lkml.org 
[lkml]   [2022]   [Sep]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    SubjectRe: RCU vs NOHZ
    On Sat, Sep 17, 2022 at 07:25:08AM -0700, Paul E. McKenney wrote:
    > On Fri, Sep 16, 2022 at 11:20:14AM +0200, Peter Zijlstra wrote:
    > > On Fri, Sep 16, 2022 at 12:58:17AM -0700, Paul E. McKenney wrote:
    > >
    > > > To the best of my knowledge at this point in time, agreed. Who knows
    > > > what someone will come up with next week? But for people running certain
    > > > types of real-time and HPC workloads, context tracking really does handle
    > > > both idle and userspace transitions.
    > >
    > > Sure, but idle != nohz. Nohz is where we disable the tick, and currently
    > > RCU can inhibit this -- rcu_needs_cpu().
    >
    > Exactly. For non-nohz userspace execution, the tick is still running
    > anyway, so RCU of course won't be inhibiting its disabling. And in that
    > case, RCU's hook is the tick interrupt itself. RCU's hook is passed a
    > flag saying whether the interrupt came from userspace or from kernel.
    >
    > > AFAICT there really isn't an RCU hook for this, not through context
    > > tracking not through anything else.
    >
    > There is a directly invoked RCU hook for any transition that enables or
    > disables the tick, namely the ct_*_enter() and ct_*_exit() functions,
    > that is, those functions formerly known as rcu_*_enter() and rcu_*_exit().
    >
    > > > It wasn't enabled for ChromeOS.
    > > >
    > > > When fully enabled, it gave them the energy-efficiency advantages Joel
    > > > described. And then Joel described some additional call_rcu_lazy()
    > > > changes that provided even better energy efficiency. Though I believe
    > > > that the application should also be changed to avoid incessantly opening
    > > > and closing that file while the device is idle, as this would remove
    > > > -all- RCU work when nearly idle. But some of the other call_rcu_lazy()
    > > > use cases would likely remain.
    > >
    > > So I'm thinking the scheme I outlined gets you most if not all of what
    > > lazy would get you without having to add the lazy thing. A CPU is never
    > > refused deep idle when it passes off the callbacks.
    > >
    > > The NOHZ thing is a nice hook for 'this-cpu-wants-to-go-idle-long-term'
    > > and do our utmost bestest to move work away from it. You *want* to break
    > > affinity at this point.
    > >
    > > If you hate on the global, push it to a per rcu_node offload list until
    > > the whole node is idle and then push it up the next rcu_node level until
    > > you reach the top.
    > >
    > > Then when the top rcu_node is full idle; you can insta progress the QS
    > > state and run the callbacks and go idle.
    >
    > Unfortunately, the overhead of doing all that tracking along with
    > resolving all the resulting race conditions will -increase- power
    > consumption. With RCU, counting CPU wakeups is not as good a predictor
    > of power consumption as one might like. Sure, it is a nice heuristic
    > in some cases, but with RCU it is absolutely -not- a replacement for
    > actually measuring power consumption on real hardware. And yes, I did
    > learn this the hard way. Why do you ask? ;-)
    >
    > And that is why the recently removed CONFIG_RCU_FAST_NO_HZ left the
    > callbacks in place and substituted a 4x slower timer for the tick.
    > -That- actually resulted in significant real measured power savings on
    > real hardware.
    >
    > Except that everything that was building with CONFIG_RCU_FAST_NO_HZ
    > was also doing nohz_full on each and every CPU. Which meant that all
    > that CONFIG_RCU_FAST_NO_HZ was doing for them was adding an additional
    > useless check on each transition to and from idle. Which in turn is why
    > CONFIG_RCU_FAST_NO_HZ was removed. No one was using it in any way that
    > made any sense.
    >
    > And more recent testing with rcu_nocbs on both ChromeOS and Android has
    > produced better savings than was produced by CONFIG_RCU_FAST_NO_HZ anyway.
    >
    > Much of the additional savings from Joel et al.'s work is not so much
    > from reducing the number of ticks, but rather from reducing the number
    > of grace periods, which are of course much heavier weight.
    >
    > And this of course means that any additional schemes to reduce RCU's
    > power consumption must be compared (with real measurements on real
    > hardware!) to Joel et al.'s work, whether in combination or as an
    > alternative. And either way, the power savings must of course justify
    > the added code and complexity.

    And here is an untested patch that in theory might allow much of the
    reduction in power with minimal complexity/overhead for kernels without
    rcu_nocbs CPUs. On the off-chance you know of someone who would be
    willing to do a realistic evaluation of it.

    Thanx, Paul

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

    commit 80fc02e80a2dfb6c7468217cff2d4494a1c4b58d
    Author: Paul E. McKenney <paulmck@kernel.org>
    Date: Wed Sep 21 13:30:24 2022 -0700

    rcu: Let non-offloaded idle CPUs with callbacks defer tick

    When a CPU goes idle, rcu_needs_cpu() is invoked to determine whether or
    not RCU needs the scheduler-clock tick to keep interrupting. Right now,
    RCU keeps the tick on for a given idle CPU if there are any non-offloaded
    callbacks queued on that CPU.

