Re: [PATCH 11/11] x86,rcu: use percpu rcu_preempt_depth

Paul E. McKenney
2019-11-01T12:58:20Z

On Thu, Oct 31, 2019 at 10:08:06AM +0000, Lai Jiangshan wrote:
> Convert x86 to use a per-cpu rcu_preempt_depth. The reason for doing so
> is that accessing per-cpu variables is a lot cheaper than accessing
> task_struct variables.
>
> We need to save/restore the actual rcu_preempt_depth when switch.
> We also place the per-cpu rcu_preempt_depth close to __preempt_count
> and current_task variable.
>
> Using the idea of per-cpu __preempt_count.
>
> No function call when using rcu_read_[un]lock().
> Single instruction for rcu_read_lock().
> 2 instructions for fast path of rcu_read_unlock().
>
> CC: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
> ---
> arch/x86/Kconfig | 2 +
> arch/x86/include/asm/rcu_preempt_depth.h | 87 ++++++++++++++++++++++++
> arch/x86/kernel/cpu/common.c | 7 ++
> arch/x86/kernel/process_32.c | 2 +
> arch/x86/kernel/process_64.c | 2 +
> include/linux/rcupdate.h | 24 +++++++
> init/init_task.c | 2 +-
> kernel/fork.c | 2 +-
> kernel/rcu/Kconfig | 3 +
> kernel/rcu/tree_exp.h | 2 +
> kernel/rcu/tree_plugin.h | 39 ++++++++---
> 11 files changed, 160 insertions(+), 12 deletions(-)
> create mode 100644 arch/x86/include/asm/rcu_preempt_depth.h
>
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index d6e1faa28c58..af9fedc0fdc4 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -18,6 +18,7 @@ config X86_32
> select MODULES_USE_ELF_REL
> select OLD_SIGACTION
> select GENERIC_VDSO_32
> + select ARCH_HAVE_RCU_PREEMPT_DEEPTH
>
> config X86_64
> def_bool y
> @@ -31,6 +32,7 @@ config X86_64
> select NEED_DMA_MAP_STATE
> select SWIOTLB
> select ARCH_HAS_SYSCALL_WRAPPER
> + select ARCH_HAVE_RCU_PREEMPT_DEEPTH
>
> config FORCE_DYNAMIC_FTRACE
> def_bool y
> diff --git a/arch/x86/include/asm/rcu_preempt_depth.h b/arch/x86/include/asm/rcu_preempt_depth.h
> new file mode 100644
> index 000000000000..88010ad59c20
> --- /dev/null
> +++ b/arch/x86/include/asm/rcu_preempt_depth.h
> @@ -0,0 +1,87 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef __ASM_RCU_PREEMPT_DEPTH_H
> +#define __ASM_RCU_PREEMPT_DEPTH_H
> +
> +#include <asm/rmwcc.h>
> +#include <asm/percpu.h>
> +
> +#ifdef CONFIG_PREEMPT_RCU
> +DECLARE_PER_CPU(int, __rcu_preempt_depth);
> +
> +/*
> + * We use the RCU_NEED_SPECIAL bit as an inverted need_special
> + * such that a decrement hitting 0 means we can and should do
> + * rcu_read_unlock_special().
> + */
> +#define RCU_NEED_SPECIAL 0x80000000
> +
> +#define INIT_RCU_PREEMPT_DEPTH (RCU_NEED_SPECIAL)
> +
> +/* We mask the RCU_NEED_SPECIAL bit so that it return real depth */
> +static __always_inline int rcu_preempt_depth(void)
> +{
> + return raw_cpu_read_4(__rcu_preempt_depth) & ~RCU_NEED_SPECIAL;

Why not raw_cpu_generic_read()?

OK, OK, I get that raw_cpu_read_4() translates directly into an "mov"
instruction on x86, but given that x86 percpu_from_op() is able to
adjust based on operand size, why doesn't something like raw_cpu_read()
also have an x86-specific definition that adjusts based on operand size?

> +}
> +
> +static __always_inline void rcu_preempt_depth_set(int pc)
> +{
> + int old, new;
> +
> + do {
> + old = raw_cpu_read_4(__rcu_preempt_depth);
> + new = (old & RCU_NEED_SPECIAL) |
> + (pc & ~RCU_NEED_SPECIAL);
> + } while (raw_cpu_cmpxchg_4(__rcu_preempt_depth, old, new) != old);

Ummm...

OK, as you know, I have long wanted _rcu_read_lock() to be inlineable.
But are you -sure- that an x86 cmpxchg is faster than a function call
and return? I have strong doubts on that score.

Plus multiplying the x86-specific code by 26 doesn't look good.

And the RCU read-side nesting depth really is a per-task thing. Copying
it to and from the task at context-switch time might make sense if we
had a serious optimization, but it does not appear that we do.

