Messages in this thread | | | From | Joel Fernandes <> | Date | Wed, 8 Aug 2018 12:24:20 -0700 | Subject | Re: [PATCH v12 3/3] tracing: Centralize preemptirq tracepoints and unify their usage |
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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
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