Re: [PATCH v2] sched: async unthrottling for cfs bandwidth

[Tejun Heo <tj@kernel.org>]

Hello,

(cc'ing Michal, Christian and Li for context)

On Tue, Nov 01, 2022 at 02:59:56PM -0700, Josh Don wrote:
> > If most of the problems were with cpu bw control, fixing that should do for
> > the time being. Otherwise, we'll have to think about finishing kernfs
> > locking granularity improvements and doing something similar to cgroup
> > locking too.
> 
> Oh we've easily hit stalls measured in multiple seconds. We
> extensively use cpu.idle to group batch tasks. One of the memory
> bandwidth mitigations implemented in userspace is cpu jailing, which
> can end up pushing lots and lots of these batch threads onto a small
> number of cpus. 5ms min gran * 200 threads is already one second :)

Ah, I see.

> We're in the process of transitioning to using bw instead for this
> instead in order to maintain parallelism. Fixing bw is definitely
> going to be useful, but I'm afraid we'll still likely have some issues
> from low throughput for non-bw reasons (some of which we can't
> directly control, since arbitrary jobs can spin up and configure their
> hierarchy/threads in antagonistic ways, in effect pushing out the
> latency of some of their threads).

Yeah, thanks for the explanation. Making the lock more granular is tedious
but definitely doable. I don't think I can work on it in the near future but
will keep it on mind. If anyone's interested in attacking it, please be my
guest.

Thanks.

-- 
tejun