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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Feb 7, 2020, at 4:43 PM, Joel Fernandes, Google joel@joelfernandes.org wrote:

> On Fri, Feb 07, 2020 at 01:24:50PM -0800, Paul E. McKenney wrote:
>> On Fri, Feb 07, 2020 at 03:56:53PM -0500, Joel Fernandes (Google) 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..
>> > 
>> > Further it occurs to me that, by using SRCU for tracepoints, we forgot that RCU
>> > is not really watching the tracepoint callbacks. This means that anyone doing
>> > preempt_disable() in their tracepoint callback, and expecting RCU to listen to
>> > them is in for a big surprise. When RCU is not watching, it does not care about
>> > preempt-disable sections on CPUs as you can see in the forced-quiescent state
>> > loop.
>> > 
>> > Since SRCU is not providing any benefit because of 865e63b04e9b2 anyway, let us
>> > revert SRCU tracepoint code to maintain the sanity of potential
>> > tracepoint callback registerers.
>> 
>> For whatever it is worth, SRCU is the exception to the "RCU needs to
>> be watching" rule.  You can have SRCU readers on idle CPUs, offline
>> CPUs, CPUs executing in userspace, whatever.
> 
> Yes sure. My concern was that callbacks are still using regular RCU somewhere
> and RCU isn't watching. I believe BPF is using RCU that way (not sure). But
> could be other out-of-tree kernel modules etc.

Tracepoint users should issue "tracepoint_synchronize_unregister()" rather than
expect tracepoints to rely on RCU.

I cannot find a good reason for perf to issue a redundant rcu_read_lock/unlock from
a tracepoint probe already providing RCU or SRCU synchronization. Same goes for BPF.
If they _really_ need to do it, then they could implement their own idle probes
which do an additional rcu_irq_enter/exit_irqson(), but this work-around does not
belong in tracepoint.h.

If out-of-tree modules fail to use the API properly, the burden of getting fixed
is on their shoulders, as it has always been.

Thanks,

Mathieu

> 
> thanks,
> 
> - Joel
> 
>> 
>> 							Thanx, Paul
>> 
>> > Joel Fernandes (Google) (3):
>> > Revert "tracepoint: Use __idx instead of idx in DO_TRACE macro to make
>> > it unique"
>> > Revert "tracing: Add back in rcu_irq_enter/exit_irqson() for rcuidle
>> > tracepoints"
>> > Revert "tracepoint: Make rcuidle tracepoint callers use SRCU"
>> > 
>> > include/linux/tracepoint.h | 40 ++++++--------------------------------
>> > kernel/tracepoint.c        | 10 +---------
>> > 2 files changed, 7 insertions(+), 43 deletions(-)
>> > 
>> > --
>> > 2.25.0.341.g760bfbb309-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com