On Wed, Sep 23 2020 at 11:52, Steven Rostedt wrote:
> On Wed, 23 Sep 2020 10:40:32 +0200
> pet...@infradead.org wrote:
>
>> However, with migrate_disable() we can have each task preempted in a
>> migrate_disable() region, worse we can stack them all on the _same_ CPU
>> (super ridiculous odds, sure). And then we end up only able to run one
>> task, with the rest of the CPUs picking their nose.
>
> What if we just made migrate_disable() a local_lock() available for !RT?
>
> I mean make it a priority inheritance PER CPU lock.
>
> That is, no two tasks could do a migrate_disable() on the same CPU? If
> one task does a migrate_disable() and then gets preempted and the
> preempting task does a migrate_disable() on the same CPU, it will block
> and wait for the first task to do a migrate_enable().
>
> No two tasks on the same CPU could enter the migrate_disable() section
> simultaneously, just like no two tasks could enter a preempt_disable()
> section.
>
> In essence, we just allow local_lock() to be used for both RT and !RT.
>
> Perhaps make migrate_disable() an anonymous local_lock()?
>
> This should lower the SHC in theory, if you can't have stacked migrate
> disables on the same CPU.
I'm pretty sure this ends up in locking hell pretty fast and aside of
that it's not working for scenarios like:
kmap_local();
migrate_disable();
...
copy_from_user()
-> #PF
-> schedule()
which brought us into that discussion in the first place. You would stop
any other migrate disable user from running until the page fault is
resolved...
Thanks,
tglx
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