
|
|
|
a) netif_rx_schedule(dev)
|
|
Called by an IRQ handler to schedule a poll for device
|
|
|
|
b) netif_rx_schedule_prep(dev)
|
|
puts the device in a state which allows for it to be added to the
|
|
CPU polling list if it is up and running. You can look at this as
|
|
the first half of netif_rx_schedule(dev) above; the second half
|
|
being c) below.
|
|
|
|
c) __netif_rx_schedule(dev)
|
|
Add device to the poll list for this CPU; assuming that _prep above
|
|
has already been called and returned 1.
|
|
|
|
d) netif_rx_reschedule(dev, undo)
|
|
Called to reschedule polling for device specifically for some
|
|
deficient hardware. Read Appendix 2 for more details.
|
|
|
|
e) netif_rx_complete(dev)
|
|
|
|
Remove interface from the CPU poll list: it must be in the poll list
|
|
on current cpu. This primitive is called by dev->poll(), when
|
|
it completes its work. The device cannot be out of poll list at this
|
|
call, if it is then clearly it is a BUG(). You'll know ;->
|
|
|
|
All these above nethods are used below. So keep reading for clarity.
|
|
|
|
Device driver changes to be made when porting NAPI
|
|
==================================================
|
|
|
|
Below we describe what kind of changes are required for NAPI to work.
|
|
|
|
1) introduction of dev->poll() method
|
|
=====================================
|
|
|
|
This is the method that is invoked by the network core when it requests
|
|
for new packets from the driver. A driver is allowed to send upto
|
|
dev->quota packets by the current CPU before yielding to the network
|
|
subsystem (so other devices can also get opportunity to send to the stack).
|
|
|
|
dev->poll() prototype looks as follows:
|
|
int my_poll(struct net_device *dev, int *budget)
|
|
|
|
budget is the remaining number of packets the network subsystem on the
|
|
current CPU can send up the stack before yielding to other system tasks.
|
|
*Each driver is responsible for decrementing budget by the total number of
|
|
packets sent.
|
|
Total number of packets cannot exceed dev->quota.
|
|
|
|
dev->poll() method is invoked by the top layer, the driver just sends if it
|
|
can to the stack the packet quantity requested.
|
|
|
|
more on dev->poll() below after the interrupt changes are explained.
|
|
|
|
2) registering dev->poll() method
|
|
===================================
|
|
|
|
dev->poll should be set in the dev->probe() method.
|
|
e.g:
|
|
dev->open = my_open;
|
|
.
|
|
.
|
|
/* two new additions */
|
|
/* first register my poll method */
|
|
dev->poll = my_poll;
|
|
/* next register my weight/quanta; can be overridden in /proc */
|
|
dev->weight = 16;
|
|
.
|
|
.
|
|
dev->stop = my_close;
|
|
|
|
|
|
|
|
3) scheduling dev->poll()
|
|
=============================
|
|
This involves modifying the interrupt handler and the code
|
|
path which takes the packet off the NIC and sends them to the
|
|
stack.
