linux-kernel - Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request

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Date:   Mon, 24 Oct 2022 17:56:15 +0100
From:   John Garry <john.garry@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <linux-kernel@...r.kernel.org>,
        <linux-block@...r.kernel.org>, <hch@....de>,
        Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH] blk-mq: Properly init bios from
 blk_mq_alloc_request_hctx()

On 24/10/2022 14:27, Ming Lei wrote:
>>>> -	rq->bio = rq->biotail = NULL;
>>> This patch looks not good, why do you switch to initialize the three fields
>>> twice in fast path?
>> Can you please show me how these are initialized twice?
> blk_mq_bio_to_request() is one which setup these fields, then you add
> another one in blk_mq_rq_ctx_init().

ok, understood.

> 
>> If there is a real concern with this then we go with my original idea, which
>> was to copy the init method of blk_mq_alloc_request() (in
>> blk_mq_alloc_request_hctx())
>>
>>> BTW, we know blk_mq_alloc_request_hctx() has big trouble, so please
>>> avoid to extend it to other use cases.
>> Yeah, I know this,
> Did you know the exact issue on nvme-tcp, nvme-rdma or nvme-fc maybe
> with blk_mq_alloc_request_hctx()?

I thought that the original issue was an OoO bounds issue, fixed in 
14dc7a18. Now there is still some issue in the following link, which is 
still unresolved as I understand:
https://lore.kernel.org/linux-block/5bd886f1-a7c6-b765-da29-777be0328bc2@grimberg.me/#t

But I think that 14dc7a18 may still leave undesirable scenario:
- all cpus in HW queue cpumask may go offline after cpu_online_mask read 
in blk_mq_alloc_request_hctx() and before we get the driver tag and set 
rq->hctx

> 
>> but sometimes we just need to allocate for a specific HW
>> queue...
>>
>> For my usecase of interest, it should not impact if the cpumask of the HW
>> queue goes offline after selecting the cpu in blk_mq_alloc_request_hctx(),
>> so any race is ok ... I think.
>>
>> However it should be still possible to make blk_mq_alloc_request_hctx() more
>> robust. How about using something like work_on_cpu_safe() to allocate and
>> execute the request with blk_mq_alloc_request() on a cpu associated with the
>> HW queue, such that we know the cpu is online and stays online until we
>> execute it? Or also extent to work_on_cpumask_safe() variant, so that we
>> don't need to try all cpus in the mask (to see if online)?
> But all cpus on this hctx->cpumask could become offline.

If all hctx->cpumask are offline then we should not allocate a request 
and this is acceptable. Maybe I am missing your point.

Thanks,
John