Paolo Bonzini <pbonz...@redhat.com> writes:
> Set cpu->running without taking the cpu_list lock, only look at it if
> there is a concurrent exclusive section. This requires adding a new
> field to CPUState, which records whether a running CPU is being counted
> in pending_cpus. When an exclusive section is started concurrently with
> cpu_exec_start, cpu_exec_start can use the new field to wait for the end
> of the exclusive section.
>
> This a separate patch for easier bisection of issues.
>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
> ---
> cpus-common.c | 72 +++++++++++++--
> docs/tcg-exclusive.promela | 224
> +++++++++++++++++++++++++++++++++++++++++++++
> include/qom/cpu.h | 5 +-
> 3 files changed, 289 insertions(+), 12 deletions(-)
> create mode 100644 docs/tcg-exclusive.promela
>
> diff --git a/cpus-common.c b/cpus-common.c
> index 88cf5ec..443617a 100644
> --- a/cpus-common.c
> +++ b/cpus-common.c
> @@ -170,8 +170,12 @@ void start_exclusive(void)
>
> /* Make all other cpus stop executing. */
> pending_cpus = 1;
> +
> + /* Write pending_cpus before reading other_cpu->running. */
> + smp_mb();
> CPU_FOREACH(other_cpu) {
> if (other_cpu->running) {
> + other_cpu->has_waiter = true;
> pending_cpus++;
> qemu_cpu_kick(other_cpu);
> }
> @@ -192,25 +196,73 @@ void end_exclusive(void)
> /* Wait for exclusive ops to finish, and begin cpu execution. */
> void cpu_exec_start(CPUState *cpu)
> {
> - qemu_mutex_lock(&qemu_cpu_list_mutex);
> - exclusive_idle();
> cpu->running = true;
> - qemu_mutex_unlock(&qemu_cpu_list_mutex);
> +
> + /* Write cpu->running before reading pending_cpus. */
> + smp_mb();
> +
> + /* 1. start_exclusive saw cpu->running == true and pending_cpus >= 1.
> + * After taking the lock we'll see cpu->has_waiter == true and run---not
> + * for long because start_exclusive kicked us. cpu_exec_end will
> + * decrement pending_cpus and signal the waiter.
> + *
> + * 2. start_exclusive saw cpu->running == false but pending_cpus >= 1.
> + * This includes the case when an exclusive item is running now.
> + * Then we'll see cpu->has_waiter == false and wait for the item to
> + * complete.
> + *
> + * 3. pending_cpu == 0. Then start_exclusive is definitely going to
> + * see cpu->running == true, and it will kick the CPU.
> + */
> + if (pending_cpus) {
> + qemu_mutex_lock(&qemu_cpu_list_mutex);
> + if (!cpu->has_waiter) {
> + /* Not counted in pending_cpus, let the exclusive item
> + * run. Since we have the lock, set cpu->running to true
> + * while holding it instead of retrying.
> + */
> + cpu->running = false;
> + exclusive_idle();
> + cpu->running = true;
> + } else {
> + /* Counted in pending_cpus, go ahead. */
> + }
> + qemu_mutex_unlock(&qemu_cpu_list_mutex);
> + }
> }
>
> /* Mark cpu as not executing, and release pending exclusive ops. */
> void cpu_exec_end(CPUState *cpu)
> {
> - qemu_mutex_lock(&qemu_cpu_list_mutex);
> cpu->running = false;
> - if (pending_cpus > 1) {
> - pending_cpus--;
> - if (pending_cpus == 1) {
> - qemu_cond_signal(&exclusive_cond);
> +
> + /* Write cpu->running before reading pending_cpus. */
> + smp_mb();
> +
> + /* 1. start_exclusive saw cpu->running == true. Then it will increment
> + * pending_cpus and wait for exclusive_cond. After taking the lock
> + * we'll see cpu->has_waiter == true.
> + *
> + * 2. start_exclusive saw cpu->running == false but here pending_cpus >=
> 1.
> + * This includes the case when an exclusive item is running now.
> + * Then we'll see cpu->has_waiter == false and not touch pending_cpus,
> + * but will run exclusive_idle to wait for the item to complete.
> + *
> + * 3. pending_cpu == 0. Then start_exclusive is definitely going to
> + * see cpu->running == false, and it can ignore this CPU until the
> + * next cpu_exec_start.
