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Message-ID: <20140722110721.GS8690@linux.vnet.ibm.com> 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 > -- 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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