![Paul E. McKenney](/assets/img/avatar_default.png)
This patch fixes a hierarchical-RCU performance bug located by Anton Blanchard. The problem stems from a misguided attempt to provide a work-around for jiffies-counter failure. This work-around uses a per-CPU n_rcu_pending counter, which is incremented on each call to rcu_pending(), which in turn is called from each scheduling-clock interrupt. Each CPU then treats this counter as a surrogate for the jiffies counter, so that if the jiffies counter fails to advance, the per-CPU n_rcu_pending counter will cause RCU to invoke force_quiescent_state(), which in turn will (among other things) send resched IPIs to CPUs that have thus far failed to pass through an RCU quiescent state. Unfortunately, each CPU resets only its own counter after sending a batch of IPIs. This means that the other CPUs will also (needlessly) send -another- round of IPIs, for a full N-squared set of IPIs in the worst case every three scheduler-clock ticks until the grace period finally ends. It is not reasonable for a given CPU to reset each and every n_rcu_pending for all the other CPUs, so this patch instead simply disables the jiffies-counter "training wheels", thus eliminating the excessive IPIs. Note that the jiffies-counter IPIs do not have this problem due to the fact that the jiffies counter is global, so that the CPU sending the IPIs can easily reset things, thus preventing the other CPUs from sending redundant IPIs. Note also that the n_rcu_pending counter remains, as it will continue to be used for tracing. It may also see use to update the jiffies counter, should an appropriate kick-the-jiffies-counter API appear. Located-by: Anton Blanchard <anton@au1.ibm.com> Tested-by: Anton Blanchard <anton@au1.ibm.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: anton@samba.org Cc: akpm@linux-foundation.org Cc: dipankar@in.ibm.com Cc: manfred@colorfullife.com Cc: cl@linux-foundation.org Cc: josht@linux.vnet.ibm.com Cc: schamp@sgi.com Cc: niv@us.ibm.com Cc: dvhltc@us.ibm.com Cc: ego@in.ibm.com Cc: laijs@cn.fujitsu.com Cc: rostedt@goodmis.org Cc: peterz@infradead.org Cc: penberg@cs.helsinki.fi Cc: andi@firstfloor.org Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> LKML-Reference: <12396834793575-git-send-email-> Signed-off-by: Ingo Molnar <mingo@elte.hu>
312 lines
11 KiB
C
312 lines
11 KiB
C
/*
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* Read-Copy Update mechanism for mutual exclusion (tree-based version)
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright IBM Corporation, 2008
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*
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* Author: Dipankar Sarma <dipankar@in.ibm.com>
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* Paul E. McKenney <paulmck@linux.vnet.ibm.com> Hierarchical algorithm
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*
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* Based on the original work by Paul McKenney <paulmck@us.ibm.com>
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* and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU
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*/
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#ifndef __LINUX_RCUTREE_H
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#define __LINUX_RCUTREE_H
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#include <linux/cache.h>
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#include <linux/spinlock.h>
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#include <linux/threads.h>
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#include <linux/cpumask.h>
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#include <linux/seqlock.h>
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/*
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* Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
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* In theory, it should be possible to add more levels straightforwardly.
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* In practice, this has not been tested, so there is probably some
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* bug somewhere.
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*/
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#define MAX_RCU_LVLS 3
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#define RCU_FANOUT (CONFIG_RCU_FANOUT)
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#define RCU_FANOUT_SQ (RCU_FANOUT * RCU_FANOUT)
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#define RCU_FANOUT_CUBE (RCU_FANOUT_SQ * RCU_FANOUT)
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#if NR_CPUS <= RCU_FANOUT
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# define NUM_RCU_LVLS 1
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# define NUM_RCU_LVL_0 1
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# define NUM_RCU_LVL_1 (NR_CPUS)
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# define NUM_RCU_LVL_2 0
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# define NUM_RCU_LVL_3 0
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#elif NR_CPUS <= RCU_FANOUT_SQ
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# define NUM_RCU_LVLS 2
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# define NUM_RCU_LVL_0 1
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# define NUM_RCU_LVL_1 (((NR_CPUS) + RCU_FANOUT - 1) / RCU_FANOUT)
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# define NUM_RCU_LVL_2 (NR_CPUS)
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# define NUM_RCU_LVL_3 0
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#elif NR_CPUS <= RCU_FANOUT_CUBE
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# define NUM_RCU_LVLS 3
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# define NUM_RCU_LVL_0 1
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# define NUM_RCU_LVL_1 (((NR_CPUS) + RCU_FANOUT_SQ - 1) / RCU_FANOUT_SQ)
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# define NUM_RCU_LVL_2 (((NR_CPUS) + (RCU_FANOUT) - 1) / (RCU_FANOUT))
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# define NUM_RCU_LVL_3 NR_CPUS
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#else
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# error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
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#endif /* #if (NR_CPUS) <= RCU_FANOUT */
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#define RCU_SUM (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
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#define NUM_RCU_NODES (RCU_SUM - NR_CPUS)
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/*
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* Dynticks per-CPU state.