> + */
> + if (pending_cpus) {
> + qemu_mutex_lock(&qemu_cpu_list_mutex);
> + if (cpu->has_waiter) {
> + cpu->has_waiter = false;
> + if (--pending_cpus == 1) {
> + qemu_cond_signal(&exclusive_cond);
> + }
> + exclusive_idle();
> }
> + qemu_mutex_unlock(&qemu_cpu_list_mutex);
> }
> - exclusive_idle();
> - qemu_mutex_unlock(&qemu_cpu_list_mutex);
> }
>
> static void async_safe_run_on_cpu_fn(CPUState *cpu, void *data)
> diff --git a/docs/tcg-exclusive.promela b/docs/tcg-exclusive.promela
> new file mode 100644
> index 0000000..293b530
> --- /dev/null
> +++ b/docs/tcg-exclusive.promela
> @@ -0,0 +1,224 @@
> +/*
> + * This model describes the implementation of exclusive sections in
> + * cpus-common.c (start_exclusive, end_exclusive, cpu_exec_start,
> + * cpu_exec_end).
\o/ nice to have a model ;-)
> + *
> + * Author: Paolo Bonzini <pbonz...@redhat.com>
> + *
> + * This file is in the public domain. If you really want a license,
> + * the WTFPL will do.
> + *
> + * To verify it:
> + * spin -a docs/event.promela
wrong docs name
> + * ./a.out -a
Which version of spin did you run? I grabbed the latest src release
(http://spinroot.com/spin/Src/src645.tar.gz) and had to manually build
the output:
~/src/spin/Src6.4.5/spin -a docs/tcg-exclusive.promela
gcc pan.c
../a.out
> + *
> + * Tunable processor macros: N_CPUS, N_EXCLUSIVE, N_CYCLES, USE_MUTEX,
> + * TEST_EXPENSIVE.
> + */
How do you pass these? I tried:
~/src/spin/Src6.4.5/spin -a docs/tcg-exclusive.promela -DN_CPUS=4
~/src/spin/Src6.4.5/spin -a docs/tcg-exclusive.promela -DN_CPUS 4
without any joy.
> +
> +// Define the missing parameters for the model
> +#ifndef N_CPUS
> +#define N_CPUS 2
> +#warning defaulting to 2 CPU processes
> +#endif
> +
> +// the expensive test is not so expensive for <= 2 CPUs
> +// If the mutex is used, it's also cheap (300 MB / 4 seconds) for 3 CPUs
> +// For 3 CPUs and the lock-free option it needs 1.5 GB of RAM
> +#if N_CPUS <= 2 || (N_CPUS <= 3 && defined USE_MUTEX)
> +#define TEST_EXPENSIVE
> +#endif
> +
> +#ifndef N_EXCLUSIVE
> +# if !defined N_CYCLES || N_CYCLES <= 1 || defined TEST_EXPENSIVE
> +# define N_EXCLUSIVE 2
> +# warning defaulting to 2 concurrent exclusive sections
> +# else
> +# define N_EXCLUSIVE 1
> +# warning defaulting to 1 concurrent exclusive sections
> +# endif
> +#endif
> +#ifndef N_CYCLES
> +# if N_EXCLUSIVE <= 1 || defined TEST_EXPENSIVE
> +# define N_CYCLES 2
> +# warning defaulting to 2 CPU cycles
> +# else
> +# define N_CYCLES 1
> +# warning defaulting to 1 CPU cycles
> +# endif
> +#endif
> +
> +
> +// synchronization primitives. condition variables require a
> +// process-local "cond_t saved;" variable.
> +
> +#define mutex_t byte
> +#define MUTEX_LOCK(m) atomic { m == 0 -> m = 1 }
> +#define MUTEX_UNLOCK(m) m = 0
> +
> +#define cond_t int
> +#define COND_WAIT(c, m) { \
> + saved = c; \
> + MUTEX_UNLOCK(m); \
> + c != saved -> MUTEX_LOCK(m); \
> + }
> +#define COND_BROADCAST(c) c++
> +
> +// this is the logic from cpus-common.c
> +
> +mutex_t mutex;
> +cond_t exclusive_cond;
> +cond_t exclusive_resume;
> +byte pending_cpus;
> +
> +byte running[N_CPUS];
> +byte has_waiter[N_CPUS];
> +
> +#define exclusive_idle() \
> + do \
> + :: pending_cpus -> COND_WAIT(exclusive_resume, mutex); \
> + :: else -> break; \
> + od
> +
> +#define start_exclusive() \
> + MUTEX_LOCK(mutex); \
> + exclusive_idle(); \
> + pending_cpus = 1; \
> + \
> + i = 0; \
> + do \
> + :: i < N_CPUS -> { \
> + if \
> + :: running[i] -> has_waiter[i] = 1; pending_cpus++; \
> + :: else -> skip; \
> + fi; \
> + i++; \
> + } \
> + :: else -> break; \
> + od; \
> + \
> + do \
