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Date: Tue, 22 Jul 2014 00:52:40 -0400 From: Pranith Kumar <bobby.prani@...il.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, 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 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@...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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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