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*/
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struct rcu_dynticks {
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int dynticks_nesting; /* Track nesting level, sort of. */
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int dynticks; /* Even value for dynticks-idle, else odd. */
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int dynticks_nmi; /* Even value for either dynticks-idle or */
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/* not in nmi handler, else odd. So this */
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/* remains even for nmi from irq handler. */
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};
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/*
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* Definition for node within the RCU grace-period-detection hierarchy.
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*/
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struct rcu_node {
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spinlock_t lock;
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unsigned long qsmask; /* CPUs or groups that need to switch in */
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/* order for current grace period to proceed.*/
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unsigned long qsmaskinit;
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/* Per-GP initialization for qsmask. */
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unsigned long grpmask; /* Mask to apply to parent qsmask. */
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int grplo; /* lowest-numbered CPU or group here. */
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int grphi; /* highest-numbered CPU or group here. */
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u8 grpnum; /* CPU/group number for next level up. */
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u8 level; /* root is at level 0. */
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struct rcu_node *parent;
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} ____cacheline_internodealigned_in_smp;
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/* Index values for nxttail array in struct rcu_data. */
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#define RCU_DONE_TAIL 0 /* Also RCU_WAIT head. */
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#define RCU_WAIT_TAIL 1 /* Also RCU_NEXT_READY head. */
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#define RCU_NEXT_READY_TAIL 2 /* Also RCU_NEXT head. */
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#define RCU_NEXT_TAIL 3
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#define RCU_NEXT_SIZE 4
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/* Per-CPU data for read-copy update. */
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struct rcu_data {
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/* 1) quiescent-state and grace-period handling : */
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long completed; /* Track rsp->completed gp number */
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/* in order to detect GP end. */
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long gpnum; /* Highest gp number that this CPU */
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/* is aware of having started. */
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long passed_quiesc_completed;
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/* Value of completed at time of qs. */
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bool passed_quiesc; /* User-mode/idle loop etc. */
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bool qs_pending; /* Core waits for quiesc state. */
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bool beenonline; /* CPU online at least once. */
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struct rcu_node *mynode; /* This CPU's leaf of hierarchy */
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unsigned long grpmask; /* Mask to apply to leaf qsmask. */
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/* 2) batch handling */
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/*
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* If nxtlist is not NULL, it is partitioned as follows.
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* Any of the partitions might be empty, in which case the
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* pointer to that partition will be equal to the pointer for
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* the following partition. When the list is empty, all of
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* the nxttail elements point to nxtlist, which is NULL.
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*
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* [*nxttail[RCU_NEXT_READY_TAIL], NULL = *nxttail[RCU_NEXT_TAIL]):
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* Entries that might have arrived after current GP ended
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* [*nxttail[RCU_WAIT_TAIL], *nxttail[RCU_NEXT_READY_TAIL]):
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* Entries known to have arrived before current GP ended
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* [*nxttail[RCU_DONE_TAIL], *nxttail[RCU_WAIT_TAIL]):
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* Entries that batch # <= ->completed - 1: waiting for current GP
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* [nxtlist, *nxttail[RCU_DONE_TAIL]):
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* Entries that batch # <= ->completed
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* The grace period for these entries has completed, and
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* the other grace-period-completed entries may be moved
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* here temporarily in rcu_process_callbacks().
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*/
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struct rcu_head *nxtlist;
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struct rcu_head **nxttail[RCU_NEXT_SIZE];
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long qlen; /* # of queued callbacks */
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long blimit; /* Upper limit on a processed batch */
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#ifdef CONFIG_NO_HZ
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/* 3) dynticks interface. */
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struct rcu_dynticks *dynticks; /* Shared per-CPU dynticks state. */
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int dynticks_snap; /* Per-GP tracking for dynticks. */
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int dynticks_nmi_snap; /* Per-GP tracking for dynticks_nmi. */
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#endif /* #ifdef CONFIG_NO_HZ */
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/* 4) reasons this CPU needed to be kicked by force_quiescent_state */
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#ifdef CONFIG_NO_HZ
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unsigned long dynticks_fqs; /* Kicked due to dynticks idle. */
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#endif /* #ifdef CONFIG_NO_HZ */
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unsigned long offline_fqs; /* Kicked due to being offline. */
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unsigned long resched_ipi; /* Sent a resched IPI. */
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/* 5) For future __rcu_pending statistics. */
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long n_rcu_pending; /* rcu_pending() calls since boot. */
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int cpu;
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};
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/* Values for signaled field in struct rcu_state. */
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#define RCU_GP_INIT 0 /* Grace period being initialized. */
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#define RCU_SAVE_DYNTICK 1 /* Need to scan dyntick state. */
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#define RCU_FORCE_QS 2 /* Need to force quiescent state. */
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#ifdef CONFIG_NO_HZ
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#define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK
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#else /* #ifdef CONFIG_NO_HZ */
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#define RCU_SIGNAL_INIT RCU_FORCE_QS
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#endif /* #else #ifdef CONFIG_NO_HZ */
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#define RCU_JIFFIES_TILL_FORCE_QS 3 /* for rsp->jiffies_force_qs */
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#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
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#define RCU_SECONDS_TILL_STALL_CHECK (10 * HZ) /* for rsp->jiffies_stall */
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#define RCU_SECONDS_TILL_STALL_RECHECK (30 * HZ) /* for rsp->jiffies_stall */
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#define RCU_STALL_RAT_DELAY 2 /* Allow other CPUs time */
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/* to take at least one */
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/* scheduling clock irq */
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/* before ratting on them. */
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#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
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/*
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* RCU global state, including node hierarchy. This hierarchy is
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* represented in "heap" form in a dense array. The root (first level)
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* of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
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* level in ->node[1] through ->node[m] (->node[1] referenced by ->level[1]),
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* and the third level in ->node[m+1] and following (->node[m+1] referenced
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* by ->level[2]). The number of levels is determined by the number of
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* CPUs and by CONFIG_RCU_FANOUT. Small systems will have a "hierarchy"
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* consisting of a single rcu_node.
