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 11:58:30 -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.

Oh, I think there is a better solution. Not adding a new bdev is possible. We
always set the BIO_THROTTLED flag after block-throttle and copy the flag in
clone. In bio_set_dev, we clear the flag. This should work I think.

Thanks,
Shaohua