|
|
|
|
it's important at this point to introduce the classical D Becker
|
|
interrupt processor:
|
|
|
|
------------------
|
|
static irqreturn_t
|
|
netdevice_interrupt(int irq, void *dev_id, struct pt_regs *regs)
|
|
{
|
|
|
|
struct net_device *dev = (struct net_device *)dev_instance;
|
|
struct my_private *tp = (struct my_private *)dev->priv;
|
|
|
|
int work_count = my_work_count;
|
|
status = read_interrupt_status_reg();
|
|
if (status == 0)
|
|
return IRQ_NONE; /* Shared IRQ: not us */
|
|
if (status == 0xffff)
|
|
return IRQ_HANDLED; /* Hot unplug */
|
|
if (status & error)
|
|
do_some_error_handling()
|
|
|
|
do {
|
|
acknowledge_ints_ASAP();
|
|
|
|
if (status & link_interrupt) {
|
|
spin_lock(&tp->link_lock);
|
|
do_some_link_stat_stuff();
|
|
spin_lock(&tp->link_lock);
|
|
}
|
|
|
|
if (status & rx_interrupt) {
|
|
receive_packets(dev);
|
|
}
|
|
|
|
if (status & rx_nobufs) {
|
|
make_rx_buffs_avail();
|
|
}
|
|
|
|
if (status & tx_related) {
|
|
spin_lock(&tp->lock);
|
|
tx_ring_free(dev);
|
|
if (tx_died)
|
|
restart_tx();
|
|
spin_unlock(&tp->lock);
|
|
}
|
|
|
|
status = read_interrupt_status_reg();
|
|
|
|
} while (!(status & error) || more_work_to_be_done);
|
|
return IRQ_HANDLED;
|
|
}
|
|
|
|
----------------------------------------------------------------------
|
|
|
|
We now change this to what is shown below to NAPI-enable it:
|
|
|
|
----------------------------------------------------------------------
|
|
static irqreturn_t
|
|
netdevice_interrupt(int irq, void *dev_id, struct pt_regs *regs)
|
|
{
|
|
struct net_device *dev = (struct net_device *)dev_instance;
|
|
struct my_private *tp = (struct my_private *)dev->priv;
|
|
|
|
status = read_interrupt_status_reg();
|
|
if (status == 0)
|
|
return IRQ_NONE; /* Shared IRQ: not us */
|
|
if (status == 0xffff)
|
|
return IRQ_HANDLED; /* Hot unplug */
|
|
if (status & error)
|
|
do_some_error_handling();
|
|
|
|
do {
|
|
/************************ start note *********************************/
|
|
acknowledge_ints_ASAP(); // dont ack rx and rxnobuff here
|
|
/************************ end note *********************************/
|
|
|
|
if (status & link_interrupt) {
|
|
spin_lock(&tp->link_lock);
|
|
do_some_link_stat_stuff();
|
|
spin_unlock(&tp->link_lock);
|
|
}
|
|
/************************ start note *********************************/
|
|
if (status & rx_interrupt || (status & rx_nobuffs)) {
|
|
if (netif_rx_schedule_prep(dev)) {
|
|
|
|
/* disable interrupts caused
|
|
* by arriving packets */
|
|
disable_rx_and_rxnobuff_ints();
|
|
/* tell system we have work to be done. */
|
|
__netif_rx_schedule(dev);
|
|
} else {
|
|
printk("driver bug! interrupt while in poll\n");
|
|
/* FIX by disabling interrupts */
|
|
disable_rx_and_rxnobuff_ints();
|
|
}
|
|
}
|
|
/************************ end note note *********************************/
|
|
|
|
if (status & tx_related) {
|
|
spin_lock(&tp->lock);
|
|
tx_ring_free(dev);
|
|
|
|
if (tx_died)
|
|
restart_tx();
|
|
spin_unlock(&tp->lock);
|
|
}
|
|
|
|
status = read_interrupt_status_reg();
|
|
|
|
/************************ start note *********************************/
|
|
} while (!(status & error) || more_work_to_be_done(status));
|
|
/************************ end note note *********************************/
|
|
return IRQ_HANDLED;
|
|
}
|
|
|
|
---------------------------------------------------------------------
|
|
|
|
|
|
We note several things from above:
|
|
|
|
I) Any interrupt source which is caused by arriving packets is now
|
|
turned off when it occurs. Depending on the hardware, there could be
|
|
several reasons that arriving packets would cause interrupts; these are the
|
|
interrupt sources we wish to avoid. The two common ones are a) a packet
|
|
arriving (rxint) b) a packet arriving and finding no DMA buffers available
|
|
(rxnobuff) .
|
|
This means also acknowledge_ints_ASAP() will not clear the status
|
|
register for those two items above; clearing is done in the place where
|
|
proper work is done within NAPI; at the poll() and refill_rx_ring()
|
|
discussed further below.
|
|
netif_rx_schedule_prep() returns 1 if device is in running state and
|
|
gets successfully added to the core poll list. If we get a zero value
|
|
we can _almost_ assume are already added to the list (instead of not running.
|
|
Logic based on the fact that you shouldn't get interrupt if not running)
|
|
We rectify this by disabling rx and rxnobuf interrupts.
|
|
|
|
II) that receive_packets(dev) and make_rx_buffs_avail() may have disappeared.
