Re: [PATCH] blk-throttle: fix repeat limit on bio with BIO_BPS_THROTTLED

["tj@kernel.org" <tj@kernel.org>]

Hello, Yu Kuai.

On Mon, Apr 22, 2024 at 11:47:41AM +0800, Yu Kuai wrote:
> Hi!
> 
> 在 2024/04/22 11:33, 周泰宇 写道:
> > What I want to do here was to set an easy to reach value to BPS_LIMIT (10M/s in this example) and an unable to reach value to IOPS_LIMIT (100000 in this example).
> > 
> > 
> > Under this setting, the iostat shows that the bps is far less than 10M/s and sometimes is far larger than 10M/s.
> 
> Yes, I know this behaviour, and this is because blk-throttle works
> before IO split, and io stats is accounting bps for rq-based disk after
> IO split, if you using Q2C for bps you'll see that bps is stable as
> limit.
>
> Hi, Tejun！
> 
> Do you think this *phenomenon* need to be fixed? If so, I don't see a
> easy way other than throttle bio after *IO split*. Perhaps ohter than
> bio merge case, this can be another motivation to move blk-throttle to
> rq_qos_throttle().

Yeah, blk-throtl is sitting too early in the pipeline to easily track how
the bios actually get issued. However, given that it's been available for
bio-based drivers for a really long time, I don't think it'd be a good idea
to move it, so iops limit is always going to be a bit unreliable w.r.t. what
actually get issued to the device. So, IMHO, if the oddity is just about how
IOs are counted, I don't think it's a critical problem on its own.

Thanks.

-- 
tejun