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

[Tim Chen]

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@xxxxxxxxxxxxx>
>>
>> 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@xxxxxxxxxxxxx>
>> Signed-off-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
>> Signed-off-by: Vineeth Remanan Pillai <vpillai@xxxxxxxxxxxxxxxx>
>> Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxxxxxxxxxx>
>> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxxxxxxx>
> 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
> 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. 

Thanks.

Tim