On 1/19/18 10:05 AM, Ming Lei wrote: > On Fri, Jan 19, 2018 at 09:52:32AM -0700, Jens Axboe wrote: >> On 1/19/18 9:47 AM, Mike Snitzer wrote: >>> On Fri, Jan 19 2018 at 11:41am -0500, >>> Jens Axboe <axboe@xxxxxxxxx> wrote: >>> >>>> On 1/19/18 9:37 AM, Ming Lei wrote: >>>>> On Fri, Jan 19, 2018 at 09:27:46AM -0700, Jens Axboe wrote: >>>>>> On 1/19/18 9:26 AM, Ming Lei wrote: >>>>>>> On Fri, Jan 19, 2018 at 09:19:24AM -0700, Jens Axboe wrote: >>>>>> >>>>>> There are no pending requests for this case, nothing to restart the >>>>>> queue. When you fail that blk_get_request(), you are idle, nothing >>>>>> is pending. >>>>> >>>>> I think we needn't worry about that, once a device is attached to >>>>> dm-rq, it can't be mounted any more, and usually user don't use the device >>>>> directly and by dm-mpath at the same time. >>>> >>>> Even if it doesn't happen for a normal dm setup, it is a case that >>>> needs to be handled. The request allocation is just one example of >>>> a wider scope resource that can be unavailable. If the driver returns >>>> NO_DEV_RESOURCE (or whatever name), it will be a possibility that >>>> the device itself is currently idle. >>> >>> How would a driver's resources be exhausted yet the device is idle (so >>> as not to be able to benefit from RESTART)? >> >> I've outlined a number of these examples already. Another case might be: >> >> 1) Device is idle >> 2) Device gets request >> 3) Device attempts to DMA map >> 4) DMA map fails because the IOMMU is out of space (nic is using it all) >> 5) Device returns STS_RESOURCE >> 6) Queue is marked as needing a restart >> >> All's well, except there is no IO on this device that will notice the >> restart bit and retry the operation. >> >> Replace IOMMU failure with any other resource that the driver might need >> for an IO, which isn't tied to a device specific resource. >> blk_get_request() on dm is an example, as is any allocation failure >> occurring in the queue IO path for the driver. > > Yeah, if we decide to take the approach of introducing NO_DEV_RESOURCE, all > the current STS_RESOURCE for non-device resource allocation(kmalloc, dma > map, get_request, ...) should be converted to NO_DEV_RESOURCE. > > And it is a generic issue, which need generic solution. Precisely. > Seems running queue after arbitrary in this case is the only way we > thought of, or other solutions? I think that is the only solution. If it's a frequent enough occurence to cause performance issues, then it's likely down to a specific resource shortage, and we can tackle that independently (we need to, since each of those will need a specialized solution). > If the decision is made, let's make/review the patch, :-) Let 'er rip. -- Jens Axboe
