Re: [PATCHSET block/for-next] blk-iocost: Implement absolute debt handling
From: Jens Axboe
Date: Tue Sep 10 2019 - 14:32:10 EST
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").
> 0001-0002 implement absolute debt handling. 0003-0005 improve the
> monitoring script and add debt reporting.