You original patch some years back, ill-received though it was at the
time, is looking rather good by comparison. Plus it did not require
architecture-specific code!

Thanx, Paul

+}
> +
> +/*
> + * We fold the RCU_NEED_SPECIAL bit into the rcu_preempt_depth such that
> + * rcu_read_unlock() can decrement and test for needing to do special
> + * with a single instruction.
> + *
> + * We invert the actual bit, so that when the decrement hits 0 we know
> + * both it just exited the outmost rcu_read_lock() critical section and
> + * we need to do specail (the bit is cleared) if it doesn't need to be
> + * deferred.
> + */
> +
> +static inline void set_rcu_preempt_need_special(void)
> +{
> + raw_cpu_and_4(__rcu_preempt_depth, ~RCU_NEED_SPECIAL);
> +}
> +
> +/*
> + * irq needs to be disabled for clearing any bits of ->rcu_read_unlock_special
> + * and calling this function. Otherwise it may clear the work done
> + * by set_rcu_preempt_need_special() in interrupt.
> + */
> +static inline void clear_rcu_preempt_need_special(void)
> +{
> + raw_cpu_or_4(__rcu_preempt_depth, RCU_NEED_SPECIAL);
> +}
> +
> +static __always_inline void rcu_preempt_depth_inc(void)
> +{
> + raw_cpu_add_4(__rcu_preempt_depth, 1);
> +}
> +
> +static __always_inline bool rcu_preempt_depth_dec_and_test(void)
> +{
> + return GEN_UNARY_RMWcc("decl", __rcu_preempt_depth, e, __percpu_arg([var]));
> +}
> +
> +/* must be macros to avoid header recursion hell */
> +#define save_restore_rcu_preempt_depth(prev_p, next_p) do { \
> + prev_p->rcu_read_lock_nesting = this_cpu_read(__rcu_preempt_depth); \
> + this_cpu_write(__rcu_preempt_depth, next_p->rcu_read_lock_nesting); \
> + } while (0)
> +
> +#define DEFINE_PERCPU_RCU_PREEMP_DEPTH \
> + DEFINE_PER_CPU(int, __rcu_preempt_depth) = INIT_RCU_PREEMPT_DEPTH; \
> + EXPORT_PER_CPU_SYMBOL(__rcu_preempt_depth)
> +#else /* #ifdef CONFIG_PREEMPT_RCU */
> +#define save_restore_rcu_preempt_depth(prev_p, next_p) do {} while (0)
> +#define DEFINE_PERCPU_RCU_PREEMP_DEPTH /* empty */
> +#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
> +
> +#endif /* __ASM_RCU_PREEMPT_DEPTH_H */
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 9ae7d1bcd4f4..0151737e196c 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -46,6 +46,7 @@
> #include <asm/asm.h>
> #include <asm/bugs.h>
> #include <asm/cpu.h>
> +#include <asm/rcu_preempt_depth.h>
> #include <asm/mce.h>
> #include <asm/msr.h>
> #include <asm/pat.h>
> @@ -1633,6 +1634,9 @@ DEFINE_PER_CPU(unsigned int, irq_count) __visible = -1;
> DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT;
> EXPORT_PER_CPU_SYMBOL(__preempt_count);
>
> +/* close to __preempt_count */
> +DEFINE_PERCPU_RCU_PREEMP_DEPTH;
> +
> /* May not be marked __init: used by software suspend */
> void syscall_init(void)
> {
> @@ -1690,6 +1694,9 @@ EXPORT_PER_CPU_SYMBOL(current_task);
> DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT;
> EXPORT_PER_CPU_SYMBOL(__preempt_count);
>
> +/* close to __preempt_count */
> +DEFINE_PERCPU_RCU_PREEMP_DEPTH;
> +
> /*
> * On x86_32, vm86 modifies tss.sp0, so sp0 isn't a reliable way to find
> * the top of the kernel stack. Use an extra percpu variable to track the
> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index b8ceec4974fe..ab1f20353663 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -51,6 +51,7 @@
> #include <asm/cpu.h>
> #include <asm/syscalls.h>
> #include <asm/debugreg.h>
> +#include <asm/rcu_preempt_depth.h>
> #include <asm/switch_to.h>
> #include <asm/vm86.h>
> #include <asm/resctrl_sched.h>
> @@ -290,6 +291,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
> if (prev->gs | next->gs)
> lazy_load_gs(next->gs);
>
> + save_restore_rcu_preempt_depth(prev_p, next_p);
> this_cpu_write(current_task, next_p);
>
> switch_fpu_finish(next_fpu);
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index af64519b2695..2e1c6e829d30 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -50,6 +50,7 @@
> #include <asm/ia32.h>
> #include <asm/syscalls.h>
> #include <asm/debugreg.h>
> +#include <asm/rcu_preempt_depth.h>
> #include <asm/switch_to.h>
> #include <asm/xen/hypervisor.h>
> #include <asm/vdso.h>
