Messages in this thread Patch in this message | | | From | Lai Jiangshan <> | Date | Mon, 29 Jan 2018 17:10:49 +0800 | Subject | Re: [PATCH RFC 01/16] prcu: Add PRCU implementation |
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On Tue, Jan 23, 2018 at 3:59 PM, <lianglihao@huawei.com> wrote: > From: Heng Zhang <heng.z@huawei.com> > > This RCU implementation (PRCU) is based on a fast consensus protocol > published in the following paper: > > Fast Consensus Using Bounded Staleness for Scalable Read-mostly Synchronization. > Haibo Chen, Heng Zhang, Ran Liu, Binyu Zang, and Haibing Guan. > IEEE Transactions on Parallel and Distributed Systems (TPDS), 2016. > https://dl.acm.org/citation.cfm?id=3024114.3024143 > > Signed-off-by: Heng Zhang <heng.z@huawei.com> > Signed-off-by: Lihao Liang <lianglihao@huawei.com> > --- > include/linux/prcu.h | 37 +++++++++++++++ > kernel/rcu/Makefile | 2 +- > kernel/rcu/prcu.c | 125 +++++++++++++++++++++++++++++++++++++++++++++++++++ > kernel/sched/core.c | 2 + > 4 files changed, 165 insertions(+), 1 deletion(-) > create mode 100644 include/linux/prcu.h > create mode 100644 kernel/rcu/prcu.c > > diff --git a/include/linux/prcu.h b/include/linux/prcu.h > new file mode 100644 > index 00000000..653b4633 > --- /dev/null > +++ b/include/linux/prcu.h > @@ -0,0 +1,37 @@ > +#ifndef __LINUX_PRCU_H > +#define __LINUX_PRCU_H > + > +#include <linux/atomic.h> > +#include <linux/mutex.h> > +#include <linux/wait.h> > + > +#define CONFIG_PRCU > + > +struct prcu_local_struct { > + unsigned int locked; > + unsigned int online; > + unsigned long long version; > +}; > + > +struct prcu_struct { > + atomic64_t global_version; > + atomic_t active_ctr; > + struct mutex mtx; > + wait_queue_head_t wait_q; > +}; > + > +#ifdef CONFIG_PRCU > +void prcu_read_lock(void); > +void prcu_read_unlock(void); > +void synchronize_prcu(void); > +void prcu_note_context_switch(void); > + > +#else /* #ifdef CONFIG_PRCU */ > + > +#define prcu_read_lock() do {} while (0) > +#define prcu_read_unlock() do {} while (0) > +#define synchronize_prcu() do {} while (0) > +#define prcu_note_context_switch() do {} while (0) > + > +#endif /* #ifdef CONFIG_PRCU */ > +#endif /* __LINUX_PRCU_H */ > diff --git a/kernel/rcu/Makefile b/kernel/rcu/Makefile > index 23803c7d..8791419c 100644 > --- a/kernel/rcu/Makefile > +++ b/kernel/rcu/Makefile > @@ -2,7 +2,7 @@ > # and is generally not a function of system call inputs. > KCOV_INSTRUMENT := n > > -obj-y += update.o sync.o > +obj-y += update.o sync.o prcu.o > obj-$(CONFIG_CLASSIC_SRCU) += srcu.o > obj-$(CONFIG_TREE_SRCU) += srcutree.o > obj-$(CONFIG_TINY_SRCU) += srcutiny.o > diff --git a/kernel/rcu/prcu.c b/kernel/rcu/prcu.c > new file mode 100644 > index 00000000..a00b9420 > --- /dev/null > +++ b/kernel/rcu/prcu.c > @@ -0,0 +1,125 @@ > +#include <linux/smp.h> > +#include <linux/prcu.h> > +#include <linux/percpu.h> > +#include <linux/compiler.h> > +#include <linux/sched.h> > + > +#include <asm/barrier.h> > + > +DEFINE_PER_CPU_SHARED_ALIGNED(struct prcu_local_struct, prcu_local); > + > +struct prcu_struct global_prcu = { > + .global_version = ATOMIC64_INIT(0), > + .active_ctr = ATOMIC_INIT(0), > + .mtx = __MUTEX_INITIALIZER(global_prcu.mtx), > + .wait_q = __WAIT_QUEUE_HEAD_INITIALIZER(global_prcu.wait_q) > +}; > +struct prcu_struct *prcu = &global_prcu; > + > +static inline void prcu_report(struct prcu_local_struct *local) > +{ > + unsigned long long global_version; > + unsigned long long local_version; > + > + global_version = atomic64_read(&prcu->global_version); > + local_version = local->version; > + if (global_version > local_version) > + cmpxchg(&local->version, local_version, global_version);
It is called with irq-disabled, and local->version can't be modified on other cpu. why cmpxchg is needed?
