Re: [PATCH v4 net 1/3] rcu: add a helper to report consolidated flavor QS

[Paul E. McKenney]

On Fri, Mar 15, 2024 at 12:55:03PM -0700, Yan Zhai wrote:
> There are several scenario in network processing that can run
> extensively under heavy traffic. In such situation, RCU synchronization
> might not observe desired quiescent states for indefinitely long period.
> Create a helper to safely raise the desired RCU quiescent states for
> such scenario.
> 
> Currently the frequency is locked at HZ/10, i.e. 100ms, which is
> sufficient to address existing problems around RCU tasks. It's unclear
> yet if there is any future scenario for it to be further tuned down.

I suggest something like the following for the commit log:

------------------------------------------------------------------------

When under heavy load, network processing can run CPU-bound for many tens
of seconds.  Even in preemptible kernels, this can block RCU Tasks grace
periods, which can cause trace-event removal to take more than a minute,
which is unacceptably long.

This commit therefore creates a new helper function that passes
through both RCU and RCU-Tasks quiescent states every 100 milliseconds.
This hard-coded value suffices for current workloads.

------------------------------------------------------------------------

> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Reviewed-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> ---
> v3->v4: comment fixup
> 
> ---
>  include/linux/rcupdate.h | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 0746b1b0b663..da224706323e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -247,6 +247,30 @@ do { \
>  	cond_resched(); \
>  } while (0)
>  
> +/**
> + * rcu_softirq_qs_periodic - Periodically report consolidated quiescent states
> + * @old_ts: last jiffies when QS was reported. Might be modified in the macro.
> + *
> + * This helper is for network processing in non-RT kernels, where there could
> + * be busy polling threads that block RCU synchronization indefinitely.  In
> + * such context, simply calling cond_resched is insufficient, so give it a
> + * stronger push to eliminate all potential blockage of all RCU types.
> + *
> + * NOTE: unless absolutely sure, this helper should in general be called
> + * outside of bh lock section to avoid reporting a surprising QS to updaters,
> + * who could be expecting RCU read critical section to end at local_bh_enable().
> + */

How about something like this for the kernel-doc comment?

/**
 * rcu_softirq_qs_periodic - Report RCU and RCU-Tasks quiescent states
 * @old_ts: jiffies at start of processing.
 *
 * This helper is for long-running softirq handlers, such as those
 * in networking.  The caller should initialize the variable passed in
 * as @old_ts at the beginning of the softirq handler.  When invoked
 * frequently, this macro will invoke rcu_softirq_qs() every 100
 * milliseconds thereafter, which will provide both RCU and RCU-Tasks
 * quiescent states.  Note that this macro modifies its old_ts argument.
 *
 * Note that although cond_resched() provides RCU quiescent states,
 * it does not provide RCU-Tasks quiescent states.
 *
 * Because regions of code that have disabled softirq act as RCU
 * read-side critical sections, this macro should be invoked with softirq
 * (and preemption) enabled.
 *
 * This macro has no effect in CONFIG_PREEMPT_RT kernels.
 */

							Thanx, Paul

> +#define rcu_softirq_qs_periodic(old_ts) \
> +do { \
> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT) && \
> +	    time_after(jiffies, (old_ts) + HZ / 10)) { \
> +		preempt_disable(); \
> +		rcu_softirq_qs(); \
> +		preempt_enable(); \
> +		(old_ts) = jiffies; \
> +	} \
> +} while (0)
> +
>  /*
>   * Infrastructure to implement the synchronize_() primitives in
>   * TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
> -- 
> 2.30.2
> 
>