> @@ -559,6 +560,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
>
> x86_fsgsbase_load(prev, next);
>
> + save_restore_rcu_preempt_depth(prev_p, next_p);
> /*
> * Switch the PDA and FPU contexts.
> */
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a35daab95d14..0d2abf08b694 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -41,6 +41,29 @@ void synchronize_rcu(void);
>
> #ifdef CONFIG_PREEMPT_RCU
>
> +#ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH
> +#include <asm/rcu_preempt_depth.h>
> +
> +#ifndef CONFIG_PROVE_LOCKING
> +extern void rcu_read_unlock_special(void);
> +
> +static inline void __rcu_read_lock(void)
> +{
> + rcu_preempt_depth_inc();
> +}
> +
> +static inline void __rcu_read_unlock(void)
> +{
> + if (unlikely(rcu_preempt_depth_dec_and_test()))
> + rcu_read_unlock_special();
> +}
> +#else
> +void __rcu_read_lock(void);
> +void __rcu_read_unlock(void);
> +#endif
> +
> +#else /* #ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
> +#define INIT_RCU_PREEMPT_DEPTH (0)
> void __rcu_read_lock(void);
> void __rcu_read_unlock(void);
>
> @@ -51,6 +74,7 @@ void __rcu_read_unlock(void);
> * types of kernel builds, the rcu_read_lock() nesting depth is unknowable.
> */
> #define rcu_preempt_depth() (current->rcu_read_lock_nesting)
> +#endif /* #else #ifdef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
>
> #else /* #ifdef CONFIG_PREEMPT_RCU */
>
> diff --git a/init/init_task.c b/init/init_task.c
> index 9e5cbe5eab7b..0a91e38fba37 100644
> --- a/init/init_task.c
> +++ b/init/init_task.c
> @@ -130,7 +130,7 @@ struct task_struct init_task
> .perf_event_list = LIST_HEAD_INIT(init_task.perf_event_list),
> #endif
> #ifdef CONFIG_PREEMPT_RCU
> - .rcu_read_lock_nesting = 0,
> + .rcu_read_lock_nesting = INIT_RCU_PREEMPT_DEPTH,
> .rcu_read_unlock_special.s = 0,
> .rcu_node_entry = LIST_HEAD_INIT(init_task.rcu_node_entry),
> .rcu_blocked_node = NULL,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index f9572f416126..7368d4ccb857 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1665,7 +1665,7 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
> static inline void rcu_copy_process(struct task_struct *p)
> {
> #ifdef CONFIG_PREEMPT_RCU
> - p->rcu_read_lock_nesting = 0;
> + p->rcu_read_lock_nesting = INIT_RCU_PREEMPT_DEPTH;
> p->rcu_read_unlock_special.s = 0;
> p->rcu_blocked_node = NULL;
> INIT_LIST_HEAD(&p->rcu_node_entry);
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 1cc940fef17c..d2ecca49a1a4 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -14,6 +14,9 @@ config TREE_RCU
> thousands of CPUs. It also scales down nicely to
> smaller systems.
>
> +config ARCH_HAVE_RCU_PREEMPT_DEEPTH
> + def_bool n
> +
> config PREEMPT_RCU
> bool
> default y if PREEMPTION
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index dc1af2073e25..b8922cb19884 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -588,6 +588,7 @@ static void wait_rcu_exp_gp(struct work_struct *wp)
> }
>
> #ifdef CONFIG_PREEMPT_RCU
> +static inline void set_rcu_preempt_need_special(void);
>
> /*
> * Remote handler for smp_call_function_single(). If there is an
> @@ -644,6 +645,7 @@ static void rcu_exp_handler(void *unused)
> if (rcu_preempt_depth() > 0) {
> rdp->exp_deferred_qs = true;
> WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, true);
> + set_rcu_preempt_need_special();
> return;
> }
> }
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 21bb04fec0d2..4d958d4b179c 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -82,7 +82,7 @@ static void __init rcu_bootup_announce_oddness(void)
> #ifdef CONFIG_PREEMPT_RCU
>
> static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
> -static void rcu_read_unlock_special(struct task_struct *t);
> +void rcu_read_unlock_special(void);
>
> /*
> * Tell them what RCU they are running.
> @@ -298,6 +298,7 @@ void rcu_note_context_switch(bool preempt)
> t->rcu_read_unlock_special.b.need_qs = false;
> t->rcu_read_unlock_special.b.blocked = true;
> t->rcu_blocked_node = rnp;
> + set_rcu_preempt_need_special();
>
> /*
> * Verify the CPU's sanity, trace the preemption, and
> @@ -345,6 +346,7 @@ static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
