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

From: Pranith Kumar
Date: Tue Jul 22 2014 - 00:36:04 EST

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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();
> -}
>

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Next message: Hugh Dickins: "[PATCH] drm/i915: fix freeze with blank screen booting highmem"
Previous message: Alexandre Courbot: "Re: [PATCH] gpio: Add support for GPIOF_ACTIVE_LOW to gpio_request_one"
Next in thread: Pranith Kumar: "Re: [PATCH RFC tip/core/rcu 1/5] rcu: Reduce overhead of cond_resched() checks for RCU"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]