> +} > + > +void prcu_read_lock(void) > +{ > + struct prcu_local_struct *local; > + > + local = get_cpu_ptr(&prcu_local); > + if (!local->online) { > + WRITE_ONCE(local->online, 1); > + smp_mb();
What's is the paired code?
> + } > + > + local->locked++; > + put_cpu_ptr(&prcu_local); > +} > +EXPORT_SYMBOL(prcu_read_lock); > + > +void prcu_read_unlock(void) > +{ > + int locked; > + struct prcu_local_struct *local; > + > + barrier(); > + local = get_cpu_ptr(&prcu_local); > + locked = local->locked; > + if (locked) { > + local->locked--; > + if (locked == 1) > + prcu_report(local); > + put_cpu_ptr(&prcu_local); > + } else { > + put_cpu_ptr(&prcu_local); > + if (!atomic_dec_return(&prcu->active_ctr)) > + wake_up(&prcu->wait_q); > + } > +} > +EXPORT_SYMBOL(prcu_read_unlock); > + > +static void prcu_handler(void *info) > +{ > + struct prcu_local_struct *local; > + > + local = this_cpu_ptr(&prcu_local); > + if (!local->locked) > + WRITE_ONCE(local->version, atomic64_read(&prcu->global_version)); > +} > + > +void synchronize_prcu(void) > +{ > + int cpu; > + cpumask_t cpus;
It might overflow the stack if the cpumask is large, please move it to struct prcu.
> + unsigned long long version; > + struct prcu_local_struct *local; > + > + version = atomic64_add_return(1, &prcu->global_version);
I think this line of code at least causes the following problem.
> + mutex_lock(&prcu->mtx); > + > + local = get_cpu_ptr(&prcu_local); > + local->version = version;
The successful orders of mutex_lock() might not be the same the orders of atomic64_add_return(). In this case, local->version will be decreased.
prcu_report() can also happen here now. It is unsure who will change successfully the local->version.
> + put_cpu_ptr(&prcu_local); > + > + cpumask_clear(&cpus); > + for_each_possible_cpu(cpu) { > + local = per_cpu_ptr(&prcu_local, cpu); > + if (!READ_ONCE(local->online)) > + continue;
It seems like reading on local->online is unreliable.
> + if (READ_ONCE(local->version) < version) {
please handle the cases when version wraps around the maximum.
> + smp_call_function_single(cpu, prcu_handler, NULL, 0);
it smells bad when it is in for_each_possible_cpu() loop.
> + cpumask_set_cpu(cpu, &cpus); > + } > + } > + > + for_each_cpu(cpu, &cpus) { > + local = per_cpu_ptr(&prcu_local, cpu); > + while (READ_ONCE(local->version) < version) > + cpu_relax(); > + } >
Ouch, the cpu_relax() loop would take a long time. Since it will wait until all the relevant cpus scheduled. relevant cpus: prcu reader active cpus. So this block of code equals to synchronze_sched() in many cases when prcu is massively used. isn't it?
smp_mb() /* A paired with B */
> + if (atomic_read(&prcu->active_ctr)) > + wait_event(prcu->wait_q, !atomic_read(&prcu->active_ctr)); > + > + mutex_unlock(&prcu->mtx); > +} > +EXPORT_SYMBOL(synchronize_prcu); > + > +void prcu_note_context_switch(void) > +{ > + struct prcu_local_struct *local; > + > + local = get_cpu_ptr(&prcu_local); > + if (local->locked) { > + atomic_add(local->locked, &prcu->active_ctr);
smp_mb() /* B paired with A */
> + local->locked = 0; > + } > + local->online = 0; > + prcu_report(local); > + put_cpu_ptr(&prcu_local); > +} > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index 326d4f88..a308581b 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -15,6 +15,7 @@ > #include <linux/init_task.h> > #include <linux/context_tracking.h> > #include <linux/rcupdate_wait.h> > +#include <linux/prcu.h> > > #include <linux/blkdev.h> > #include <linux/kprobes.h> > @@ -3383,6 +3384,7 @@ static void __sched notrace __schedule(bool preempt) > > local_irq_disable(); > rcu_note_context_switch(preempt); > + prcu_note_context_switch(); > > /* > * Make sure that signal_pending_state()->signal_pending() below > -- > 2.14.1.729.g59c0ea183 >
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