Re: [RFC PATCH 06/16] sched: Add core wide task selection and scheduling.

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jul 01, 2020 at 05:54:11PM -0700, Tim Chen wrote:
> 
> 
> On 7/1/20 4:28 PM, Joel Fernandes wrote:
> > On Tue, Jun 30, 2020 at 09:32:27PM +0000, Vineeth Remanan Pillai wrote:
> >> From: Peter Zijlstra <peterz@infradead.org>
> >>
> >> Instead of only selecting a local task, select a task for all SMT
> >> siblings for every reschedule on the core (irrespective which logical
> >> CPU does the reschedule).
> >>
> >> There could be races in core scheduler where a CPU is trying to pick
> >> a task for its sibling in core scheduler, when that CPU has just been
> >> offlined.  We should not schedule any tasks on the CPU in this case.
> >> Return an idle task in pick_next_task for this situation.
> >>
> >> NOTE: there is still potential for siblings rivalry.
> >> NOTE: this is far too complicated; but thus far I've failed to
> >>       simplify it further.
> >>
> >> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> >> Signed-off-by: Julien Desfossez <jdesfossez@digitalocean.com>
> >> Signed-off-by: Vineeth Remanan Pillai <vpillai@digitalocean.com>
> >> Signed-off-by: Aaron Lu <aaron.lu@linux.alibaba.com>
> >> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > 
> > Hi Peter, Tim, all, the below patch fixes the hotplug issue described in the
> > below patch's Link tag. Patch description below describes the issues fixed
> > and it applies on top of this patch.
> > 
> > ------8<----------
> > 
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> > Subject: [PATCH] sched: Fix CPU hotplug causing crashes in task selection logic
> > 
> > The selection logic does not run correctly if the current CPU is not in the
> > cpu_smt_mask (which it is not because the CPU is offlined when the stopper
> > finishes running and needs to switch to idle).  There are also other issues
> > fixed by the patch I think such as: if some other sibling set core_pick to
> > something, however the selection logic on current cpu resets it before
> > selecting. In this case, we need to run the task selection logic again to
> > make sure it picks something if there is something to run. It might end up
> > picking the wrong task.  Yet another issue was, if the stopper thread is an

"It might end up picking the wrong task" needs to be: "We might end up
picking a different task but that's Ok".

> > unconstrained pick, then rq->core_pick is set. The next time task selection
> > logic runs when stopper needs to switch to idle, the current CPU is not in
> > the smt_mask. This causes the previous ->core_pick to be picked again which
> > happens to be the unconstrained task! so the stopper keeps getting selected
> > forever.
> > 
> > That and there are a few more safe guards and checks around checking/setting
> > rq->core_pick. To test it, I ran rcutorture and made it tag all torture
> > threads. Then ran it in hotplug mode (hotplugging every 200ms) and it hit the
> > issue. Now it runs for an hour or so without issue. (Torture testing debug
> > changes: https://bit.ly/38htfqK ).
> > 
> > Various fixes were tried causing varying degrees of crashes.  Finally I found
> > that it is easiest to just add current CPU to the smt_mask's copy always.
> > This is so that task selection logic always runs on the current CPU which
> > called schedule().
> 
> 
> It looks good to me. 

Thank you for your review! Could I add your Reviewed-by tag to the patch?

 - Joel


> Thanks.
> 
> Tim