Re: [PATCH RFC] r8152: Ensure that napi_schedule() is handled

On Fri, 14 May 2021 23:10:43 +0200 Thomas Gleixner wrote:

On Fri, May 14 2021 at 13:46, Jakub Kicinski wrote:

quoted

On Fri, 14 May 2021 22:25:50 +0200 Thomas Gleixner wrote:  

quoted

Except that some instruction cycle beancounters might complain about
the extra conditional for the sane cases.

But yes, I'm fine with that as well. That's why this patch is marked RFC :)  

When we're in the right context (irq/bh disabled etc.) the cost is just
read of preempt_count() and jump, right? And presumably preempt_count()
is in the cache already, because those sections aren't very long. Let me
make this change locally and see if it is in any way perceivable.  

Right. Just wanted to mention it :)

quoted

Obviously if anyone sees a way to solve the problem without much
ifdefinery and force_irqthreads checks that'd be great - I don't.  

This is not related to force_irqthreads at all. This very driver invokes
it from plain thread context.

I see, but a driver calling __napi_schedule_irqoff() from its IRQ
handler _would_ be an issue, right? Or do irq threads trigger softirq
processing on exit?

quoted

I'd rather avoid pushing this kind of stuff out to the drivers.  

You could have napi_schedule_intask() or something like that which would
do the local_bh_disable()/enable() dance around the invocation of
napi_schedule(). That would also document it clearly in the drivers. A
quick grep shows a bunch of instances which could be replaced:

drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c-5704-		local_bh_disable();
drivers/net/ethernet/mellanox/mlx4/en_netdev.c-1830-		local_bh_disable();
drivers/net/usb/r8152.c-1552-	local_bh_disable();
drivers/net/virtio_net.c-1355-	local_bh_disable();
drivers/net/wireless/intel/iwlwifi/pcie/rx.c-1650-	local_bh_disable();
drivers/net/wireless/intel/iwlwifi/pcie/rx.c-2015-		local_bh_disable();
drivers/net/wireless/intel/iwlwifi/pcie/rx.c-2225-		local_bh_disable();
drivers/net/wireless/intel/iwlwifi/pcie/rx.c-2235-		local_bh_disable();
drivers/s390/net/qeth_core_main.c-3515-	local_bh_disable();

Very well aware, I've just sent a patch for mlx5 last week :)

My initial reaction was the same as yours - we should add lockdep
check, and napi_schedule_intask(). But then I started wondering
if it's all for nothing on rt or with force_irqthreads, and therefore
we should just eat the extra check.