Re: single aio thread is migrated crazily by scheduler

[Ming Lei]

On Wed, Nov 20, 2019 at 05:03:13PM -0500, Phil Auld wrote:
> Hi Peter,
> 
> On Wed, Nov 20, 2019 at 08:16:36PM +0100 Peter Zijlstra wrote:
> > On Tue, Nov 19, 2019 at 07:40:54AM +1100, Dave Chinner wrote:
> > > On Mon, Nov 18, 2019 at 10:21:21AM +0100, Peter Zijlstra wrote:
> > 
> > > > We typically only fall back to the active balancer when there is
> > > > (persistent) imbalance and we fail to migrate anything else (of
> > > > substance).
> > > > 
> > > > The tuning mentioned has the effect of less frequent scheduling, IOW,
> > > > leaving (short) tasks on the runqueue longer. This obviously means the
> > > > load-balancer will have a bigger chance of seeing them.
> > > > 
> > > > Now; it's been a while since I looked at the workqueue code but one
> > > > possible explanation would be if the kworker that picks up the work item
> > > > is pinned. That would make it runnable but not migratable, the exact
> > > > situation in which we'll end up shooting the current task with active
> > > > balance.
> > > 
> > > Yes, that's precisely the problem - work is queued, by default, on a
> > > specific CPU and it will wait for a kworker that is pinned to that
> > 
> > I'm thinking the problem is that it doesn't wait. If it went and waited
> > for it, active balance wouldn't be needed, that only works on active
> > tasks.
> 
> Since this is AIO I wonder if it should queue_work on a nearby cpu by 
> default instead of unbound.  

When the current CPU isn't busy enough, there is still cost for completing
request remotely.

Or could we change queue_work() in the following way?

 * We try to queue the work to the CPU on which it was submitted, but if the
 * CPU dies or is saturated enough it can be processed by another CPU.

Can we decide in a simple or efficient way if the current CPU is saturated
enough?

Thanks,
Ming