> + :: pending_cpus > 1 -> COND_WAIT(exclusive_cond, mutex); \
> + :: else -> break; \
> + od;
> +
> +#define end_exclusive() \
> + pending_cpus = 0; \
> + COND_BROADCAST(exclusive_resume); \
> + MUTEX_UNLOCK(mutex);
> +
> +#ifdef USE_MUTEX
> +// Simple version using mutexes
> +#define cpu_exec_start(id)
> \
> + MUTEX_LOCK(mutex);
> \
> + exclusive_idle();
> \
> + running[id] = 1;
> \
> + MUTEX_UNLOCK(mutex);
> +
> +#define cpu_exec_end(id)
> \
> + MUTEX_LOCK(mutex);
> \
> + running[id] = 0;
> \
> + if
> \
> + :: pending_cpus -> {
> \
> + pending_cpus--;
> \
> + if
> \
> + :: pending_cpus == 1 -> COND_BROADCAST(exclusive_cond);
> \
> + :: else -> skip;
> \
> + fi;
> \
> + exclusive_idle();
> \
> + }
> \
> + :: else -> skip;
> \
> + fi;
> \
> + MUTEX_UNLOCK(mutex);
> +#else
> +// Wait-free fast path, only needs mutex when concurrent with
> +// an exclusive section
> +#define cpu_exec_start(id)
> \
> + running[id] = 1;
> \
> + if
> \
> + :: pending_cpus -> {
> \
> + MUTEX_LOCK(mutex);
> \
> + if
> \
> + :: !has_waiter[id] -> {
> \
> + running[id] = 0;
> \
> + exclusive_idle();
> \
> + running[id] = 1;
> \
> + }
> \
> + :: else -> skip;
> \
> + fi;
> \
> + MUTEX_UNLOCK(mutex);
> \
> + }
> \
> + :: else -> skip;
> \
> + fi;
> +
> +#define cpu_exec_end(id)
> \
> + running[id] = 0;
> \
> + if
> \
> + :: pending_cpus -> {
> \
> + MUTEX_LOCK(mutex);
> \
> + if
> \
> + :: has_waiter[id] -> {
> \
> + has_waiter[id] = 0;
> \
> + pending_cpus--;
> \
> + if
> \
> + :: pending_cpus == 1 ->
> COND_BROADCAST(exclusive_cond); \
> + :: else -> skip;
> \
> + fi;
> \
> + exclusive_idle();
> \
> + }
> \
> + :: else -> skip;
> \
> + fi;
> \
> + MUTEX_UNLOCK(mutex);
> \
> + }
> \
> + :: else -> skip;
> \
> + fi
> +#endif
> +
> +// Promela processes
> +
> +byte done_cpu;
> +byte in_cpu;
> +active[N_CPUS] proctype cpu()
> +{
> + byte id = _pid % N_CPUS;
> + byte cycles = 0;
> + cond_t saved;
> +
> + do
> + :: cycles == N_CYCLES -> break;
> + :: else -> {
> + cycles++;
> + cpu_exec_start(id)
> + in_cpu++;
> + done_cpu++;
> + in_cpu--;
> + cpu_exec_end(id)
> + }
> + od;
> +}
> +
> +byte done_exclusive;
> +byte in_exclusive;
> +active[N_EXCLUSIVE] proctype exclusive()
> +{
> + cond_t saved;
> + byte i;
> +
> + start_exclusive();
> + in_exclusive = 1;
> + done_exclusive++;
> + in_exclusive = 0;
> + end_exclusive();
> +}
> +
> +#define LIVENESS (done_cpu == N_CPUS * N_CYCLES && done_exclusive ==
> N_EXCLUSIVE)
> +#define SAFETY !(in_exclusive && in_cpu)
> +
> +never { /* ! ([] SAFETY && <> [] LIVENESS) */
> + do
> + // once the liveness property is satisfied, this is not executable
> + // and the never clause is not accepted
> + :: ! LIVENESS -> accept_liveness: skip
> + :: 1 -> assert(SAFETY)
> + od;
> +}
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 3817a98..14cf97c 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -249,7 +249,8 @@ struct qemu_work_item {
> * @nr_threads: Number of threads within this CPU.
> * @numa_node: NUMA node this CPU is belonging to.
> * @host_tid: Host thread ID.
> - * @running: #true if CPU is currently running;
> + * @running: #true if CPU is currently running (lockless).
> + * @has_waiter: #true if a CPU is currently waiting for the cpu_exec_end;
> * valid under cpu_list_lock.
> * @created: Indicates whether the CPU thread has been successfully created.
> * @interrupt_request: Indicates a pending interrupt request.
> @@ -303,7 +304,7 @@ struct CPUState {
> #endif
> int thread_id;
> uint32_t host_tid;
> - bool running;
> + bool running, has_waiter;
> struct QemuCond *halt_cond;
> bool thread_kicked;
> bool created;
--
Alex Bennée
