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 11:54:13 -0800
From:   Shaohua Li <shli@...nel.org>
To:     Tejun Heo <tj@...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

On Mon, Nov 13, 2017 at 07:57:45AM -0800, Tejun Heo wrote:
> Hello,
> 
> On Mon, Nov 13, 2017 at 03:13:48AM -0800, Tejun Heo wrote:
> > You're right.  If we wanna take this approach, we need to keep the
> > throttled flag while cloning.  The clearing part is still correct tho.
> > Without that, I get 1/4 bw limit enforced.  Hmm... I'm not quite sure
> > where that 1/4 is coming from tho.  Will investigate more.
> 
> Okay, this is because when we spiit, the split bio is the first part
> which gets issued and then the orignal bio is wound forward and
> requeued.  So, for the splits, the original bio is the one which gets
> trimmed in the front and requeued, so not clearing BIO_THROTTLED is
> enough.  I think we should still copy BIO_THROTTLED on clones so that
> we don't get suprises w/ other bio drivers.

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.

Thanks,
Shaohua