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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Steven Rostedt <rostedt@goodmis.org>, peterz@infradead.org
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends
In-Reply-To: <20200923115251.7cc63a7e@oasis.local.home>
References: <20200919091751.011116649@linutronix.de> <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com> <87mu1lc5mp.fsf@nanos.tec.linutronix.de> <87k0wode9a.fsf@nanos.tec.linutronix.de> <CAHk-=wgbmwsTOKs23Z=71EBTrULoeaH2U3TNqT2atHEWvkBKdw@mail.gmail.com> <87eemwcpnq.fsf@nanos.tec.linutronix.de> <CAHk-=wgF-upZVpqJWK=TK7MS9H-Rp1ZxGfOG+dDW=JThtxAzVQ@mail.gmail.com> <87a6xjd1dw.fsf@nanos.tec.linutronix.de> <CAHk-=wjhxzx3KHHOMvdDj3Aw-_Mk5eRiNTUBB=tFf=vTkw1FeA@mail.gmail.com> <87sgbbaq0y.fsf@nanos.tec.linutronix.de> <20200923084032.GU1362448@hirez.programming.kicks-ass.net> <20200923115251.7cc63a7e@oasis.local.home>
Date:   Wed, 23 Sep 2020 22:55:54 +0200
Message-ID: <874kno9pr9.fsf@nanos.tec.linutronix.de>
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On Wed, Sep 23 2020 at 11:52, Steven Rostedt wrote:
> On Wed, 23 Sep 2020 10:40:32 +0200
> peterz@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