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Date:	Tue, 22 Jul 2014 04:07:21 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Josh Triplett <josh@...htriplett.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of
 cond_resched() checks for RCU

On Tue, Jul 22, 2014 at 12:52:40AM -0400, Pranith Kumar wrote:
> Doh! I figured it out *after* I sent out the mail. Sorry for the noise!

I know that feeling!  ;-)

							Thanx, Paul

> On Tue, Jul 22, 2014 at 12:35 AM, Pranith Kumar <bobby.prani@...il.com> wrote:
> > Hi Paul,
> >
> > I was going through this code and found a few inconsistencies. I git blamed it
> > and found that it was this recent commit and thought I could ask a few
> > questions. I am dropping the CC's as I am not sure since it is pretty late.
> >
> > Please find a few questions below:
> >
> > On 06/20/2014 02:33 PM, Paul E. McKenney wrote:
> >> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> >>
> >> Commit ac1bea85781e (Make cond_resched() report RCU quiescent states)
> >> fixed a problem where a CPU looping in the kernel with but one runnable
> >> task would give RCU CPU stall warnings, even if the in-kernel loop
> >> contained cond_resched() calls.  Unfortunately, in so doing, it introduced
> >> performance regressions in Anton Blanchard's will-it-scale "open1" test.
> >> The problem appears to be not so much the increased cond_resched() path
> >> length as an increase in the rate at which grace periods complete, which
> >> increased per-update grace-period overhead.
> >>
> >> This commit takes a different approach to fixing this bug, mainly by
> >> avoiding having cond_resched() do an RCU-visible quiescent state unless
> >> there is a grace period that has been in flight for a significant period
> >> of time.  This commit also reduces the common-case cond_resched() overhead
> >> to a check of a single per-CPU variable.
> >>
> > <snip>
> >> index f1ba77363fbb..2cc72ce19ff6 100644
> >> --- a/kernel/rcu/tree.c
> >> +++ b/kernel/rcu/tree.c
> >> @@ -229,6 +229,58 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
> >>  #endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> >>  };
> >>
> >> +/*
> >> + * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> >> + */
> >> +
> >> +DEFINE_PER_CPU(int, rcu_cond_resched_mask);
> >> +
> >> +/*
> >> + * Let the RCU core know that this CPU has gone through a cond_resched(),
> >> + * which is a quiescent state.
> >> + */
> >> +void rcu_resched(void)
> >> +{
> >> +     unsigned long flags;
> >> +     struct rcu_data *rdp;
> >> +     struct rcu_dynticks *rdtp;
> >> +     int resched_mask;
> >> +     struct rcu_state *rsp;
> >> +
> >> +     local_irq_save(flags);
> >> +
> >> +     /*
> >> +      * Yes, we can lose flag-setting operations.  This is OK, because
> >> +      * the flag will be set again after some delay.
> >> +      */
> >> +     resched_mask = raw_cpu_read(rcu_cond_resched_mask);
> >> +     raw_cpu_write(rcu_cond_resched_mask, 0);
> >> +
> >> +     /* Find the flavor that needs a quiescent state. */
> >> +     for_each_rcu_flavor(rsp) {
> >> +             rdp = raw_cpu_ptr(rsp->rda);
> >> +             if (!(resched_mask & rsp->flavor_mask))
> >> +                     continue;
> >
> > Here both resched_mask and flavor_mask are not being updated within the loop.
> > Are they supposed to be? It is really not clear what flavor_mask is doing in the
> > code.
> >
> >
> >> +             smp_mb(); /* ->flavor_mask before ->cond_resched_completed. */
> >> +             if (ACCESS_ONCE(rdp->mynode->completed) !=
> >> +                 ACCESS_ONCE(rdp->cond_resched_completed))
> >> +                     continue;
> >> +
> >> +             /*
> >> +              * Pretend to be momentarily idle for the quiescent state.
> >> +              * This allows the grace-period kthread to record the
> >> +              * quiescent state, with no need for this CPU to do anything
> >> +              * further.
> >> +              */
> >> +             rdtp = this_cpu_ptr(&rcu_dynticks);
> >> +             smp_mb__before_atomic(); /* Earlier stuff before QS. */
> >> +             atomic_add(2, &rdtp->dynticks);  /* QS. */
> >> +             smp_mb__after_atomic(); /* Later stuff after QS. */
> >> +             break;
> >> +     }
> >> +     local_irq_restore(flags);
> >> +}
> >> +
> >>  static long blimit = 10;     /* Maximum callbacks per rcu_do_batch. */
> >>  static long qhimark = 10000; /* If this many pending, ignore blimit. */
> >>  static long qlowmark = 100;  /* Once only this many pending, use blimit. */
> >> @@ -853,6 +905,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >>                                   bool *isidle, unsigned long *maxj)
> >>  {
> >>       unsigned int curr;
> >> +     int *rcrmp;
> >>       unsigned int snap;
> >>
> >>       curr = (unsigned int)atomic_add_return(0, &rdp->dynticks->dynticks);
> >> @@ -893,13 +946,20 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
> >>       }
> >>
> >>       /*
> >> -      * There is a possibility that a CPU in adaptive-ticks state
> >> -      * might run in the kernel with the scheduling-clock tick disabled
> >> -      * for an extended time period.  Invoke rcu_kick_nohz_cpu() to
> >> -      * force the CPU to restart the scheduling-clock tick in this
> >> -      * CPU is in this state.
> >> +      * A CPU running for an extended time within the kernel can
> >> +      * delay RCU grace periods.  When the CPU is in NO_HZ_FULL mode,