|
|
These functionalities are still around actually......
|
|
|
|
infact, receive_packets(dev) is very close to my_poll() and
|
|
make_rx_buffs_avail() is invoked from my_poll()
|
|
|
|
4) converting receive_packets() to dev->poll()
|
|
===============================================
|
|
|
|
We need to convert the classical D Becker receive_packets(dev) to my_poll()
|
|
|
|
First the typical receive_packets() below:
|
|
-------------------------------------------------------------------
|
|
|
|
/* this is called by interrupt handler */
|
|
static void receive_packets (struct net_device *dev)
|
|
{
|
|
|
|
struct my_private *tp = (struct my_private *)dev->priv;
|
|
rx_ring = tp->rx_ring;
|
|
cur_rx = tp->cur_rx;
|
|
int entry = cur_rx % RX_RING_SIZE;
|
|
int received = 0;
|
|
int rx_work_limit = tp->dirty_rx + RX_RING_SIZE - tp->cur_rx;
|
|
|
|
while (rx_ring_not_empty) {
|
|
u32 rx_status;
|
|
unsigned int rx_size;
|
|
unsigned int pkt_size;
|
|
struct sk_buff *skb;
|
|
/* read size+status of next frame from DMA ring buffer */
|
|
/* the number 16 and 4 are just examples */
|
|
rx_status = le32_to_cpu (*(u32 *) (rx_ring + ring_offset));
|
|
rx_size = rx_status >> 16;
|
|
pkt_size = rx_size - 4;
|
|
|
|
/* process errors */
|
|
if ((rx_size > (MAX_ETH_FRAME_SIZE+4)) ||
|
|
(!(rx_status & RxStatusOK))) {
|
|
netdrv_rx_err (rx_status, dev, tp, ioaddr);
|
|
return;
|
|
}
|
|
|
|
if (--rx_work_limit < 0)
|
|
break;
|
|
|
|
/* grab a skb */
|
|
skb = dev_alloc_skb (pkt_size + 2);
|
|
if (skb) {
|
|
.
|
|
.
|
|
netif_rx (skb);
|
|
.
|
|
.
|
|
} else { /* OOM */
|
|
/*seems very driver specific ... some just pass
|
|
whatever is on the ring already. */
|
|
}
|
|
|
|
/* move to the next skb on the ring */
|
|
entry = (++tp->cur_rx) % RX_RING_SIZE;
|
|
received++ ;
|
|
|
|
}
|
|
|
|
/* store current ring pointer state */
|
|
tp->cur_rx = cur_rx;
|
|
|
|
/* Refill the Rx ring buffers if they are needed */
|
|
refill_rx_ring();
|
|
.
|
|
.
|
|
|
|
}
|
|
-------------------------------------------------------------------
|
|
We change it to a new one below; note the additional parameter in
|
|
the call.
|
|
|
|
-------------------------------------------------------------------
|
|
|
|
/* this is called by the network core */
|
|
static int my_poll (struct net_device *dev, int *budget)
|
|
{
|
|
|
|
struct my_private *tp = (struct my_private *)dev->priv;
|
|
rx_ring = tp->rx_ring;
|
|
cur_rx = tp->cur_rx;
|
|
int entry = cur_rx % RX_BUF_LEN;
|
|
/* maximum packets to send to the stack */
|
|
/************************ note note *********************************/
|
|
int rx_work_limit = dev->quota;
|
|
|
|
/************************ end note note *********************************/
|
|
do { // outer beginning loop starts here
|
|
|
|
clear_rx_status_register_bit();
|
|
|
|
while (rx_ring_not_empty) {
|
|
u32 rx_status;
|
|
unsigned int rx_size;
|
|
unsigned int pkt_size;
|
|
struct sk_buff *skb;
|
|
/* read size+status of next frame from DMA ring buffer */
|
|
/* the number 16 and 4 are just examples */
|
|
rx_status = le32_to_cpu (*(u32 *) (rx_ring + ring_offset));
|
|
rx_size = rx_status >> 16;
|
|
pkt_size = rx_size - 4;
|
|
|
|
/* process errors */
|
|
if ((rx_size > (MAX_ETH_FRAME_SIZE+4)) ||
|
|
(!(rx_status & RxStatusOK))) {
|
|
netdrv_rx_err (rx_status, dev, tp, ioaddr);
|
|
return 1;
|
|
}
|
|
|
|
/************************ note note *********************************/
|
|
if (--rx_work_limit < 0) { /* we got packets, but no quota */
|
|
/* store current ring pointer state */
|
|
tp->cur_rx = cur_rx;
|
|
|
|
/* Refill the Rx ring buffers if they are needed */
|
|
refill_rx_ring(dev);
|
|
goto not_done;
|
|
}
|
|
/********************** end note **********************************/
|
|
|
|
/* grab a skb */
|
|
skb = dev_alloc_skb (pkt_size + 2);
|
|
if (skb) {
|
|
.
