Re: [PATCH -next 4/5] blk-iocost: bypass if only one cgroup issues io

[Yu Kuai]

Hi, Tejun!

在 2022/10/12 1:02, Tejun Heo 写道:
> On Tue, Oct 11, 2022 at 04:35:46PM +0800, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > In this special case, there is no need to throttle io.
> >
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   block/blk-iocost.c | 9 +++++++--
> >   1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> > index 5acc5f13bbd6..32e7e416d67c 100644
> > --- a/block/blk-iocost.c
> > +++ b/block/blk-iocost.c
> > @@ -2564,8 +2564,13 @@ static void ioc_rqos_throttle(struct rq_qos *rqos, struct bio *bio)
> >   	bool use_debt, ioc_locked;
> >   	unsigned long flags;
> >   
> > -	/* bypass IOs if disabled, still initializing, or for root cgroup */
> > -	if (!ioc->enabled || !iocg || !iocg->level)
> > +	/*
> > +	 * bypass IOs if disabled, still initializing, for root cgroup,
> > +	 * or the cgroup is the only cgroup with io.
> > +	 */
> > +	if (!ioc->enabled || !iocg || !iocg->level ||
> > +	    (iocg->hweight_inuse == WEIGHT_ONE &&
> > +	     atomic_read(&ioc->hweight_gen) == iocg->hweight_gen))
>
> I'm not sure about this one. Bypassing here means that we lose track of how
> much IO it's issuing which can affect future throttling decisions, right?

Yes, you're right, this patch doesn't look good in this case.

The reason why I tried to do this is because during test, I found that
io performance is affected when I only issue io from one cgroup（only
happened in some environment with default configuration）, and I found
out that each io is throttled for some time before dispatching.

Perhaps a suitable configuration can avoid this problem.

Thanks,
Kuai

> Thanks.