    But if all of these callbacks are waiting for a grace period to finish,
    there is no point in scheduling a tick before that grace period has any
    reasonable chance of completing. This commit therefore delays the tick
    in the case where all the callbacks are waiting for a specific grace
    period to elapse. In theory, this should result in a 50-70% reduction in
    RCU-induced scheduling-clock ticks on mostly-idle CPUs. In practice, TBD.

    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>

    diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
    index 9bc025aa79a3..84e930c11065 100644
    --- a/include/linux/rcutiny.h
    +++ b/include/linux/rcutiny.h
    @@ -133,7 +133,7 @@ static inline void rcu_softirq_qs(void)
    rcu_tasks_qs(current, (preempt)); \
    } while (0)

    -static inline int rcu_needs_cpu(void)
    +static inline int rcu_needs_cpu(u64 basemono, u64 *nextevt)
    {
    return 0;
    }
    diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
    index 70795386b9ff..3066e0975022 100644
    --- a/include/linux/rcutree.h
    +++ b/include/linux/rcutree.h
    @@ -19,7 +19,7 @@

    void rcu_softirq_qs(void);
    void rcu_note_context_switch(bool preempt);
    -int rcu_needs_cpu(void);
    +int rcu_needs_cpu(u64 basemono, u64 *nextevt);
    void rcu_cpu_stall_reset(void);

    /*
    diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
    index 5ec97e3f7468..47cd3b0d2a07 100644
    --- a/kernel/rcu/tree.c
    +++ b/kernel/rcu/tree.c
    @@ -676,12 +676,33 @@ void __rcu_irq_enter_check_tick(void)
    * scheduler-clock interrupt.
    *
    * Just check whether or not this CPU has non-offloaded RCU callbacks
    - * queued.
    + * queued that need immediate attention.
    */
    -int rcu_needs_cpu(void)
    +int rcu_needs_cpu(u64 basemono, u64 *nextevt)
    {
    - return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
    - !rcu_rdp_is_offloaded(this_cpu_ptr(&rcu_data));
    + struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
    + struct rcu_segcblist *rsclp = &rdp->cblist;
    +
    + // Disabled, empty, or offloaded means nothing to do.
    + if (!rcu_segcblist_is_enabled(rsclp) ||
    + rcu_segcblist_empty(rsclp) || rcu_rdp_is_offloaded(rdp)) {
    + *nextevt = KTIME_MAX;
    + return 0;
    + }
    +
    + // Callbacks ready to invoke or that have not already been
    + // assigned a grace period need immediate attention.
    + if (!rcu_segcblist_segempty(rsclp, RCU_DONE_TAIL) ||
    + !rcu_segcblist_segempty(rsclp, RCU_NEXT_TAIL))
    + return 1;
    +
    + // There are callbacks waiting for some later grace period.
    + // Wait for about a grace period or two for the next tick, at which
    + // point there is high probability that this CPU will need to do some
    + // work for RCU.
    + *nextevt = basemono + TICK_NSEC * (READ_ONCE(jiffies_till_first_fqs) +
    + READ_ONCE(jiffies_till_next_fqs) + 1);
    + return 0;
    }

    /*
    diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
    index b0e3c9205946..303ea15cdb96 100644
    --- a/kernel/time/tick-sched.c
    +++ b/kernel/time/tick-sched.c
    @@ -784,7 +784,7 @@ static inline bool local_timer_softirq_pending(void)

    static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
    {
    - u64 basemono, next_tick, delta, expires;
    + u64 basemono, next_tick, next_tmr, next_rcu, delta, expires;
    unsigned long basejiff;
    unsigned int seq;

    @@ -807,7 +807,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
    * minimal delta which brings us back to this place
    * immediately. Lather, rinse and repeat...
    */
    - if (rcu_needs_cpu() || arch_needs_cpu() ||
    + if (rcu_needs_cpu(basemono, &next_rcu) || arch_needs_cpu() ||
    irq_work_needs_cpu() || local_timer_softirq_pending()) {
    next_tick = basemono + TICK_NSEC;
    } else {
    @@ -818,8 +818,10 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
    * disabled this also looks at the next expiring
    * hrtimer.
    */
    - next_tick = get_next_timer_interrupt(basejiff, basemono);
    - ts->next_timer = next_tick;
    + next_tmr = get_next_timer_interrupt(basejiff, basemono);
    + ts->next_timer = next_tmr;
    + /* Take the next rcu event into account */
    + next_tick = next_rcu < next_tmr ? next_rcu : next_tmr;
    }

    /*
    \
     
     \ /
      Last update: 2022-09-21 23:37    [W:4.312 / U:0.016 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site