|
|
.
|
|
/************************ note note *********************************/
|
|
netif_receive_skb (skb);
|
|
/********************** end note **********************************/
|
|
.
|
|
.
|
|
} else { /* OOM */
|
|
/*seems very driver specific ... common is just pass
|
|
whatever is on the ring already. */
|
|
}
|
|
|
|
/* move to the next skb on the ring */
|
|
entry = (++tp->cur_rx) % RX_RING_SIZE;
|
|
received++ ;
|
|
|
|
}
|
|
|
|
/* store current ring pointer state */
|
|
tp->cur_rx = cur_rx;
|
|
|
|
/* Refill the Rx ring buffers if they are needed */
|
|
refill_rx_ring(dev);
|
|
|
|
/* no packets on ring; but new ones can arrive since we last
|
|
checked */
|
|
status = read_interrupt_status_reg();
|
|
if (rx status is not set) {
|
|
/* If something arrives in this narrow window,
|
|
an interrupt will be generated */
|
|
goto done;
|
|
}
|
|
/* done! at least that's what it looks like ;->
|
|
if new packets came in after our last check on status bits
|
|
they'll be caught by the while check and we go back and clear them
|
|
since we havent exceeded our quota */
|
|
} while (rx_status_is_set);
|
|
|
|
done:
|
|
|
|
/************************ note note *********************************/
|
|
dev->quota -= received;
|
|
*budget -= received;
|
|
|
|
/* If RX ring is not full we are out of memory. */
|
|
if (tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
|
|
goto oom;
|
|
|
|
/* we are happy/done, no more packets on ring; put us back
|
|
to where we can start processing interrupts again */
|
|
netif_rx_complete(dev);
|
|
enable_rx_and_rxnobuf_ints();
|
|
|
|
/* The last op happens after poll completion. Which means the following:
|
|
* 1. it can race with disabling irqs in irq handler (which are done to
|
|
* schedule polls)
|
|
* 2. it can race with dis/enabling irqs in other poll threads
|
|
* 3. if an irq raised after the beginning of the outer beginning
|
|
* loop (marked in the code above), it will be immediately
|
|
* triggered here.
|
|
*
|
|
* Summarizing: the logic may result in some redundant irqs both
|
|
* due to races in masking and due to too late acking of already
|
|
* processed irqs. The good news: no events are ever lost.
|
|
*/
|
|
|
|
return 0; /* done */
|
|
|
|
not_done:
|
|
if (tp->cur_rx - tp->dirty_rx > RX_RING_SIZE/2 ||
|
|
tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
|
|
refill_rx_ring(dev);
|
|
|
|
if (!received) {
|
|
printk("received==0\n");
|
|
received = 1;
|
|
}
|
|
dev->quota -= received;
|
|
*budget -= received;
|
|
return 1; /* not_done */
|
|
|
|
oom:
|
|
/* Start timer, stop polling, but do not enable rx interrupts. */
|
|
start_poll_timer(dev);
|
|
return 0; /* we'll take it from here so tell core "done"*/
|
|
|
|
/************************ End note note *********************************/
|
|
}
|
|
-------------------------------------------------------------------
|
|
|
|
From above we note that:
|
|
0) rx_work_limit = dev->quota
|
|
1) refill_rx_ring() is in charge of clearing the bit for rxnobuff when
|
|
it does the work.
|
|
2) We have a done and not_done state.
