netdev - Re: [PATCH] net: raise RCU qs after each threaded NAPI poll

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Date: Thu, 29 Feb 2024 12:41:55 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Yan Zhai <yan@...udflare.com>,
 Eric Dumazet <edumazet@...gle.com>,
 Network Development <netdev@...r.kernel.org>,
 "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
 Simon Horman <horms@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Lorenzo Bianconi <lorenzo@...nel.org>, Coco Li <lixiaoyan@...gle.com>,
 Wei Wang <weiwan@...gle.com>, Alexander Duyck <alexanderduyck@...com>,
 Hannes Frederic Sowa <hannes@...essinduktion.org>,
 LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org,
 bpf <bpf@...r.kernel.org>, kernel-team <kernel-team@...udflare.com>,
 Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] net: raise RCU qs after each threaded NAPI poll



> On Feb 29, 2024, at 11:57 AM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Thu, Feb 29, 2024 at 09:21:48AM -0500, Joel Fernandes wrote:
>> 
>> 
>>> On 2/28/2024 5:58 PM, Paul E. McKenney wrote:
>>> On Wed, Feb 28, 2024 at 02:48:44PM -0800, Alexei Starovoitov wrote:
>>>> On Wed, Feb 28, 2024 at 2:31 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>> 
>>>>> On Wed, 28 Feb 2024 14:19:11 -0800
>>>>> "Paul E. McKenney" <paulmck@...nel.org> wrote:
>>>>> 
>>>>>>>> 
>>>>>>>> Well, to your initial point, cond_resched() does eventually invoke
>>>>>>>> preempt_schedule_common(), so you are quite correct that as far as
>>>>>>>> Tasks RCU is concerned, cond_resched() is not a quiescent state.
>>>>>>> 
>>>>>>> Thanks for confirming. :-)
>>>>>> 
>>>>>> However, given that the current Tasks RCU use cases wait for trampolines
>>>>>> to be evacuated, Tasks RCU could make the choice that cond_resched()
>>>>>> be a quiescent state, for example, by adjusting rcu_all_qs() and
>>>>>> .rcu_urgent_qs accordingly.
>>>>>> 
>>>>>> But this seems less pressing given the chance that cond_resched() might
>>>>>> go away in favor of lazy preemption.
>>>>> 
>>>>> Although cond_resched() is technically a "preemption point" and not truly a
>>>>> voluntary schedule, I would be happy to state that it's not allowed to be
>>>>> called from trampolines, or their callbacks. Now the question is, does BPF
>>>>> programs ever call cond_resched()? I don't think they do.
>>>>> 
>>>>> [ Added Alexei ]
>>>> 
>>>> I'm a bit lost in this thread :)
>>>> Just answering the above question.
>>>> bpf progs never call cond_resched() directly.
>>>> But there are sleepable (aka faultable) bpf progs that
>>>> can call some helper or kfunc that may call cond_resched()
>>>> in some path.
>>>> sleepable bpf progs are protected by rcu_tasks_trace.
>>>> That's a very different one vs rcu_tasks.
>>> 
>>> Suppose that the various cond_resched() invocations scattered throughout
>>> the kernel acted as RCU Tasks quiescent states, so that as soon as a
>>> given task executed a cond_resched(), synchronize_rcu_tasks() might
>>> return or call_rcu_tasks() might invoke its callback.
>>> 
>>> Would that cause BPF any trouble?
>>> 
>>> My guess is "no", because it looks like BPF is using RCU Tasks (as you
>>> say, as opposed to RCU Tasks Trace) only to wait for execution to leave a
>>> trampoline.  But I trust you much more than I trust myself on this topic!
>> 
>> But it uses RCU Tasks Trace as well (for sleepable bpf programs), not just
>> Tasks? Looks like that's what Alexei said above as well, and I confirmed it in
>> bpf/trampoline.c
>> 
>>        /* The trampoline without fexit and fmod_ret progs doesn't call original
>>         * function and doesn't use percpu_ref.
>>         * Use call_rcu_tasks_trace() to wait for sleepable progs to finish.
>>         * Then use call_rcu_tasks() to wait for the rest of trampoline asm
>>         * and normal progs.
>>         */
>>        call_rcu_tasks_trace(&im->rcu, __bpf_tramp_image_put_rcu_tasks);
>> 
>> The code comment says it uses both.
> 
> BPF does quite a few interesting things with these.
> 
> But would you like to look at the update-side uses of RCU Tasks Rude
> to see if lazy preemption affects them?  I don't believe that there
> are any problems here, but we do need to check.

Sure I will be happy to. I am planning look at it in detail over the 3 day weekend. Too much fun! ;-)

thanks,

- Joel



> 
>                            Thanx, Paul