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

[Mark Rutland]

On Fri, Mar 15, 2024 at 10:40:56PM -0700, Paul E. McKenney wrote:
> 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.

FWIW, this sounds good to me.

> 
> ------------------------------------------------------------------------
> 
> > 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.
>  */

Considering the note about cond_resched(), does does cond_resched() actually
provide an RCU quiescent state for fully-preemptible kernels? IIUC for those
cond_resched() expands to:

	__might_resched();
	klp_sched_try_switch()

.. and AFAICT neither reports an RCU quiescent state.

So maybe it's worth dropping the note?

Seperately, what's the rationale for not doing this on PREEMPT_RT? Does that
avoid the problem through other means, or are people just not running effected
workloads on that?

Mark.

> 
> 							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
> > 
> >