> /* Bias and limit values for ->rcu_read_lock_nesting. */
> #define RCU_NEST_PMAX (INT_MAX / 2)
>
> +#ifndef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH
> static inline void rcu_preempt_depth_inc(void)
> {
> current->rcu_read_lock_nesting++;
> @@ -352,7 +354,12 @@ static inline void rcu_preempt_depth_inc(void)
>
> static inline bool rcu_preempt_depth_dec_and_test(void)
> {
> - return --current->rcu_read_lock_nesting == 0;
> + if (--current->rcu_read_lock_nesting == 0) {
> + /* check speical after dec ->rcu_read_lock_nesting */
> + barrier();
> + return READ_ONCE(current->rcu_read_unlock_special.s);
> + }
> + return 0;
> }
>
> static inline void rcu_preempt_depth_set(int val)
> @@ -360,6 +367,12 @@ static inline void rcu_preempt_depth_set(int val)
> current->rcu_read_lock_nesting = val;
> }
>
> +static inline void clear_rcu_preempt_need_special(void) {}
> +static inline void set_rcu_preempt_need_special(void) {}
> +
> +#endif /* #ifndef CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH */
> +
> +#if !defined(CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH) || defined (CONFIG_PROVE_LOCKING)
> /*
> * Preemptible RCU implementation for rcu_read_lock().
> * Just increment ->rcu_read_lock_nesting, shared state will be updated
> @@ -383,18 +396,16 @@ EXPORT_SYMBOL_GPL(__rcu_read_lock);
> */
> void __rcu_read_unlock(void)
> {
> - struct task_struct *t = current;
> -
> if (rcu_preempt_depth_dec_and_test()) {
> - barrier(); /* critical section before exit code. */
> - if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
> - rcu_read_unlock_special(t);
> + rcu_read_unlock_special();
> }
> if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
> - WARN_ON_ONCE(rcu_preempt_depth() < 0);
> + WARN_ON_ONCE(rcu_preempt_depth() < 0 ||
> + rcu_preempt_depth() > RCU_NEST_PMAX);
> }
> }
> EXPORT_SYMBOL_GPL(__rcu_read_unlock);
> +#endif /* #if !defined(CONFIG_ARCH_HAVE_RCU_PREEMPT_DEEPTH) || defined (CONFIG_PROVE_LOCKING) */
>
> /*
> * Advance a ->blkd_tasks-list pointer to the next entry, instead
> @@ -445,6 +456,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> */
> special = t->rcu_read_unlock_special;
> t->rcu_read_unlock_special.s = 0;
> + clear_rcu_preempt_need_special();
> if (special.b.need_qs) {
> rcu_qs();
> }
> @@ -577,8 +589,9 @@ static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
> * notify RCU core processing or task having blocked during the RCU
> * read-side critical section.
> */
> -static void rcu_read_unlock_special(struct task_struct *t)
> +void rcu_read_unlock_special(void)
> {
> + struct task_struct *t = current;
> unsigned long flags;
> bool preempt_bh_were_disabled =
> !!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
> @@ -610,6 +623,8 @@ static void rcu_read_unlock_special(struct task_struct *t)
> // call rcu_read_unlock_special() and then wake_up()
> // recursively and deadlock if deferred_qs is still false.
> // To avoid it, deferred_qs has to be set beforehand.
> + // rcu_preempt_need_special is already set, so it doesn't
> + // need to call set_rcu_preempt_need_special()
> t->rcu_read_unlock_special.b.deferred_qs = true;
> if (irqs_were_disabled && use_softirq &&
> (in_interrupt() || (exp && !deferred_qs))) {
> @@ -636,6 +651,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> }
> rcu_preempt_deferred_qs_irqrestore(t, flags);
> }
> +EXPORT_SYMBOL_GPL(rcu_read_unlock_special);
>
> /*
> * Check that the list of blocked tasks for the newly completed grace
> @@ -699,8 +715,10 @@ static void rcu_flavor_sched_clock_irq(int user)
> __this_cpu_read(rcu_data.core_needs_qs) &&
> __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
> !t->rcu_read_unlock_special.b.need_qs &&
> - time_after(jiffies, rcu_state.gp_start + HZ))
> + time_after(jiffies, rcu_state.gp_start + HZ)) {
> t->rcu_read_unlock_special.b.need_qs = true;
> + set_rcu_preempt_need_special();
> + }
> }
>
> /*
> @@ -719,6 +737,7 @@ void exit_rcu(void)
> rcu_preempt_depth_set(1);
> barrier();
> WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
> + set_rcu_preempt_need_special();
> } else if (unlikely(rcu_preempt_depth())) {
> rcu_preempt_depth_set(1);
> } else {
> --
> 2.20.1
>