https://github.com/hisilicon/kernel-dev/commits/private-topic-blk-mq-
shared-tags-rfc-v8
I tested this repo + megaraid_sas shared hosttag driver. This repo (5.8-rc)
has CPU hotplug patch.
" bf0beec0607d blk-mq: drain I/O when all CPUs in a hctx are offline"
Looking at description of above patch and changes, it looks like
megaraid_sas driver can still work without shared host tag for this feature.
I observe CPU hotplug works irrespective of shared host tag
Can you be clear exactly what you mean by "irrespective of shared host tag"?
Do you mean that for your test Scsi_Host.nr_hw_queues is set to expose
hw queues and scsi_host_template.map_queues = blk_mq_pci_map_queues(),
but you just don't set the host_tagset flag?
in megaraid_sas
on 5.8-rc.
Without shared host tag, megaraid driver will expose single hctx and all the
CPU will be mapped to hctx0.
right
Any CPU offline event will have " blk_mq_hctx_notify_offline" callback in
blk-mq module. If we do not have this callback/patch, we will see IO
timeout.
blk_mq_hctx_notify_offline callback will make sure all the outstanding on
hctx0 is cleared and only after it is cleared, CPU will go offline.
But that is only for when the last CPU for the hctx is going offline. If
nr_hw_queues == 1, then hctx0 would cover all CPUs, so that would never
occur during normal operation. See initial check in
blk_mq_hctx_notify_offline():
static int blk_mq_hctx_notify_offline(unsigned int cpu, struct
hlist_node *node)
{
if (!cpumask_test_cpu(cpu, hctx->cpumask) ||
!blk_mq_last_cpu_in_hctx(cpu, hctx))
return 0;
megaraid_sas driver has internal reply_queue mapping which helps to get IO
completion on same cpu. Driver get msix index from that table based on "
raw_smp_processor_id".
If table is mapped correctly at probe time, It is not possible to pick
entry of offline CPU.
Am I missing anything ?
Not sure, I think I need to be clear exactly what you're doing.
If you can help me to understand why we need shared host tag for CPU
hotplug, I can try to frame some test case for possible reproduction.
I think it's best explained in cover letter for "blk-mq: Facilitate a
shared sbitmap per tagset".
See points "HBA HW queues are required to be mapped to that of the
blk-mq hctx", "HBA LLDD would have to generate this tag internally", and
"blk-mq assumes the host may accept (Scsi_host.can_queue * #hw queue)
commands".
I just updated to include the change to have Scsi_Host.host_tagset in
4291f617a02b commit ("scsi: Add host and host template flag
'host_tagset'")
megaraid sas support is not on the branch yet, but I think everything else
required is. And it is mutable, so I'd clone it now if I were you - or
just replace
the required patch onto your v7 branch.
I am working on this.
Great, thanks
