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+ioprio. Please add or correct any of the assumptions I have made. + +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. + +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 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. + +> Thanks. +> +> -- +> tejun + +Take care, +Adam diff --git a/a/content_digest b/N1/content_digest index 82c18c2..7dd9af9 100644 --- a/a/content_digest +++ b/N1/content_digest @@ -5,88 +5,75 @@ "Subject\0Re: [PATCH 1/3] block: Add iocontext priority to request\0" "Date\0Fri, 30 Sep 2016 09:02:17 -0700\0" "To\0Tejun Heo <tj@kernel.org>\0" - "Cc\0<axboe@kernel.dk>" - <linux-block@vger.kernel.org> - " <linux-ide@vger.kernel.org>\0" + "Cc\0axboe@kernel.dk" + linux-block@vger.kernel.org + " linux-ide@vger.kernel.org\0" "\00:1\0" "b\0" - "SGVsbG8gVGVqdW4sCgpUaGUgMDkvMjkvMjAxNiAxMDo0MCwgVGVqdW4gSGVvIHdyb3RlOgo+IEhl\n" - "bGxvLAo+IAo+IE9uIFR1ZSwgU2VwIDI3LCAyMDE2IGF0IDExOjE0OjU0QU0gLTA3MDAsIEFkYW0g\n" - "TWFuemFuYXJlcyB3cm90ZToKPiA+IFBhdGNoIGFkZHMgYXNzb2NpYXRpb24gYmV0d2VlbiBpb2Nv\n" - "bnRleHQgYW5kIGEgcmVxdWVzdC4KPiA+IAo+ID4gU2lnbmVkLW9mZi1ieTogQWRhbSBNYW56YW5h\n" - "cmVzIDxhZGFtLm1hbnphbmFyZXNAaGdzdC5jb20+Cj4gCj4gQ2FuIHlvdSBwbGVhc2UgZGVzY3Jp\n" - 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Please add or correct any of the assumptions I have made. \n" + " \n" + "In blk-core, the behavior before the patch is to get the ioprio for the request \n" + "from the bio. The only references I found to bio_set_prio are in bcache. Both \n" + "of these references are in low priority operations (gc, bg writeback) so the \n" + "iopriority of the bio is set to IO_PRIOCLASS_IDLE in these cases. \n" + " \n" + "A kernel thread is used to submit these bios so the ioprio is going to come \n" + "from the current running task if the iocontext exists. This could be a problem \n" + "if we have set a task with high priority and some background work ends up \n" + "getting generated in the bcache layer. I propose that we check if the \n" + "iopriority of the bio is valid and if so, then we keep the priorirty from the \n" + "bio. \n" + " \n" + "The second area that I see a potential problem is in the merging code code in \n" + "blk-core when a bio is queued. If there is a request that is mergeable then \n" + "the merge code takes the highest priority of the bio and the request. This \n" + "could wipe out the values set by bio_set_prio. I think it would be \n" + "best to set the request as non mergeable when we see that it is a high \n" + "priority IO request. \n" + " \n" + "The third area that is of interest is in the CFQ scheduler and the ioprio is \n" + "only used in the case of async IO and I found that the priority is only \n" + "obtained from the task and not from the request. This leads me to believe that \n" + "the changes made in the blk-core to add the priority to the request will not \n" + "impact the CFQ scheduler. \n" + "\n" + "The fourth area that might be concerning is the drivers. virtio_block copies \n" + "the request priority into a virtual block request. I am assuming that this \n" + "eventually makes it to another device driver so we don't need to worry about \n" + "this. null block device driver also uses the ioprio, but this is also not a \n" + "concern. lightnvm also sets the ioprio to build a request that is passed onto \n" + "another driver. The last driver that uses the request ioprio is the fusion \n" + "mptsas driver and I don't understand how it is using the ioprio. From what I \n" + "can tell it is taking a request of IOPRIO_CLASS_NONE with data of 0x7 and \n" + "calling this high priority IO. This could be impacted by the code I have \n" + "proposed, but I believe the authors intended to treat this particular ioprio \n" + "value as high priority. The driver will pass the request to the device \n" + "with high priority if the appropriate ioprio values is seen on the request. \n" + " \n" + "The fifth area that I noticed may be impacted is file systems. btrfs uses low \n" + "priority IO for read ahead. Ext4 uses ioprio for journaling. Both of these \n" + "issues are not a problem because the ioprio is set on the task and not on a \n" + "bio.\n" + "\n" + "> Thanks.\n" + "> \n" + "> -- \n" + "> tejun\n" + "\n" + "Take care,\n" + Adam -878c3c8731fdaf83016ff6ec3ddc0d27ac3bbeff38eeb630d38518e90c06f732 +bc8465216deac9dbe7a158a139213c082bef3125c13d60dc2efc45161315a39a
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