linux-kernel - Re: [PATCH 7/7] blk-throtl: don't throttle the same IO multiple times

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Date:   Mon, 13 Nov 2017 11:58:54 -0800
From:   Tejun Heo <tj@...nel.org>
To:     Shaohua Li <shli@...nel.org>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, kernel-team@...com,
        lizefan@...wei.com, hannes@...xchg.org, cgroups@...r.kernel.org,
        guro@...com
Subject: Re: [PATCH 7/7] blk-throtl: don't throttle the same IO multiple times

Hello,

On Mon, Nov 13, 2017 at 11:54:13AM -0800, Shaohua Li wrote:
> I'm not sure how you are going to make this correct. The mechanism is very
> fragile. So for example, 'q->make_request_fn(q, bio)' could just queue the bio
> somewhere and handle in other context (both dm and md do this). The bio will be
> called again with generic_make_request. In this case, the second time shouldn't
> throttle the bio. The bio could be called again with generic_make_request but
> with bdev changed. In this case, the second time should throttle the bio
> (against the new bdev). There are a lot of different usages of bio. I'd rather
> not depend on generic_make_request dispatches new bio immediately. That's why I
> add a bdev in my patch.

I see.  Yeah, that makes sense.  The cloned bios may get queued to a
different request_queue asynchronously and without remembering the
last queue, you can't tell whether it's a new queue or not.  I'll drop
this one and test with your patch.

Thanks.

-- 
tejun