[PATCH] slip: fix NOHZ local_softirq_pending 08 warning

When using nanosleep() in an userspace application we get a ratelimit warning:

	NOHZ: local_softirq_pending 08

According to 481a8199142c050b72bff8a1956a49fd0a75bbe0 the problem is caused by
netif_rx() function. This patch replaces netif_rx() with netif_rx_ni() which
has to be used from process/softirq context.

Signed-off-by: Matvejchikov Ilya <redacted>
---
 drivers/net/slip.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/net/slip.c b/drivers/net/slip.c
index f11b3f3..4c61753 100644
--- a/drivers/net/slip.c
+++ b/drivers/net/slip.c

@@ -367,7 +367,7 @@ static void sl_bump(struct slip *sl)
 	memcpy(skb_put(skb, count), sl->rbuff, count);
 	skb_reset_mac_header(skb);
 	skb->protocol = htons(ETH_P_IP);
-	netif_rx(skb);
+	netif_rx_ni(skb);
 	dev->stats.rx_packets++;
 }

-- 
1.7.4.1

From: Matvejchikov Ilya <redacted>
Date: Fri, 5 Aug 2011 23:23:51 +0400

When using nanosleep() in an userspace application we get a ratelimit warning:

	NOHZ: local_softirq_pending 08

According to 481a8199142c050b72bff8a1956a49fd0a75bbe0 the problem is caused by
netif_rx() function. This patch replaces netif_rx() with netif_rx_ni() which
has to be used from process/softirq context.

Signed-off-by: Matvejchikov Ilya <redacted>

Applied, thanks.

Hello Ilya,

is the processing of characters in the tty input stream for line disciplines
*always* done in softirq context???

If so, i would send a patch for drivers/net/can/slcan.c too ...

Thanks,
Oliver

On 05.08.2011 21:23, Matvejchikov Ilya wrote:

Oliver,

As I see it, there is the following call chain of the netif_rx_ni() in
the slip (or slcan) driver:
[flush_to_ldisc] -> [disc->ops->receive_buf] -> [slip_unesc] ->
[sl_bump] -> [netif_rx_ni]

So flush_to_ldisc() may be called:
  1) From tty_flush_to_ldisc() with flush_work and as it mentioned
must not be called from IRQ context.
  2) From input_available_p() which can be called from n_tty_read() or
n_tty_poll(), so it's the user's context.
  2) From tty_flip_buffer_push() which schedules the flush_to_ldisc to
work or calls it directly if the tty->low_latency set.

So the only thing we must take into account that with tty->low_latency
set we can be in IRQ context when doing with netif_rx_ni(). But if we
are using netif_rx_ni() we can handle both IRQ and softirq contexts
well as if there are any softirq pending netif_rx_ni() calls
do_softirq()  with preemtion disabled. Then do_softirq() checks if
there is an IRQ context and does nothing if so.

So, I think it's correct to change netif_rx() with the netif_rx_ni().

2011/8/9 Oliver Hartkopp [off-list ref]:

Hello Ilya,

is the processing of characters in the tty input stream for line disciplines
*always* done in softirq context???

If so, i would send a patch for drivers/net/can/slcan.c too ...

Thanks,
Oliver

On 05.08.2011 21:23, Matvejchikov Ilya wrote:

quoted

When using nanosleep() in an userspace application we get a ratelimit warning:

      NOHZ: local_softirq_pending 08

According to 481a8199142c050b72bff8a1956a49fd0a75bbe0 the problem is caused by
netif_rx() function. This patch replaces netif_rx() with netif_rx_ni() which
has to be used from process/softirq context.

Signed-off-by: Matvejchikov Ilya <redacted>
---
 drivers/net/slip.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/drivers/net/slip.c b/drivers/net/slip.c
index f11b3f3..4c61753 100644
--- a/drivers/net/slip.c
+++ b/drivers/net/slip.c

@@ -367,7 +367,7 @@ static void sl_bump(struct slip *sl)

      memcpy(skb_put(skb, count), sl->rbuff, count);
      skb_reset_mac_header(skb);
      skb->protocol = htons(ETH_P_IP);
-     netif_rx(skb);
+     netif_rx_ni(skb);
      dev->stats.rx_packets++;
 }

  2) From tty_flip_buffer_push() which schedules the flush_to_ldisc to
work or calls it directly if the tty->low_latency set.

So the only thing we must take into account that with tty->low_latency
set we can be in IRQ context when doing with netif_rx_ni(). But if we

A driver is not permitted to directly call into flush_to_ldisc from an
interrupt so that case doesn't happen.

Alan

On 10.08.2011 11:28, Alan Cox wrote:

quoted

  2) From tty_flip_buffer_push() which schedules the flush_to_ldisc to
work or calls it directly if the tty->low_latency set.

So the only thing we must take into account that with tty->low_latency
set we can be in IRQ context when doing with netif_rx_ni(). But if we

A driver is not permitted to directly call into flush_to_ldisc from an
interrupt so that case doesn't happen.

Thanks to both of you for the explanation!

I'll send a patch for slcan.c then.

Best regards,
Oliver