Re: [PATCH] lost softirq, 2.6.24-rc7

From: Steven Rostedt
Date: Tue Jan 15 2008 - 19:40:39 EST


On Tue, Jan 15, 2008 at 02:15:26PM -0800, Frank Rowand wrote:
> From: Frank Rowand <frank.rowand@xxxxxxxxxxx>
>
> (Ingo, there is a question for you after the description, just before the
> patch.)
>
> When running an interrupt and network intensive stress test with PREEMPT_RT
> enabled, the target system stopped processing received network packets.
> skbs from received packets were being queued by net_rx_action(), but the
> NET_RX_SOFTIRQ softirq was never running to remove the skbs from the queue.
> Since the target system root file system is NFS mounted, the system is now
> effectively hung.
>
> A pseudocode description of how this state was reached follows.
> Each level of indentation represents a function call from the previous line.
>
>
> ethernet driver irq handler receives packet
> netif_rx()
> queues skb (qlen == 1), raises NET_RX_SOFTIRQ
>
> on return from irq
> ___do_softirq() [ 1 ]
> Reset the pending bitmask

Frank,

This path should not be hit when running with PREEMPT_RT. The softirqs
are now all separate, and are not run in batch in ksoftirqd. In fact,
ksoftirqd should not be running at all with PREEMPT_RT.

-- Steve

> net_rx_action()
> dequeues skb (qlen == 0)
> jiffies incremented, so
> break out of processing
> and raise NET_RX_SOFTIRQ
> (but don't deassert NAPI_STATE_SCHED)
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> ksoftirqd thread runs
> process TIMER_SOFTIRQ
> process RCU_SOFTIRQ
> << ksoftirqd sleeps >>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> ___do_softirq() [ 2 ]
> Reset the pending bitmask
> finds NET_RX_SOFTIRQ raised but already running
> << ___do_softirq() [ 2 ] completes >>
>
> << ___do_softirq() [ 1 ] resumes >>
> the pending bitmask is empty, so NET_RX_SOFTIRQ is lost
>
>
>
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