Re: [PATCH 0/2] blk-mq: fix blk_mq_alloc_request_hctx

[Hannes Reinecke <hare@suse.de>]

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
hare@suse.de                              +49 911 74053 688
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