|
|
3) instead of netif_rx() we call netif_receive_skb() to pass the skb.
|
|
4) we have a new way of handling oom condition
|
|
5) A new outer for (;;) loop has been added. This serves the purpose of
|
|
ensuring that if a new packet has come in, after we are all set and done,
|
|
and we have not exceeded our quota that we continue sending packets up.
|
|
|
|
|
|
-----------------------------------------------------------
|
|
Poll timer code will need to do the following:
|
|
|
|
a)
|
|
|
|
if (tp->cur_rx - tp->dirty_rx > RX_RING_SIZE/2 ||
|
|
tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
|
|
refill_rx_ring(dev);
|
|
|
|
/* If RX ring is not full we are still out of memory.
|
|
Restart the timer again. Else we re-add ourselves
|
|
to the master poll list.
|
|
*/
|
|
|
|
if (tp->rx_buffers[tp->dirty_rx % RX_RING_SIZE].skb == NULL)
|
|
restart_timer();
|
|
|
|
else netif_rx_schedule(dev); /* we are back on the poll list */
|
|
|
|
5) dev->close() and dev->suspend() issues
|
|
==========================================
|
|
The driver writer needn't worry about this; the top net layer takes
|
|
care of it.
|
|
|
|
6) Adding new Stats to /proc
|
|
=============================
|
|
In order to debug some of the new features, we introduce new stats
|
|
that need to be collected.
|
|
TODO: Fill this later.
|
|
|
|
APPENDIX 1: discussion on using ethernet HW FC
|
|
==============================================
|
|
Most chips with FC only send a pause packet when they run out of Rx buffers.
|
|
Since packets are pulled off the DMA ring by a softirq in NAPI,
|
|
if the system is slow in grabbing them and we have a high input
|
|
rate (faster than the system's capacity to remove packets), then theoretically
|
|
there will only be one rx interrupt for all packets during a given packetstorm.
|
|
Under low load, we might have a single interrupt per packet.
|
|
FC should be programmed to apply in the case when the system cant pull out
|
|
packets fast enough i.e send a pause only when you run out of rx buffers.
|
|
Note FC in itself is a good solution but we have found it to not be
|
|
much of a commodity feature (both in NICs and switches) and hence falls
|
|
under the same category as using NIC based mitigation. Also, experiments
|
|
indicate that it's much harder to resolve the resource allocation
|
|
issue (aka lazy receiving that NAPI offers) and hence quantify its usefulness
|
|
proved harder. In any case, FC works even better with NAPI but is not
|
|
necessary.
|
|
|
|
|
|
APPENDIX 2: the "rotting packet" race-window avoidance scheme
|
|
=============================================================
|
|
|
|
There are two types of associations seen here
|
|
|
|
1) status/int which honors level triggered IRQ
|
|
|
|
If a status bit for receive or rxnobuff is set and the corresponding
|
|
interrupt-enable bit is not on, then no interrupts will be generated. However,
|
|
as soon as the "interrupt-enable" bit is unmasked, an immediate interrupt is
|
|
generated. [assuming the status bit was not turned off].
|
|
Generally the concept of level triggered IRQs in association with a status and
|
|
interrupt-enable CSR register set is used to avoid the race.
|
|
|
|
If we take the example of the tulip:
|
|
"pending work" is indicated by the status bit(CSR5 in tulip).
|
|
the corresponding interrupt bit (CSR7 in tulip) might be turned off (but
|
|
the CSR5 will continue to be turned on with new packet arrivals even if
|
|
we clear it the first time)
|
|
Very important is the fact that if we turn on the interrupt bit on when
|
|
status is set that an immediate irq is triggered.
|
|
|
|
If we cleared the rx ring and proclaimed there was "no more work
|
|
to be done" and then went on to do a few other things; then when we enable
|
|
interrupts, there is a possibility that a new packet might sneak in during
|
|
this phase. It helps to look at the pseudo code for the tulip poll
|
|
routine:
|
|
|
|
--------------------------
|
|
do {
|
|
ACK;
|
|
while (ring_is_not_empty()) {
|
|
work-work-work
|
|
if quota is exceeded: exit, no touching irq status/mask
|
|
}
|
|
/* No packets, but new can arrive while we are doing this*/
|
|
CSR5 := read
|
|
if (CSR5 is not set) {
|
|
/* If something arrives in this narrow window here,
|
|
* where the comments are ;-> irq will be generated */
|
|
unmask irqs;
|
|
exit poll;
|
|
}
|
|
} while (rx_status_is_set);
|
|
------------------------
|
|
|
|
CSR5 bit of interest is only the rx status.
