On 6/30/21 8:59 PM, Sagi Grimberg wrote:
Shouldn't we rather modify the tagset to only refer to the current
online
CPUs _only_, thereby never submit a connect request for hctx with only
offline CPUs?
Then you may setup very less io queues, and performance may suffer even
though lots of CPUs become online later.
;
Only if we stay with the reduced number of I/O queues. Which is not
what I'm
proposing; I'd rather prefer to connect and disconnect queues from
the cpu
hotplug handler. For starters we could even trigger a reset once the
first
cpu within a hctx is onlined.
Yeah, that need one big/complicated patchset, but not see any advantages
over this simple approach.
I tend to agree with Ming here.
Actually, Daniel and me came to a slightly different idea: use cpu
hotplug notifier.
Thing is, blk-mq already has cpu hotplug notifier, which should ensure
that no I/O is pending during cpu hotplug.
If we now add a nvme cpu hotplug notifier which essentially kicks off a
reset once all cpu in a hctx are offline the reset logic will rearrange
the queues to match the current cpu layout.
And when the cpus are getting onlined we'll do another reset.
Daniel is currently preparing a patch; let's see how it goes.
Cheers,
Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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