lkml.org 
[lkml]   [2021]   [Jan]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH -v3 8/9] sched: Fix CPU hotplug / tighten is_per_cpu_kthread()
Date
On 21/01/21 15:18, Peter Zijlstra wrote:
> On Thu, Jan 21, 2021 at 02:01:03PM +0000, Valentin Schneider wrote:
>> On 21/01/21 11:17, Peter Zijlstra wrote:
>> > @@ -7504,6 +7525,9 @@ int sched_cpu_deactivate(unsigned int cp
>> > * preempt-disabled and RCU users of this state to go away such that
>> > * all new such users will observe it.
>> > *
>> > + * Specifically, we rely on ttwu to no longer target this CPU, see
>> > + * ttwu_queue_cond() and is_cpu_allowed().
>> > + *
>>
>> So the last time ttwu_queue_wakelist() can append a task onto a dying
>> CPU's wakelist is before sched_cpu_deactivate()'s synchronize_rcu()
>> returns.
>>
>> As discussed on IRC, paranoia would have us issue a
>>
>> flush_smp_call_function_from_idle()
>>
>> upon returning from said sync, but this will require further surgery.
>
> Right, specifically RCU needs a little more help there.
>
>> Do we want something like the below in the meantime? Ideally we'd warn on
>> setting rq->ttwu_pending when !cpu_active(), but as per the above this is
>> allowed before the synchronize_rcu() returns.
>
> I'm not sure I'm brave enough to add that just now :/

I get you; I couldn't come up with a better scheme that would give us a bit
more info than the sched_cpu_dying() splat :/

\
 
 \ /
  Last update: 2021-01-21 15:38    [W:0.042 / U:1.796 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site