Messages in this thread | | | Date | Tue, 25 Oct 2022 08:40:38 +0100 | From | John Garry <> | Subject | Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request_hctx() |
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On 25/10/2022 01:34, Ming Lei wrote: >>>> 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. > As you saw, this API has the above problem too, but any one of CPUs > may become online later, maybe just during blk_mq_alloc_request_hctx(), > and it is easy to cause inconsistence. > > You didn't share your use case, but for nvme connection request, if it > is 1:1 mapping, if any one of CPU becomes offline, the controller > initialization could be failed, that isn't good from user viewpoint at > all.
My use case is in SCSI EH domain. For my HBA controller of interest, to abort an erroneous IO we must send a controller proprietary abort command on same HW queue as original command. So we would need to allocate this abort request for a specific HW queue.
I mentioned before that if no hctx->cpumask is online then we don't need to allocate a request. That is because if no hctx->cpumask is online, this means that original erroneous IO must be completed due to nature of how blk-mq cpu hotplug handler works, i.e. drained, and then we don't actually need to abort it any longer, so ok to not get a request.
I have an RFC series for this work in which I am using blk_mq_alloc_request_hctx(). However, as I mentioned before, I can experiment with using something like work_on_cpu_safe() to alloc and execute the abort request to safeguard against cpu hotplug events.
Thanks, John
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