> >> +      * even context-switching back and forth between a pair of
> >> +      * in-kernel CPU-bound tasks cannot advance grace periods.
> >> +      * So if the grace period is old enough, make the CPU pay attention.
> >>        */
> >> -     rcu_kick_nohz_cpu(rdp->cpu);
> >> +     if (ULONG_CMP_GE(jiffies, rdp->rsp->gp_start + 7)) {
> >> +             rcrmp = &per_cpu(rcu_cond_resched_mask, rdp->cpu);
> >> +             ACCESS_ONCE(rdp->cond_resched_completed) =
> >> +                     ACCESS_ONCE(rdp->mynode->completed);
> >> +             smp_mb(); /* ->cond_resched_completed before *rcrmp. */
> >> +             ACCESS_ONCE(*rcrmp) =
> >> +                     ACCESS_ONCE(*rcrmp) + rdp->rsp->flavor_mask;
> >> +     }
> >>
> >>       /*
> >>        * Alternatively, the CPU might be running in the kernel
> >> @@ -3491,6 +3551,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
> >>                              "rcu_node_fqs_1",
> >>                              "rcu_node_fqs_2",
> >>                              "rcu_node_fqs_3" };  /* Match MAX_RCU_LVLS */
> >> +     static u8 fl_mask = 0x1;
> >
> > What does 0x1 mean here? Is it for a particular flavor? This could use a
> > comment.
> >
> >>       int cpustride = 1;
> >>       int i;
> >>       int j;
> >> @@ -3509,6 +3570,8 @@ static void __init rcu_init_one(struct rcu_state *rsp,
> >>       for (i = 1; i < rcu_num_lvls; i++)
> >>               rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> >>       rcu_init_levelspread(rsp);
> >> +     rsp->flavor_mask = fl_mask;
> >> +     fl_mask <<= 1;
> >
> > Something looks off here. fl_mask is not being used after this. Was it supposed
> > to be used or is it just a stray statement?
> >
> > The flavor_mask operations could really use some comments as it is not really
> > clear what is being achieved by that.
> >
> > --
> > Pranith
> >
> >>
> >>       /* Initialize the elements themselves, starting from the leaves. */
> >>
> >> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> >> index bf2c1e669691..0f69a79c5b7d 100644
> >> --- a/kernel/rcu/tree.h
> >> +++ b/kernel/rcu/tree.h
> >> @@ -307,6 +307,9 @@ struct rcu_data {
> >>       /* 4) reasons this CPU needed to be kicked by force_quiescent_state */
> >>       unsigned long dynticks_fqs;     /* Kicked due to dynticks idle. */
> >>       unsigned long offline_fqs;      /* Kicked due to being offline. */
> >> +     unsigned long cond_resched_completed;
> >> +                                     /* Grace period that needs help */
> >> +                                     /*  from cond_resched(). */
> >>
> >>       /* 5) __rcu_pending() statistics. */
> >>       unsigned long n_rcu_pending;    /* rcu_pending() calls since boot. */
> >> @@ -392,6 +395,7 @@ struct rcu_state {
> >>       struct rcu_node *level[RCU_NUM_LVLS];   /* Hierarchy levels. */
> >>       u32 levelcnt[MAX_RCU_LVLS + 1];         /* # nodes in each level. */
> >>       u8 levelspread[RCU_NUM_LVLS];           /* kids/node in each level. */
> >> +     u8 flavor_mask;                         /* bit in flavor mask. */
> >>       struct rcu_data __percpu *rda;          /* pointer of percu rcu_data. */
> >>       void (*call)(struct rcu_head *head,     /* call_rcu() flavor. */
> >>                    void (*func)(struct rcu_head *head));
> >> @@ -563,7 +567,7 @@ static bool rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp);
> >>  static void do_nocb_deferred_wakeup(struct rcu_data *rdp);
> >>  static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
> >>  static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
> >> -static void rcu_kick_nohz_cpu(int cpu);
> >> +static void __maybe_unused rcu_kick_nohz_cpu(int cpu);
> >>  static bool init_nocb_callback_list(struct rcu_data *rdp);
> >>  static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
> >>  static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
> >> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> >> index cbc2c45265e2..02ac0fb186b8 100644
> >> --- a/kernel/rcu/tree_plugin.h
> >> +++ b/kernel/rcu/tree_plugin.h
> >> @@ -2404,7 +2404,7 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
> >>   * if an adaptive-ticks CPU is failing to respond to the current grace
> >>   * period and has not be idle from an RCU perspective, kick it.
> >>   */
> >> -static void rcu_kick_nohz_cpu(int cpu)
> >> +static void __maybe_unused rcu_kick_nohz_cpu(int cpu)
> >>  {
> >>  #ifdef CONFIG_NO_HZ_FULL
> >>       if (tick_nohz_full_cpu(cpu))
> >> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> >> index a2aeb4df0f60..d22309cae9f5 100644
> >> --- a/kernel/rcu/update.c
> >> +++ b/kernel/rcu/update.c
> >> @@ -350,21 +350,3 @@ static int __init check_cpu_stall_init(void)
> >>  early_initcall(check_cpu_stall_init);
> >>
> >>  #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
> >> -
> >> -/*
> >> - * Hooks for cond_resched() and friends to avoid RCU CPU stall warnings.
> >> - */
> >> -
> >> -DEFINE_PER_CPU(int, rcu_cond_resched_count);
> >> -
> >> -/*
> >> - * Report a set of RCU quiescent states, for use by cond_resched()
> >> - * and friends.  Out of line due to being called infrequently.
> >> - */
> >> -void rcu_resched(void)
> >> -{
> >> -     preempt_disable();
> >> -     __this_cpu_write(rcu_cond_resched_count, 0);
> >> -     rcu_note_context_switch(smp_processor_id());
> >> -     preempt_enable();
> >> -}
> >>
> >
> 
> 
> 
> -- 
> Pranith
> 

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