Subject: Re: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)

From: Brian King <[email-masked]>
Date: Tue, 18 May 2010 16:52:50 -0500

Is IRQF_NODELAY something specific to the RT kernel? I don't see it in mainline...

-Brian

On 05/18/2010 04:33 PM, [email-masked] wrote:
>>>From ad81664794e33d785f533c5edee37aaba20dd92d Mon Sep 17 00:00:00 2001
> From: Darren Hart <[email-masked]>
> Date: Tue, 18 May 2010 11:07:13 -0700
> Subject: [PATCH RT] ehea: make receive irq handler non-threaded (IRQF_NODELAY)
> 
> The underlying hardware is edge triggered but presented by XICS as level
> triggered. The edge triggered interrupts are not reissued after masking. This
> is not a problem in mainline which does not mask the interrupt (relying on the
> EOI mechanism instead). The threaded interrupts in PREEMPT_RT do mask the
> interrupt, and can lose interrupts that occurred while masked, resulting in a
> hung ethernet interface.
> 
> The receive handler simply calls napi_schedule(), as such, there is no
> significant additional overhead in making this non-threaded, since we either
> wakeup the threaded irq handler to call napi_schedule(), or just call
> napi_schedule() directly to wakeup the softirqs.  As the receive handler is
> lockless, there is no need to convert any of the ehea spinlock_t's to
> atomic_spinlock_t's.
> 
> Without this patch, a simple scp file copy loop would fail quickly (usually
> seconds). We have over two hours of sustained scp activity with the patch
> applied.
> 
> Credit goes to Will Schmidt for lots of instrumentation and tracing which
> clarified the scenario and to Thomas Gleixner for the incredibly simple
> solution.
> 
> Signed-off-by: Darren Hart <[email-masked]>
> Acked-by: Will Schmidt <[email-masked]>
> Cc: Thomas Gleixner <[email-masked]>
> Cc: Jan-Bernd Themann <[email-masked]>
> Cc: Nivedita Singhvi <[email-masked]>
> Cc: Brian King <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Cc: Doug Maxey <[email-masked]>
> ---
>  drivers/net/ehea/ehea_main.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/ehea/ehea_main.c b/drivers/net/ehea/ehea_main.c
> index 977c3d3..2c53df2 100644
> --- a/drivers/net/ehea/ehea_main.c
> +++ b/drivers/net/ehea/ehea_main.c
> @@ -1263,7 +1263,7 @@ static int ehea_reg_interrupts(struct net_device *dev)
>  			 "%s-queue%d", dev->name, i);
>  		ret = ibmebus_request_irq(pr->eq->attr.ist1,
>  					  ehea_recv_irq_handler,
> -					  IRQF_DISABLED, pr->int_send_name,
> +					  IRQF_DISABLED | IRQF_NODELAY, pr->int_send_name,
>  					  pr);
>  		if (ret) {
>  			ehea_error("failed registering irq for ehea_queue "

-- 
Brian King
Linux on Power Virtualization
IBM Linux Technology Center