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    SubjectRe: [PATCH v5 net 1/3] rcu: add a helper to report consolidated flavor QS
    On 2024-03-19 13:44:34 [-0700], Yan Zhai wrote:
    > index 16f519914415..17d7ed5f3ae6 100644
    > --- a/include/linux/rcupdate.h
    > +++ b/include/linux/rcupdate.h
    > @@ -247,6 +247,37 @@ do { \
    > cond_resched(); \
    > } while (0)
    >
    > +/**
    > + * 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 NAPI threads 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.
    > + *
    > + * 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.
    > + *
    > + * The macro is not needed when CONFIG_PREEMPT_RT is defined. RT kernels would
    > + * have more chance to invoke schedule() calls and provide necessary quiescent
    > + * states. As a contrast, calling cond_resched() only won't achieve the same
    > + * effect because cond_resched() does not provide RCU-Tasks quiescent states.
    > + */

    Paul, so CONFIG_PREEMPTION is affected but CONFIG_PREEMPT_RT is not.
    Why does RT have more scheduling points?
    The RCU-Tasks thread is starving and yet there is no wake-up, correct?
    Shouldn't cond_resched() take care of RCU-Tasks's needs, too?
    This function is used by napi_threaded_poll() which is not invoked in
    softirq it is a simple thread which does disable BH but this is it. Any
    pending softirqs are served before the cond_resched().

    This napi_threaded_poll() case _basically_ a busy thread doing a lot of
    work and delaying RCU-Tasks as far as I understand. The same may happen
    to other busy-worker which have cond_resched() between works, such as
    the kworker. Therefore I would expect to have some kind of timeout at
    which point NEED_RESCHED is set so that cond_resched() can do its work.

    > +#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.

    Sebastian

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