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Date: Wed, 21 Oct 2020 13:34:00 +0100 From: Russell King - ARM Linux admin <linux@...linux.org.uk> To: Arnd Bergmann <arnd@...nel.org> Cc: Maninder Singh <maninder1.s@...sung.com>, v.narang@...sung.com, a.sahrawat@...sung.com, Andrew Morton <akpm@...ux-foundation.org>, Marc Zyngier <maz@...nel.org>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Vincent Whitchurch <vincent.whitchurch@...s.com>, Nick Desaulniers <ndesaulniers@...gle.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Valentin Schneider <valentin.schneider@....com>, Dmitry Safonov <0x7f454c46@...il.com>, Thomas Gleixner <tglx@...utronix.de>, Nathan Huckleberry <nhuck@...gle.com>, Will Deacon <will@...nel.org>, Jian Cai <caij2003@...il.com>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, Uwe Kleine-König <u.kleine-koenig@...gutronix.de> Subject: Re: [PATCH 0/3] IRQ stack support for ARM On Wed, Oct 21, 2020 at 01:58:21PM +0200, Arnd Bergmann wrote: > (replying to my own mail, apparently my normal outgoing email server is > blacklisted, so resending from @kernel.org) > > On Fri, Oct 16, 2020 at 12:09 PM Arnd Bergmann <arnd@...db.de> wrote: > > > > On Thu, Oct 8, 2020 at 10:32 AM Russell King - ARM Linux admin > > <linux@...linux.org.uk> wrote: > > > On Thu, Oct 08, 2020 at 12:45:30PM +0530, Maninder Singh wrote: > > > > Observed Stack Overflow on 8KB kernel stack on ARM specially > > > > incase on network interrupts, which results in undeterministic behaviour. > > > > So there is need for per cpu dedicated IRQ stack for ARM. > > > > > > > > As ARm does not have extra co-processor register > > > > to save thread info pointer, IRQ stack will be at some > > > > performance cost, so code is under CONFIG_IRQ_STACK. > > > > > > > > and we don't have much knowledge and set up for CLANG > > > > and ARM_UNWIND, so dependency added for both cases. > > > > > > > > Tested patch set with QEMU for latest kernel > > > > and 4.1 kernel for ARM target with same patch set. > > > > > > You need to investigate and show where and why this is happening. My > > > guess is you have a network driver that uses a lot of kernel stack > > > space, which itself would be a bug. > > > > Agreed. > > > > > Note that there are compiler versions out there that mis-optimise and > > > eat stack space - the kernel build should be warning if a function > > > uses a large amount of stack. > > > > Some more ideas for figuring it out: > > > > CONFIG_DEBUG_STACK_USAGE may also be helpful in identifying > > code paths that are deeply nested with multiple functions taking a > > lot of stack space, but each one staying under the limit. > > > > CONFIG_DEBUG_STACKOVERFLOW would also help here but > > is not supported on Arm at the moment. There was a patch[1] from > > Uwe Kleine-König to add this, and I suppose we should still add > > that, in particular if it helps debug this problem. > > > > CONFIG_VMAP_STACK is probably the best way to debug > > random runtime stack overflows because using a guard page > > turns random memory corruption into an immediate oops, > > but I don't think there is an implementation for Arm yet and > > using a lot of vmalloc space means we might not be able to > > default to this. > > > > Regardless of identifying and fixing the bug Maninder found, I > > also think that supporting separate async stacks on Arm is useful > > for determinism. Most of the popular architectures use irqstack > > for this reason, and I was actually surprised that we don't do it > > on arch/arm/. > > > > Arnd > > > > [1] https://lore.kernel.org/linux-arm-kernel/20200108082913.29710-1-u.kleine-koenig@pengutronix.de/ We don't do it because we don't have a separate register to be able to store the thread_info pointer, and copying that lump between the SVC and IRQ stack will add massively to IRQ latency, especially for older machines. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
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