Hello Tejun, 10/02/2016 10:53, Tejun Heo wrote: > Hello, Adam. > > On Fri, Sep 30, 2016 at 09:02:17AM -0700, Adam Manzanares wrote: > > I'll start with the changes I made and work my way through a grep of > > ioprio. Please add or correct any of the assumptions I have made. > > Well, it looks like you're the one who's most familiar with ioprio > handling at this point. :) > > > In blk-core, the behavior before the patch is to get the ioprio for the request > > from the bio. The only references I found to bio_set_prio are in bcache. Both > > of these references are in low priority operations (gc, bg writeback) so the > > iopriority of the bio is set to IO_PRIOCLASS_IDLE in these cases. > > > > A kernel thread is used to submit these bios so the ioprio is going to come > > from the current running task if the iocontext exists. This could be a problem > > if we have set a task with high priority and some background work ends up > > getting generated in the bcache layer. I propose that we check if the > > iopriority of the bio is valid and if so, then we keep the priorirty from the > > bio. > > I wonder whether the right thing to do is adding bio->bi_ioprio which > is initialized on bio submission and carried through req->ioprio. > I looked around and thought about this and I'm not sure if this will help. I dug into the bio submission code and I thought generic_make_request was the best place to save the ioprio information. This is quite close in the call stack to init_request_from bio. Bcache sets the bio priority before the submission, so we would have to check to see if the bio priority was valid on bio submission leaving us with the same problem. Leaving the priority in the upper bits of bio->bi_rw is fine with me. It may help to have the bio->bi_ioprio for clarity, but I think we will still face the issue of having to check if this value is set when we submit the bio or init the request so I'm leaning towards leaving it as is. > > The second area that I see a potential problem is in the merging code code in > > blk-core when a bio is queued. If there is a request that is mergeable then > > the merge code takes the highest priority of the bio and the request. This > > could wipe out the values set by bio_set_prio. I think it would be > > best to set the request as non mergeable when we see that it is a high > > priority IO request. > > The current behavior should be fine for most non-pathological cases > but I have no objection to not merging ios with differing priorities. > > > The third area that is of interest is in the CFQ scheduler and the ioprio is > > only used in the case of async IO and I found that the priority is only > > obtained from the task and not from the request. This leads me to believe that > > the changes made in the blk-core to add the priority to the request will not > > impact the CFQ scheduler. > > > > The fourth area that might be concerning is the drivers. virtio_block copies > > the request priority into a virtual block request. I am assuming that this > > eventually makes it to another device driver so we don't need to worry about > > this. null block device driver also uses the ioprio, but this is also not a > > concern. lightnvm also sets the ioprio to build a request that is passed onto > > another driver. The last driver that uses the request ioprio is the fusion > > mptsas driver and I don't understand how it is using the ioprio. From what I > > can tell it is taking a request of IOPRIO_CLASS_NONE with data of 0x7 and > > calling this high priority IO. This could be impacted by the code I have > > proposed, but I believe the authors intended to treat this particular ioprio > > value as high priority. The driver will pass the request to the device > > with high priority if the appropriate ioprio values is seen on the request. > > > > The fifth area that I noticed may be impacted is file systems. btrfs uses low > > priority IO for read ahead. Ext4 uses ioprio for journaling. Both of these > > issues are not a problem because the ioprio is set on the task and not on a > > bio. > > Yeah, looks good to me. Care to include a brief summary of expected > (non)impacts in the patch description? > I'm going to send out an updated series of patches summarizing some of this discussion and I'll also include some performance results that we have measured. > Thanks. > > -- > tejun Take care, Adam -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