|
|
If you look at the last if statement:
|
|
you just finished grabbing all the packets from the rx ring .. you check if
|
|
status bit says there are more packets just in ... it says none; you then
|
|
enable rx interrupts again; if a new packet just came in during this check,
|
|
we are counting that CSR5 will be set in that small window of opportunity
|
|
and that by re-enabling interrupts, we would actually trigger an interrupt
|
|
to register the new packet for processing.
|
|
|
|
[The above description nay be very verbose, if you have better wording
|
|
that will make this more understandable, please suggest it.]
|
|
|
|
2) non-capable hardware
|
|
|
|
These do not generally respect level triggered IRQs. Normally,
|
|
irqs may be lost while being masked and the only way to leave poll is to do
|
|
a double check for new input after netif_rx_complete() is invoked
|
|
and re-enable polling (after seeing this new input).
|
|
|
|
Sample code:
|
|
|
|
---------
|
|
.
|
|
.
|
|
restart_poll:
|
|
while (ring_is_not_empty()) {
|
|
work-work-work
|
|
if quota is exceeded: exit, not touching irq status/mask
|
|
}
|
|
.
|
|
.
|
|
.
|
|
enable_rx_interrupts()
|
|
netif_rx_complete(dev);
|
|
if (ring_has_new_packet() && netif_rx_reschedule(dev, received)) {
|
|
disable_rx_and_rxnobufs()
|
|
goto restart_poll
|
|
} while (rx_status_is_set);
|
|
---------
|
|
|
|
Basically netif_rx_complete() removes us from the poll list, but because a
|
|
new packet which will never be caught due to the possibility of a race
|
|
might come in, we attempt to re-add ourselves to the poll list.
|
|
|
|
|
|
|
|
|
|
APPENDIX 3: Scheduling issues.
|
|
==============================
|
|
As seen NAPI moves processing to softirq level. Linux uses the ksoftirqd as the
|
|
general solution to schedule softirq's to run before next interrupt and by putting
|
|
them under scheduler control. Also this prevents consecutive softirq's from
|
|
monopolize the CPU. This also have the effect that the priority of ksoftirq needs
|
|
to be considered when running very CPU-intensive applications and networking to
|
|
get the proper balance of softirq/user balance. Increasing ksoftirq priority to 0
|
|
(eventually more) is reported cure problems with low network performance at high
|
|
CPU load.
|
|
|
|
Most used processes in a GIGE router:
|
|
USER PID %CPU %MEM SIZE RSS TTY STAT START TIME COMMAND
|
|
root 3 0.2 0.0 0 0 ? RWN Aug 15 602:00 (ksoftirqd_CPU0)
|
|
root 232 0.0 7.9 41400 40884 ? S Aug 15 74:12 gated
|
|
|
|
--------------------------------------------------------------------
|
|
|
|
relevant sites:
|
|
==================
|
|
ftp://robur.slu.se/pub/Linux/net-development/NAPI/
|
|
|
|
|
|
--------------------------------------------------------------------
|
|
TODO: Write net-skeleton.c driver.
|
|
-------------------------------------------------------------
|
|
|
|
Authors:
|
|
========
|
|
Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
|
|
Jamal Hadi Salim <hadi@cyberus.ca>
|
|
Robert Olsson <Robert.Olsson@data.slu.se>
|
|
|
|
Acknowledgements:
|
|
================
|
|
People who made this document better:
|
|
|
|
Lennert Buytenhek <buytenh@gnu.org>
|
|
Andrew Morton <akpm@zip.com.au>
|
|
Manfred Spraul <manfred@colorfullife.com>
|
|
Donald Becker <becker@scyld.com>
|
|
Jeff Garzik <jgarzik@pobox.com>
|