Re: [RFC 0/3] Revert SRCU from tracepoint infrastructure

[Peter Zijlstra]

On Sat, Feb 08, 2020 at 11:31:25AM -0500, Mathieu Desnoyers wrote:
> ----- On Feb 7, 2020, at 3:56 PM, Joel Fernandes, Google joel@xxxxxxxxxxxxxxxxx wrote:
> 
> > Hi,
> > These patches remove SRCU usage from tracepoints. The reason for proposing the
> > reverts is because the whole point of SRCU was to avoid having to call
> > rcu_irq_enter_irqson(). However this was added back in 865e63b04e9b2 ("tracing:
> > Add back in rcu_irq_enter/exit_irqson() for rcuidle tracepoints") because perf
> > was breaking..
> 
> I think the original patch re-enabling the rcu_irq_enter/exit_irqson() is a
> tracepoint band-aid over what should actually been fixed within perf instead.
> 
> Perf should not do rcu_read_lock/unlock()/synchronize_rcu(), but rather use
> tracepoint_synchronize_unregister() to match the read-side provided by
> tracepoints.
> 
> If perf can then just rely on the underlying synchronization provided by each
> instrumentation providers (tracepoint, kprobe, ...) and not explicitly add its own
> unneeded synchronization on top (e.g. rcu_read_lock/unlock), then it should simplify
> all this.

It can't. At this point it doesn't know where the event came from. Also,
the whole perf stuff is per definition non-preemptible, as it needs to
run from NMI context.

Furthermore, using srcu would be detrimental, because of how it has
smp_mb() in the read side primitives.

The best we can do is move that rcu_irq_enter/exit_*() crud into the
perf tracepoint glue I suppose.