[lttng-dev] call_rcu seems inefficient without futex

[Mathieu Desnoyers]

----- On Jan 27, 2020, at 10:45 PM, paulmck paulmck at kernel.org wrote:

> On Mon, Jan 27, 2020 at 10:38:05AM -0500, Mathieu Desnoyers wrote:
>> ----- On Jan 23, 2020, at 7:19 PM, lttng-dev lttng-dev at lists.lttng.org wrote:
>> 
>> > Hi,
>> > 
>> > I recently installed knot dns for a very small FreeBSD server. I noticed
>> > that it uses a surprising amount of CPU, even when there is no load:
>> > about 0.25%. That's not huge, but it seems unnecessarily high when my
>> > QPS is less than 0.01.
>> > 
>> > After some profiling, I came to the conclusion that this is caused by
>> > call_rcu_wait using futex_async to repeatedly wait. Since there is no
>> > futex on FreeBSD (without the Linux compatibility layer), this
>> > effectively turns into a permanent busy waiting loop.
>> > 
>> > I think futex_noasync can be used here instead. call_rcu_wait is only
>> > supposed to be called from call_rcu_thread, never from a signal context.
>> > call_rcu calls get_call_rcu_data, which may call
>> > get_default_call_rcu_data, which calls pthread_mutex_lock through
>> > call_rcu_lock. Therefore, call_rcu is not async-signal-safe already.
>> 
>> call_rcu() is meant to be async-signal-safe and lock-free after that
>> initialization has been performed on first use. Paul, do you know where
>> we have documented this in liburcu ?
> 
> Lock freedom is the goal, but when not in real-time mode, call_rcu()
> does invoke futex_async(), which can acquire locks within the Linux
> kernel.
> 
> Should BSD instead use POSIX condvars for the call_rcu() waits and
> wakeups?

There are two distinct benefit to lock-freedom which I think are relevant
here (at least):

- As you stated, lock-freedom is useful for real-time algorithms because it
does not require careful handling of locks (priority inversion and so on),

- Moreover, another characteristic of lock-free algorithms which is useful
beyond the scope of real-time systems is its ability to fail gracefully.
Basically, if a lock-free algorithm crashes at any point, the rest of the
system can still go on. This is especially useful for data structures over
shared memory between processes.

This last point highlights why being lock-free in user-space vs being lock-free
over the entire system (including kernel system call implementation) do not
cover exactly the same requirements. For RT, indeed, the requirement is to
be lock-free on both sides of user/kernel boundary, because timings are what
matter. However, if lock-freedom is used as a mean to recover from failure
gracefully, it can be sufficient to achieve lock-freedom in the userspace
part of the algorithm, and then rely on non-lock-free algorithms within the
kernel, because failure within the kernel is an internal kernel failure
which affects the entire system anyways.

> 
>> > Also, I think it only makes sense to use call_rcu around a RCU write,
>> > which contradicts the README saying that only RCU reads are allowed in
>> > signal handlers.
> 
> I do not believe that it is always safe to invoke call_rcu() from within
> a signal handler.  If you made sure to invoke it outside a signal handler
> the first time, and then used real-time mode, that should work.  But in
> that case, you aren't invoking the futex code.

Other that the initialization, what prevents using non-rt call_rcu() from a
signal handler context ? AFAIU it should be safe to issue futex WAKEUP from
a signal handler context.

> 
>> Not sure what you mean by "use call_rcu around a RCU write" ?
> 
> I confess to some curiosity on this point as well.  Maybe what is meant
> is "around a RCU write" as in "near to an RCU write" as in "in place of
> using synchronize_rcu()"?

>From Alex Xu's reply:

"I mean that in general, the pattern is usually to do an RCU write (to
remove an item from a list, for example), then do call_rcu to
aynchronously clean up the item."

> 
>> Is there anything similar to sys_futex on FreeBSD ?

Alex Xu provided a patch set in a separate thread implementing "umtx"
support to basically provide OS support for futex on FreeBSD and
DragonflyBSD.

https://lists.lttng.org/pipermail/lttng-dev/2020-January/029507.html
https://lists.lttng.org/pipermail/lttng-dev/2020-January/029510.html

>> 
>> It would be good to look into alternative ways to fix this that do not
>> involve changing the guarantees provided by call_rcu() for that fallback
>> scenario (no futex available). Perhaps in your use-case you may want to
>> tweak the retry delay for compat_futex_async(). Currently
>> src/compat_futex.c:compat_futex_async() has a 10ms delay. Would 100ms
>> be more acceptable ?
> 
> If this works for knot dns, it would of course be simpler.

I think we should not put too much effort in tweaking the fallback for
scenarios where futex is missing. The proper approach seems to be to
implement proper support for futex-like APIs provided by each OS kernel.

Thanks,

Mathieu

> 
>							Thanx, Paul
> 
>> Thanks,
>> 
>> Mathieu
>> 
>> > 
>> > I applied "sed -i -e 's/futex_async/futex_noasync/'
>> > src/urcu-call-rcu-impl.h" and knot seems to work correctly with only
>> > 0.01% CPU now. I also ran tests/unit and tests/regression with default
>> > and signal backends and all completed successfully.
>> > 
>> > I think that the other two usages of futex_async are also a little
>> > suspicious, but I didn't look too closely.
>> > 
>> > Thanks,
>> > Alex.
>> > _______________________________________________
>> > lttng-dev mailing list
>> > lttng-dev at lists.lttng.org
>> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

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