Re: [PATCH] rcu: Avoid unneeded function call in rcu_read_unlock()

[Andrii Nakryiko]

On Fri, Aug 27, 2021 at 11:34 AM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Thu, Aug 26, 2021 at 10:21:22PM -0400, Waiman Long wrote:
> > Since commit aa40c138cc8f3 ("rcu: Report QS for outermost
> > PREEMPT=n rcu_read_unlock() for strict GPs"). A real function call
> > rcu_read_unlock_strict() is added to the inlined rcu_read_unlock().
> > The rcu_read_unlock_strict() call is only needed if the performance
> > sagging CONFIG_RCU_STRICT_GRACE_PERIOD option is set. This config
> > option isn't set for most production kernels while the function call
> > overhead remains.
> >
> > To provide a slight performance improvement, the
> > CONFIG_RCU_STRICT_GRACE_PERIOD config check is moved from
> > rcu_read_unlock_strict() to __rcu_read_unlock() so that the function
> > call can be compiled out in most cases.
> >
> > Besides, the GPL exported rcu_read_unlock_strict() also impact the
> > the compilation of non-GPL kernel modules as rcu_read_unlock() is a
> > frequently used kernel API.
> >
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
>
> Nice, and good eyes!!!
>
> I have queued this for v5.16, that is, not the upcoming merge window
> but the one after that.
>
> I did my usual wordsmithing, so please check the following in case I
> messed something up.  I intentionally omitted the EXPORT_SYMBOL_GPL()
> discussion because:
>
> 1.      Kernels built with CONFIG_PREEMPT=y have the same issue
>         with the __rcu_read_lock() and __rcu_read_unlock() functions.
>
> 2.      Many other RCU functions are EXPORT_SYMBOL_GPL() and have
>         been for almost two decades.
>
> But if someone does use RCU readers within CONFIG_PREEMPT=n kernels from
> a binary module, I will happily refer them to you for any RCU issues
> that they encounter.  ;-)
>
> I am also CCing the BPF guys in case my interpretation of the code in
> the BPF verifier is incorrect.
>

LGTM from the BPF side, nothing really changed about when
rcu_read_unlock_strict is an actual function vs no-op macro. It's also
important to minimize the number of function calls in the context of
recent LBR on-demand work done by Song, so this is a great
improvement!

>                                                         Thanx, Paul
>
> ------------------------------------------------------------------------
>
> commit 4a9f53b997b809c0256838e31c604aeeded2345a
> Author: Waiman Long <longman@xxxxxxxxxx>
> Date:   Thu Aug 26 22:21:22 2021 -0400
>
>     rcu: Avoid unneeded function call in rcu_read_unlock()
>
>     Since commit aa40c138cc8f3 ("rcu: Report QS for outermost PREEMPT=n
>     rcu_read_unlock() for strict GPs") the function rcu_read_unlock_strict()
>     is invoked by the inlined rcu_read_unlock() function.  However,
>     rcu_read_unlock_strict() is an empty function in production kernels,
>     which are built with CONFIG_RCU_STRICT_GRACE_PERIOD=n.
>
>     There is a mention of rcu_read_unlock_strict() in the BPF verifier,
>     but this is in a deny-list, meaning that BPF does not care whether
>     rcu_read_unlock_strict() is ever called.
>
>     This commit therefore provides a slight performance improvement
>     by hoisting the check of CONFIG_RCU_STRICT_GRACE_PERIOD from
>     rcu_read_unlock_strict() into rcu_read_unlock(), thus avoiding the
>     pointless call to an empty function.
>
>     Cc: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
>     Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
>     Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
>     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 434d12fe2d4f..5e0beb5c5659 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -71,7 +71,8 @@ static inline void __rcu_read_lock(void)
>  static inline void __rcu_read_unlock(void)
>  {
>         preempt_enable();
> -       rcu_read_unlock_strict();
> +       if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
> +               rcu_read_unlock_strict();
>  }
>
>  static inline int rcu_preempt_depth(void)
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 7a4876a3a882..0b55c647ab80 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -814,8 +814,7 @@ void rcu_read_unlock_strict(void)
>  {
>         struct rcu_data *rdp;
>
> -       if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
> -          irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
> +       if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
>                 return;
>         rdp = this_cpu_ptr(&rcu_data);
>         rcu_report_qs_rdp(rdp);