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*/
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struct rcu_state {
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struct rcu_node node[NUM_RCU_NODES]; /* Hierarchy. */
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struct rcu_node *level[NUM_RCU_LVLS]; /* Hierarchy levels. */
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u32 levelcnt[MAX_RCU_LVLS + 1]; /* # nodes in each level. */
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u8 levelspread[NUM_RCU_LVLS]; /* kids/node in each level. */
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struct rcu_data *rda[NR_CPUS]; /* array of rdp pointers. */
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/* The following fields are guarded by the root rcu_node's lock. */
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u8 signaled ____cacheline_internodealigned_in_smp;
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/* Force QS state. */
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long gpnum; /* Current gp number. */
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long completed; /* # of last completed gp. */
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spinlock_t onofflock; /* exclude on/offline and */
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/* starting new GP. */
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spinlock_t fqslock; /* Only one task forcing */
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/* quiescent states. */
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unsigned long jiffies_force_qs; /* Time at which to invoke */
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/* force_quiescent_state(). */
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unsigned long n_force_qs; /* Number of calls to */
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/* force_quiescent_state(). */
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unsigned long n_force_qs_lh; /* ~Number of calls leaving */
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/* due to lock unavailable. */
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unsigned long n_force_qs_ngp; /* Number of calls leaving */
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/* due to no GP active. */
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#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
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unsigned long gp_start; /* Time at which GP started, */
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/* but in jiffies. */
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unsigned long jiffies_stall; /* Time at which to check */
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/* for CPU stalls. */
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#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
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#ifdef CONFIG_NO_HZ
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long dynticks_completed; /* Value of completed @ snap. */
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#endif /* #ifdef CONFIG_NO_HZ */
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};
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extern void rcu_qsctr_inc(int cpu);
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extern void rcu_bh_qsctr_inc(int cpu);
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extern int rcu_pending(int cpu);
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extern int rcu_needs_cpu(int cpu);
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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extern struct lockdep_map rcu_lock_map;
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# define rcu_read_acquire() \
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lock_acquire(&rcu_lock_map, 0, 0, 2, 1, NULL, _THIS_IP_)
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# define rcu_read_release() lock_release(&rcu_lock_map, 1, _THIS_IP_)
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#else
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# define rcu_read_acquire() do { } while (0)
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# define rcu_read_release() do { } while (0)
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#endif
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static inline void __rcu_read_lock(void)
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{
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preempt_disable();
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__acquire(RCU);
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rcu_read_acquire();
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}
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static inline void __rcu_read_unlock(void)
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{
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rcu_read_release();
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__release(RCU);
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preempt_enable();
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}
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static inline void __rcu_read_lock_bh(void)
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{
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local_bh_disable();
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__acquire(RCU_BH);
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rcu_read_acquire();
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}
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static inline void __rcu_read_unlock_bh(void)
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{
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rcu_read_release();
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__release(RCU_BH);
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local_bh_enable();
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}
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#define __synchronize_sched() synchronize_rcu()
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#define call_rcu_sched(head, func) call_rcu(head, func)
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static inline void rcu_init_sched(void)
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{
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}
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extern void __rcu_init(void);
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extern void rcu_check_callbacks(int cpu, int user);
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extern void rcu_restart_cpu(int cpu);
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extern long rcu_batches_completed(void);
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extern long rcu_batches_completed_bh(void);
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#ifdef CONFIG_NO_HZ
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void rcu_enter_nohz(void);
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void rcu_exit_nohz(void);
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#else /* CONFIG_NO_HZ */
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static inline void rcu_enter_nohz(void)
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{
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}
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static inline void rcu_exit_nohz(void)
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{
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}
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#endif /* CONFIG_NO_HZ */
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/* A context switch is a grace period for rcutree. */
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static inline int rcu_blocking_is_gp(void)
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{
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return num_online_cpus() == 1;
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}
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#endif /* __LINUX_RCUTREE_H */
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