Re: [PATCH,RFC] iommu/vt-d: Convert dmar_fault IRQ to a threaded IRQ

[Baolu Lu]

On 10/25/22 4:08 PM, Lennert Buytenhek wrote:
> Under a high enough I/O page fault load, the dmar_fault hardirq handler
> can end up starving other tasks that wanted to run on the CPU that the
> IRQ is being routed to.  On an i7-6700 CPU this seems to happen at
> around 2.5 million I/O page faults per second, and at a fraction of
> that rate on some of the lower-end CPUs that we use.
>
> An I/O page fault rate of 2.5 million per second may seem like a very
> high number, but when we get an I/O page fault for every cache line
> touched by a DMA operation, this I/O page fault rate can be the result
> of a confused PCIe device DMAing to RAM at 2.5 * 64 = 160 MB/sec, which
> is not an unlikely rate to be DMAing things to RAM at.  And, in fact,
> when we do see PCIe devices getting confused like this, this sort of
> I/O page fault rate is not uncommon.
>
> A peripheral device continuously DMAing to RAM at 160 MB/s is
> inarguably a bug, either in the kernel driver for the device or in the
> firmware for the device, and should be fixed there, but it's the sort
> of bug that iommu/vt-d could be handling better than it currently does,
> and there is a fairly simple way to achieve that.
>
> This patch changes the dmar_fault IRQ handler to be a threaded IRQ
> handler.  This is a pretty minimal code change, and comes with the
> advantage that Intel IOMMU I/O page fault handling work is now subject
> to RT throttling, which allows it to be kept under control using the
> sched_rt_period_us / sched_rt_runtime_us parameters.

Thanks for the patch! I like it, but also have some concerns.

If you look at the commit history, you will find that the opposite
change took place 10+ years ago.

commit 477694e71113fd0694b6bb0bcc2d006b8ac62691
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date:   Tue Jul 19 16:25:42 2011 +0200

    x86, iommu: Mark DMAR IRQ as non-threaded

    Mark this lowlevel IRQ handler as non-threaded. This prevents a boot
    crash when "threadirqs" is on the kernel commandline. Also the
    interrupt handler is handling hardware critical events which should
    not be delayed into a thread.

    Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
    Cc: stable@xxxxxxxxxx
    Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

I am not sure whether the "boot crash" mentioned above is due to that
"trying to setup a threaded IRQ handler before kthreadd is started".

> iommu/amd already uses a threaded IRQ handler for its I/O page fault
> reporting, and so it already has this advantage.
>
> When IRQ remapping is enabled, iommu/vt-d will try to set up its
> dmar_fault IRQ handler from start_kernel() -> x86_late_time_init()
> -> apic_intr_mode_init() -> apic_bsp_setup() ->
> irq_remap_enable_fault_handling() -> enable_drhd_fault_handling(),
> which happens before kthreadd is started, and trying to set up a
> threaded IRQ handler this early on will oops.  However, there
> doesn't seem to be a reason why iommu/vt-d needs to set up its fault
> reporting IRQ handler this early, and if we remove the IRQ setup code
> from enable_drhd_fault_handling(), the IRQ will be registered instead
> from pci_iommu_init() -> intel_iommu_init() -> init_dmars(), which
> seems to work just fine.

At present, we cannot do so. Because the VT-d interrupt remapping and
DMA remapping can be independently enabled. In another words, it's a
possible case where interrupt remapping is enabled while DMA remapping
is not.

> Suggested-by: Scarlett Gourley <scarlett@xxxxxxxxxx>
> Suggested-by: James Sewart <jamessewart@xxxxxxxxxx>
> Suggested-by: Jack O'Sullivan <jack@xxxxxxxxxx>
> Signed-off-by: Lennert Buytenhek <buytenh@xxxxxxxxxx>
> ---
>   drivers/iommu/intel/dmar.c | 27 ++-------------------------
>   1 file changed, 2 insertions(+), 25 deletions(-)
>
> diff --git a/drivers/iommu/intel/dmar.c b/drivers/iommu/intel/dmar.c
> index 5a8f780e7ffd..d0871fe9d04d 100644
> --- a/drivers/iommu/intel/dmar.c
> +++ b/drivers/iommu/intel/dmar.c
> @@ -2043,7 +2043,8 @@ int dmar_set_interrupt(struct intel_iommu *iommu)
>   		return -EINVAL;
>   	}
>   
> -	ret = request_irq(irq, dmar_fault, IRQF_NO_THREAD, iommu->name, iommu);
> +	ret = request_threaded_irq(irq, NULL, dmar_fault, IRQF_ONESHOT,
> +				   iommu->name, iommu);
>   	if (ret)
>   		pr_err("Can't request irq\n");
>   	return ret;
> @@ -2051,30 +2052,6 @@ int dmar_set_interrupt(struct intel_iommu *iommu)
>   
>   int __init enable_drhd_fault_handling(void)
>   {
> -	struct dmar_drhd_unit *drhd;
> -	struct intel_iommu *iommu;
> -
> -	/*
> -	 * Enable fault control interrupt.
> -	 */
> -	for_each_iommu(iommu, drhd) {
> -		u32 fault_status;
> -		int ret = dmar_set_interrupt(iommu);
> -
> -		if (ret) {
> -			pr_err("DRHD %Lx: failed to enable fault, interrupt, ret %d\n",
> -			       (unsigned long long)drhd->reg_base_addr, ret);
> -			return -1;
> -		}
> -
> -		/*
> -		 * Clear any previous faults.
> -		 */
> -		dmar_fault(iommu->irq, iommu);
> -		fault_status = readl(iommu->reg + DMAR_FSTS_REG);
> -		writel(fault_status, iommu->reg + DMAR_FSTS_REG);
> -	}
> -
>   	return 0;
>   }
>   

Best regards,
baolu