linux-kernel - Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

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Date:   Fri, 19 Jan 2018 10:32:13 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Bart Van Assche <Bart.VanAssche@....com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "hch@...radead.org" <hch@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "osandov@...com" <osandov@...com>
Subject: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle

On Thu, Jan 18, 2018 at 01:11:01PM -0700, Jens Axboe wrote:
> On 1/18/18 11:47 AM, Bart Van Assche wrote:
> >> This is all very tiresome.
> > 
> > Yes, this is tiresome. It is very annoying to me that others keep
> > introducing so many regressions in such important parts of the kernel.
> > It is also annoying to me that I get blamed if I report a regression
> > instead of seeing that the regression gets fixed.
> 
> I agree, it sucks that any change there introduces the regression. I'm
> fine with doing the delay insert again until a new patch is proven to be
> better.

That way is still buggy as I explained, since rerun queue before adding
request to hctx->dispatch_list isn't correct. Who can make sure the request
is visible when __blk_mq_run_hw_queue() is called?

Not mention this way will cause performance regression again.

> 
> From the original topic of this email, we have conditions that can cause
> the driver to not be able to submit an IO. A set of those conditions can
> only happen if IO is in flight, and those cases we have covered just
> fine. Another set can potentially trigger without IO being in flight.
> These are cases where a non-device resource is unavailable at the time
> of submission. This might be iommu running out of space, for instance,
> or it might be a memory allocation of some sort. For these cases, we
> don't get any notification when the shortage clears. All we can do is
> ensure that we restart operations at some point in the future. We're SOL
> at that point, but we have to ensure that we make forward progress.

Right, it is a generic issue, not DM-specific one, almost all drivers
call kmalloc(GFP_ATOMIC) in IO path.

IMO, there is enough time for figuring out a generic solution before
4.16 release.

> 
> That last set of conditions better not be a a common occurence, since
> performance is down the toilet at that point. I don't want to introduce
> hot path code to rectify it. Have the driver return if that happens in a
> way that is DIFFERENT from needing a normal restart. The driver knows if
> this is a resource that will become available when IO completes on this
> device or not. If we get that return, we have a generic run-again delay.

Now most of times both NVMe and SCSI won't return BLK_STS_RESOURCE, and
it should be DM-only which returns STS_RESOURCE so often.

> 
> This basically becomes the same as doing the delay queue thing from DM,
> but just in a generic fashion.

Yeah, it is right.

-- 
Ming