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Message-ID: <5614F9F9.90607@oracle.com>
Date: Wed, 07 Oct 2015 18:54:49 +0800
From: Bob Liu <bob.liu@...cle.com>
To: Roger Pau Monné <roger.pau@...rix.com>
CC: xen-devel@...ts.xen.org, david.vrabel@...rix.com,
linux-kernel@...r.kernel.org, konrad.wilk@...cle.com,
felipe.franciosi@...rix.com, axboe@...com, hch@...radead.org,
avanzini.arianna@...il.com, rafal.mielniczuk@...rix.com,
boris.ostrovsky@...cle.com, jonathan.davies@...rix.com
Subject: Re: [PATCH v3 9/9] xen/blkback: get number of hardware queues/rings
from blkfront
On 10/05/2015 11:15 PM, Roger Pau Monné wrote:
> El 05/09/15 a les 14.39, Bob Liu ha escrit:
>> Backend advertises "multi-queue-max-queues" to front, and then read back the
>> final negotiated queues/rings from "multi-queue-num-queues" which is wrote by
>> blkfront.
>>
>> Signed-off-by: Bob Liu <bob.liu@...cle.com>
>> ---
>> drivers/block/xen-blkback/blkback.c | 8 ++++++++
>> drivers/block/xen-blkback/xenbus.c | 36 ++++++++++++++++++++++++++++++-----
>> 2 files changed, 39 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
>> index fd02240..b904fe05f0 100644
>> --- a/drivers/block/xen-blkback/blkback.c
>> +++ b/drivers/block/xen-blkback/blkback.c
>> @@ -83,6 +83,11 @@ module_param_named(max_persistent_grants, xen_blkif_max_pgrants, int, 0644);
>> MODULE_PARM_DESC(max_persistent_grants,
>> "Maximum number of grants to map persistently");
>>
>> +unsigned int xenblk_max_queues;
>> +module_param_named(max_queues, xenblk_max_queues, uint, 0644);
>> +MODULE_PARM_DESC(max_queues,
>> + "Maximum number of hardware queues per virtual disk");
>> +
>> /*
>> * Maximum order of pages to be used for the shared ring between front and
>> * backend, 4KB page granularity is used.
>> @@ -1458,6 +1463,9 @@ static int __init xen_blkif_init(void)
>> xen_blkif_max_ring_order = XENBUS_MAX_RING_PAGE_ORDER;
>> }
>>
>> + /* Allow as many queues as there are CPUs, by default */
>> + xenblk_max_queues = num_online_cpus();
>
> Hm, I'm not sure of the best way to set a default value for this.
> Consider for example a scenario were Dom0 is limited to 2vCPUs, but DomU
> has 8 vCPUs. Are we going to limit the number of queues to two? Is that
> the most appropriate value from a performance PoV?
>
> I have to admit I don't have a clear idea of a default value for this
> field, and maybe the number of CPUs on the backend is indeed what works
> better, but there needs to be a comment explaining the reasoning behind
> this setting.
>
It looks like xen-netback also chose num online cpus as the default value.
Anyway, that's not a big problem and can be fixed easily in future.
Thanks again for reviewing this big patch set!
Regards,
-Bob
>> rc = xen_blkif_interface_init();
>> if (rc)
>> goto failed_init;
>> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
>> index 04b8d0d..aa97ea5 100644
>> --- a/drivers/block/xen-blkback/xenbus.c
>> +++ b/drivers/block/xen-blkback/xenbus.c
>> @@ -28,6 +28,8 @@
>> #define RINGREF_NAME_LEN (20)
>> #define RINGREF_NAME_LEN (20)
>>
>> +extern unsigned int xenblk_max_queues;
>
> This should live in blkback/common.h
>
>> +
>> struct backend_info {
>> struct xenbus_device *dev;
>> struct xen_blkif *blkif;
>> @@ -191,11 +193,6 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
>> atomic_set(&blkif->drain, 0);
>> INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
>>
>> - blkif->nr_rings = 1;
>> - if (xen_blkif_alloc_rings(blkif)) {
>> - kmem_cache_free(xen_blkif_cachep, blkif);
>> - return ERR_PTR(-ENOMEM);
>> - }
>> return blkif;
>> }
>>
>> @@ -618,6 +615,14 @@ static int xen_blkbk_probe(struct xenbus_device *dev,
>> goto fail;
>> }
>>
>> + /* Multi-queue: wrte how many queues backend supported. */
> ^ write how many queues are supported by the
> backend.
>> + err = xenbus_printf(XBT_NIL, dev->nodename,
>> + "multi-queue-max-queues", "%u", xenblk_max_queues);
>> + if (err) {
>> + pr_debug("Error writing multi-queue-num-queues\n");
> ^ pr_warn at least.
>> + goto fail;
>> + }
>> +
>> /* setup back pointer */
>> be->blkif->be = be;
>>
>> @@ -1008,6 +1013,7 @@ static int connect_ring(struct backend_info *be)
>> char *xspath;
>> size_t xspathsize;
>> const size_t xenstore_path_ext_size = 11; /* sufficient for "/queue-NNN" */
>> + unsigned int requested_num_queues = 0;
>>
>> pr_debug("%s %s\n", __func__, dev->otherend);
>>
>> @@ -1035,6 +1041,26 @@ static int connect_ring(struct backend_info *be)
>> be->blkif->vbd.feature_gnt_persistent = pers_grants;
>> be->blkif->vbd.overflow_max_grants = 0;
>>
>> + /*
>> + * Read the number of hardware queus from frontend.
> ^ queues
>> + */
>> + err = xenbus_scanf(XBT_NIL, dev->otherend, "multi-queue-num-queues", "%u", &requested_num_queues);
>> + if (err < 0) {
>> + requested_num_queues = 1;
>> + } else {
>> + if (requested_num_queues > xenblk_max_queues
>> + || requested_num_queues == 0) {
>> + /* buggy or malicious guest */
>> + xenbus_dev_fatal(dev, err,
>> + "guest requested %u queues, exceeding the maximum of %u.",
>> + requested_num_queues, xenblk_max_queues);
>> + return -1;
>> + }
>> + }
>> + be->blkif->nr_rings = requested_num_queues;
>> + if (xen_blkif_alloc_rings(be->blkif))
>> + return -ENOMEM;
>> +
>> pr_info("nr_rings:%d protocol %d (%s) %s\n", be->blkif->nr_rings,
>> be->blkif->blk_protocol, protocol,
>> pers_grants ? "persistent grants" : "");
>>
--
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