Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of cond_resched() checks for RCU

[Pranith Kumar]

Doh! I figured it out *after* I sent out the mail. Sorry for the noise!

On Tue, Jul 22, 2014 at 12:35 AM, Pranith Kumar <bobby.prani@xxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
>>
>> 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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