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Message-ID: <1d8b7394-09bc-63ef-5bb7-de0f2ff2c75a@kernel.dk>
Date: Tue, 10 Sep 2019 12:32:02 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Tejun Heo <tj@...nel.org>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
kernel-team@...r.kernel.org, cgroups@...r.kernel.org,
newella@...com
Subject: Re: [PATCHSET block/for-next] blk-iocost: Implement absolute debt
handling
On 9/4/19 1:45 PM, Tejun Heo wrote:
> Currently, when a given cgroup doesn't have enough budget, a forced or
> merged bio will advance the cgroup's vtime by the cost calculated
> according to the hierarchical weight at the time of issue. Once vtime
> is advanced, how the cgroup's weight changes doesn't matter. It has
> to wait until global vtime catches up with the cgroup's.
>
> This means that the cost is calculated based on the hweight at the
> time of issuing but may later be paid at the wrong hweight. This, for
> example, can lead to a scenario like the following.
>
> 1. A cgroup with a very low hweight runs out of budget.
>
> 2. A storm of swap-out happens on it. All of them are scaled
> according to the current low hweight and charged to vtime pushing
> it to a far future.
>
> 3. All other cgroups go idle and now the above cgroup has access to
> the whole device. However, because vtime is already wound using
> the past low hweight, what its current hweight is doesn't matter
> until global vtime catches up to the local vtime.
>
> 4. As a result, either vrate gets ramped up extremely or the IOs stall
> while the underlying device is idle.
>
> This patchset fixes the behavior by accounting the cost of forced or
> merged bios in absolute vtime rather than cgroup-relative. This
> allows the cgroup to pay back the debt with whatever actual budget it
> has each period removing the hweight discrepancy.
>
> Note that !forced bios' costs are already accounted in absolute vtime.
> This patchset puts forced charges on the same ground.
>
> This patchset contains the following five patches and is on top of the
> current linux-block.git for-next 35e7ae82f62b ("Merge branch
> 'for-5.4/block' into for-next").
>
> 0001-blk-iocost-Account-force-charged-overage-in-absolute.patch
> 0002-blk-iocost-Don-t-let-merges-push-vtime-into-the-futu.patch
> 0003-iocost_monitor-Always-use-strings-for-json-values.patch
> 0004-iocost_monitor-Report-more-info-with-higher-accuracy.patch
> 0005-iocost_monitor-Report-debt.patch
>
> 0001-0002 implement absolute debt handling. 0003-0005 improve the
> monitoring script and add debt reporting.
Applied, thanks.
--
Jens Axboe
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