RE: [RFC 16/16] lpfc: vmid: Introducing vmid in io path.

Muneendra Kumar M
2020-08-06T17:10:01Z

Hi James,

As Tejun and Ming is suggesting to have interface close to the usage as
fabric infrasture I was thinking of this approach from the inputs provided
by you in the below mail.
Please provide your inputs on this approach.

1) blkcg will have a new field to store driver specific information as
"blkio_cg_ priv_data"(in the current patch it is app_identifier) as Tejun
said he doesn’t mind cgroup data structs carrying extra bits for stuff.
2) scsi transport will provide a new interface(sysfs) as register_vm_fabric
3) As part of this interface user/deamon will provide the details of VM such
as UUID,PID on VM creation to the transport .
4) With VM PID information we need to find the associated blkcg and needs to
update the UUID info in blkio_cg_ priv_data.
5) Once we update the blkio_cg_ priv_data with vmid all the io’s issued from
VM will have the UUID info as part of blkcg.

With this we can also address the concerns raised by Tejun and the
existing lpfc patches still holds good.

Regards,
Muneendra.

-----Original Message-----
From: James Smart [mailto:james.smart@broadcom.com]
Sent: Thursday, August 6, 2020 5:08 AM
To: Hannes Reinecke <hare@suse.de>; Muneendra
<muneendra.kumar@broadcom.com>; linux-block@vger.kernel.org;
linux-scsi@vger.kernel.org
Cc: pbonzini@redhat.com; emilne@redhat.com; mkumar@redhat.com; Gaurav
Srivastava <gaurav.srivastava@broadcom.com>; James Smart
<jsmart2021@gmail.com>
Subject: Re: [RFC 16/16] lpfc: vmid: Introducing vmid in io path.

That's actually what's supposed to be happening. fastpath uses the uuid to
look up a vmid tag. If no vmid tag, kick off the fabric traffic that will
get one but don't wait for it to complete. Any io issued while that process
is occurring will be not be vmid tagged. I'll circle back on lpfc to make
sure this is happening.

In the mean time - the most important patch to review is the cgroup patch -
patch1.

If we wanted to speed the driver's io path up, one thing to consider is
adding a driver-settable value on the blkcg structure. Once the fabric
traffic obtained the vmid, the driver would set the blkcg structure with the
value. In this scenario though, as the vmid is destroyed as of link down,
the driver needs a way, independent of an io, to reach into the blkcg struct
to clear the vmid value. We also need to be sure the blkcg struct won't be
on top of a multipath device or something such that the blkcg struct may be
referenced by a different scsi host - I assume we're good in that area.

-- james