Re: [PATCH v3 03/11] ARC: Allow irq threading

From: Vineet Gupta
Date: Fri Aug 25 2017 - 16:45:30 EST


+CC Peter, Tglx, Steven

On 06/15/2017 01:43 AM, Noam Camus wrote:
From: Noam Camus <noamc@xxxxxxxxxx>

Working with NPS400 we noticed that there is a possibility of L1
interrupt nesting that may run out kernel stack.
The scenario include serving invoke_softirqs() from irq_exit()
and once local_irq_enable() called can hit another one before we
managed to restore last one and pop some place from kernel stack.

Serving softirqs at dedicated kernel thread may mitigate this.
We see that many architectures, including x86, behave like this.

Note 1: All interrupts which must be non threaded
should be marked IRQF_NO_THREAD.
Note 2: using kernel param "threadirqs" is needed to actually
turn this on. This configuration is only a preperation.

Signed-off-by: Noam Camus <noamc@xxxxxxxxxx>
---
arch/arc/Kconfig | 1 +
1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
index a545969..f464f97 100644
--- a/arch/arc/Kconfig
+++ b/arch/arc/Kconfig
@@ -33,6 +33,7 @@ config ARC
select HAVE_OPROFILE
select HAVE_PERF_EVENTS
select HANDLE_DOMAIN_IRQ
+ select IRQ_FORCED_THREADING
select IRQ_DOMAIN
select MODULES_USE_ELF_RELA
select NO_BOOTMEM>

As Noam notes above and looking at work needed in other arches prior to switching to this: we need mark the low level ints as NO_THREAD.

ARC ipi, timer, perf interrupts happen to use request_percpu_irq() which doesn't take flags and doesn't seem to force NO_THREAD internally either. Does that mean we first need to convert all of these sites to __request_percpu_irq(.. NO_THREAD ...) or is there a better way of doing this !

Thx,
-Vineet