Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage

[joel@joelfernandes.org]

On August 8, 2018 6:47:16 PM EDT, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>On Wed, Aug 08, 2018 at 03:15:31PM -0700, Joel Fernandes wrote:
>> On Wed, Aug 8, 2018 at 1:18 PM, Paul E. McKenney
>> <paulmck@linux.vnet.ibm.com> wrote:
>> [...]
>> >> >> >> It does start to seem like a show stopper :-(
>> >> >> >
>> >> >> > I suppose that an srcu_read_lock_nmi() and
>srcu_read_unlock_nmi() could
>> >> >> > be added, which would do atomic ops on
>sp->sda->srcu_lock_count.  Not sure
>> >> >> > whether this would be fast enough to be useful, but easy to
>provide:
>> >> >> >
>> >> >> > int __srcu_read_lock_nmi(struct srcu_struct *sp)  /*
>UNTESTED. */
>> >> >> > {
>> >> >> >         int idx;
>> >> >> >
>> >> >> >         idx = READ_ONCE(sp->srcu_idx) & 0x1;
>> >> >> >         atomic_inc(&sp->sda->srcu_lock_count[idx]);
>> >> >> >         smp_mb__after_atomic(); /* B */  /* Avoid leaking
>critical section. */
>> >> >> >         return idx;
>> >> >> > }
>> >> >> >
>> >> >> > void __srcu_read_unlock_nmi(struct srcu_struct *sp, int idx)
>> >> >> > {
>> >> >> >         smp_mb__before_atomic(); /* C */  /* Avoid leaking
>critical section. */
>> >> >> >         atomic_inc(&sp->sda->srcu_unlock_count[idx]);
>> >> >> > }
>> >> >> >
>> >> >> > With appropriate adjustments to also allow Tiny RCU to also
>work.
>> >> >> >
>> >> >> > Note that you have to use _nmi() everywhere, not just in NMI
>handlers.
>> >> >> > In fact, the NMI handlers are the one place you -don't- need
>to use
>> >> >> > _nmi(), strangely enough.
>> >> >> >
>> >> >> > Might be worth a try -- smp_mb__{before,after}_atomic() is a
>no-op on
>> >> >> > some architectures, for example.
>> >> >>
>> >> >> Continuing Steve's question on regular interrupts, do we need
>to use
>> >> >> this atomic_inc API for regular interrupts as well? So I guess
>> >> >
>> >> > If NMIs use one srcu_struct and non-NMI uses another, the
>current
>> >> > srcu_read_lock() and srcu_read_unlock() will work just fine.  If
>any given
>> >> > srcu_struct needs both NMI and non-NMI readers, then we really
>do need
>> >> > __srcu_read_lock_nmi() and __srcu_read_unlock_nmi() for that
>srcu_struct.
>> >>
>> >> Yes, I believe as long as in_nmi() works reliably, we can use the
>> >> right srcu_struct (NMI vs non-NMI) and it would be fine.
>> >>
>> >> Going through this thread, it sounds though that this_cpu_inc may
>not
>> >> be reliable on all architectures even for non-NMI interrupts and
>> >> local_inc may be the way to go.
>> >
>> > My understanding is that this_cpu_inc() is defined to handle
>interrupts,
>> > so any architecture on which it is unreliable needs to fix its bug.
> ;-)
>> 
>> Yes that's my understanding as well.
>> 
>> Then may be I'm missing something about yours/Steve's conversations
>in
>> the morning, about why we need bother with the local_inc then. So the
>> current SRCU code with the separate NMI handle should work fine (for
>> future merge windows) as long as we're using a separate srcu_struct
>> for NMI. :-)
>
>I do believe that to be true.  But only as long as that separate
>srcu_struct is -only- used for NMI.
>
>If this is what you have been pushing for all along, please accept my
>apologies for my being slow!
>

That's ok, sorry I initially didn't describe it well which may have caused confusion, but yes that's what I was pushing for.

>That said, your approach does require you to have a perfect way to
>distinguish between NMI and not-NMI.  If the distinguishing is even
>in the slightest imperfect, then some sort of NMI-safe SRCU reader
>approach is of course required.
>

Thanks Paul, agreed with everything and we are on the same page.

- Joel



>							Thanx, Paul
>
>> >> For next merge window (not this one), lets do that then? Paul, if
>you
>> >> could provide me an SRCU API that uses local_inc, then I believe
>that
>> >> coupled with this patch should be all that's needed:
>> >> https://lore.kernel.org/patchwork/patch/972657/
>> >>
>> >> Steve did express concern though if in_nmi() works reliably (i.e.
>> >> tracepoint doesn't fire from "thunk" code before in_nmi() is
>> >> available). Any thoughts on that Steve?
>> >
>> > Agreed, not the upcoming merge window.  But we do need to work out
>> > exactly what is the right way to do this.
>> 
>> Agreed, thanks!
>> 
>>  - Joel
>> 

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