Re: [PATCH v4] iommu: Optimise PCI SAC address trick

[Vasant Hegde <vasant.hegde@amd.com>]

Robin,


On 4/18/2023 4:27 PM, Robin Murphy wrote:
> On 2023-04-18 10:23, Vasant Hegde wrote:
>> Robin,
>>
>>
>> On 4/13/2023 7:10 PM, Robin Murphy wrote:
>>> Per the reasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for
>>> PCI SAC address trick") and its subsequent revert, this mechanism no
>>> longer serves its original purpose, but now only works around broken
>>> hardware/drivers in a way that is unfortunately too impactful to remove.
>>>
>>> This does not, however, prevent us from solving the performance impact
>>> which that workaround has on large-scale systems that don't need it.
>>> Once the 32-bit IOVA space fills up and a workload starts allocating and
>>> freeing on both sides of the boundary, the opportunistic SAC allocation
>>> can then end up spending significant time hunting down scattered
>>> fragments of free 32-bit space, or just reestablishing max32_alloc_size.
>>> This can easily be exacerbated by a change in allocation pattern, such
>>> as by changing the network MTU, which can increase pressure on the
>>> 32-bit space by leaving a large quantity of cached IOVAs which are now
>>> the wrong size to be recycled, but also won't be freed since the
>>> non-opportunistic allocations can still be satisfied from the whole
>>> 64-bit space without triggering the reclaim path.
>>>
>>> However, in the context of a workaround where smaller DMA addresses
>>> aren't simply a preference but a necessity, if we get to that point at
>>> all then in fact it's already the endgame. The nature of the allocator
>>> is currently such that the first IOVA we give to a device after the
>>> 32-bit space runs out will be the highest possible address for that
>>> device, ever. If that works, then great, we know we can optimise for
>>> speed by always allocating from the full range. And if it doesn't, then
>>> the worst has already happened and any brokenness is now showing, so
>>> there's little point in continuing to try to hide it.
>>>
>>> To that end, implement a flag to refine the SAC business into a
>>> per-device policy that can automatically get itself out of the way if
>>> and when it stops being useful.
>>>
>>> CC: Linus Torvalds <torvalds@linux-foundation.org>
>>> CC: Jakub Kicinski <kuba@kernel.org>
>>> Reviewed-by: John Garry <john.g.garry@oracle.com>
>>> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
>>
>> We hit kernel softlockup while running stress-ng test system having 384 CPU and
>> NVMe disk. This patch helped to solve one soft lockup in allocation path.
>>
>>> ---
>>>
>>> v4: Rebase to use the new bitfield in dev_iommu, expand commit message.
>>>
>>>   drivers/iommu/dma-iommu.c | 26 ++++++++++++++++++++------
>>>   drivers/iommu/dma-iommu.h |  8 ++++++++
>>>   drivers/iommu/iommu.c     |  3 +++
>>>   include/linux/iommu.h     |  2 ++
>>>   4 files changed, 33 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
>>> index 99b2646cb5c7..9193ad5bc72f 100644
>>> --- a/drivers/iommu/dma-iommu.c
>>> +++ b/drivers/iommu/dma-iommu.c
>>> @@ -630,7 +630,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct
>>> iommu_domain *domain,
>>>   {
>>>       struct iommu_dma_cookie *cookie = domain->iova_cookie;
>>>       struct iova_domain *iovad = &cookie->iovad;
>>> -    unsigned long shift, iova_len, iova = 0;
>>> +    unsigned long shift, iova_len, iova;
>>>         if (cookie->type == IOMMU_DMA_MSI_COOKIE) {
>>>           cookie->msi_iova += size;
>>> @@ -645,15 +645,29 @@ static dma_addr_t iommu_dma_alloc_iova(struct
>>> iommu_domain *domain,
>>>       if (domain->geometry.force_aperture)
>>>           dma_limit = min(dma_limit, (u64)domain->geometry.aperture_end);
>>>   -    /* Try to get PCI devices a SAC address */
>>> -    if (dma_limit > DMA_BIT_MASK(32) && !iommu_dma_forcedac && dev_is_pci(dev))
>>> +    /*
>>> +     * Try to use all the 32-bit PCI addresses first. The original SAC vs.
>>> +     * DAC reasoning loses relevance with PCIe, but enough hardware and
>>> +     * firmware bugs are still lurking out there that it's safest not to
>>> +     * venture into the 64-bit space until necessary.
>>> +     *
>>> +     * If your device goes wrong after seeing the notice then likely either
>>> +     * its driver is not setting DMA masks accurately, the hardware has
>>> +     * some inherent bug in handling >32-bit addresses, or not all the
>>> +     * expected address bits are wired up between the device and the IOMMU.
>>> +     */
>>> +    if (dma_limit > DMA_BIT_MASK(32) && dev->iommu->pci_32bit_workaround) {
>>>           iova = alloc_iova_fast(iovad, iova_len,
>>>                          DMA_BIT_MASK(32) >> shift, false);
>>> +        if (iova)
>>> +            goto done;
>>>   -    if (!iova)
>>> -        iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift,
>>> -                       true);
>>> +        dev->iommu->pci_32bit_workaround = false;
>>> +        dev_notice(dev, "Using %d-bit DMA addresses\n", bits_per(dma_limit));
>>
>> May be dev_notice_once? Otherwise we may see this message multiple time for same
>> device like below:
> 
> Oh, that's a bit irritating. Of course multiple threads can reach this
> in parallel, silly me :(
> 
> I would really prefer the notice to be once per device rather than once
> globally, since there's clearly no guarantee that the first device to

Agree. Makes sense.

> hit this case is going to be the one which is liable to go wrong. Does
> the (untested) diff below do any better?


Thanks for the patch. I have tested and its working fine.

-Vasant