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

[Joel Fernandes <joelaf@google.com>]

On Wed, Aug 8, 2018 at 7:49 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
[...]
>
>> In that case based on what you're saying, the patch I sent to using
>> different srcu_struct for NMI is still good I guess...
>
> As long as you wait for both SRCU grace periods.  Hmmm...  Maybe that means
> that there is still a use for synchronize_rcu_mult():
>
>         void call_srcu_nmi(struct rcu_head *rhp, rcu_callback_t func)
>         {
>                 call_srcu(&trace_srcu_struct_nmi, rhp, func);
>         }
>
>         void call_srcu_nonmi(struct rcu_head *rhp, rcu_callback_t func)
>         {
>                 call_srcu(&trace_srcu_struct_nonmi, rhp, func);
>         }
>
>         ...
>
>         /* Wait concurrently on the two grace periods. */
>         synchronize_rcu_mult(call_srcu_nmi, call_srcu_nonmi);
>
> On the other hand, I bet that doing this is just fine in your use case:
>
>         synchronize_srcu(&trace_srcu_struct_nmi);
>         synchronize_srcu(&trace_srcu_struct_nonmi);
>
> But please note that synchronize_rcu_mult() is no more in my -rcu tree,
> so if you do want it please let me know (and please let me know why it
> is important).

I did the chaining thing (one callback calling another), that should
work too right? I believe that is needed so that the tracepoint
callbacks are freed at one point and only when both NMI and non-NMI
read sections have completed.

>> >> 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.

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?

thanks!

- Joel