Re: [Qemu-devel] [RFC v1 09/11] tcg: drop global lock during TCG code execution

[Alex Bennée]

KONRAD Frederic <address@hidden> writes:

> Hi Alex,
>
> Thanks for having pulling all that together!
> About this patch the original author was Jan Kiszka
> (https://lists.gnu.org/archive/html/qemu-devel/2011-09/msg03323.html)
>
> This has probably been dropped during a rebase.

I'll amend the author on v2.

Jan,

The original didn't have a s-o-b tag from you. Are you happy to give one
for its current iteration?

>
> Thanks,
> Fred
>
> Le 18/03/2016 17:18, Alex Bennée a écrit :
>> From: KONRAD Frederic <address@hidden>
>>
>> This finally allows TCG to benefit from the iothread introduction: Drop
>> the global mutex while running pure TCG CPU code. Reacquire the lock
>> when entering MMIO or PIO emulation, or when leaving the TCG loop.
>>
>> We have to revert a few optimization for the current TCG threading
>> model, namely kicking the TCG thread in qemu_mutex_lock_iothread and not
>> kicking it in qemu_cpu_kick. We also need to disable RAM block
>> reordering until we have a more efficient locking mechanism at hand.
>>
>> Still, a Linux x86 UP guest and my Musicpal ARM model boot fine here.
>> These numbers demonstrate where we gain something:
>>
>> 20338 jan       20   0  331m  75m 6904 R   99  0.9   0:50.95 qemu-system-arm
>> 20337 jan       20   0  331m  75m 6904 S   20  0.9   0:26.50 qemu-system-arm
>>
>> The guest CPU was fully loaded, but the iothread could still run mostly
>> independent on a second core. Without the patch we don't get beyond
>>
>> 32206 jan       20   0  330m  73m 7036 R   82  0.9   1:06.00 qemu-system-arm
>> 32204 jan       20   0  330m  73m 7036 S   21  0.9   0:17.03 qemu-system-arm
>>
>> We don't benefit significantly, though, when the guest is not fully
>> loading a host CPU.
>>
>> Message-Id: <address@hidden>
>> Signed-off-by: KONRAD Frederic <address@hidden>
>> [AJB: -smp single-threaded fix, rm old info from commit msg]
>> Signed-off-by: Alex Bennée <address@hidden>
>>
>> ---
>> v1 (ajb):
>>    - SMP failure now fixed by previous commit
>> Changes from Fred Konrad (mttcg-v7 via paolo):
>>    * Rebase on the current HEAD.
>>    * Fixes a deadlock in qemu_devices_reset().
>>    * Remove the mutex in address_space_*
>> ---
>>   cpu-exec.c         | 10 ++++++++++
>>   cpus.c             | 26 +++-----------------------
>>   cputlb.c           |  4 ++++
>>   exec.c             | 12 ++++++++++++
>>   hw/i386/kvmvapic.c |  3 +++
>>   softmmu_template.h | 17 +++++++++++++++++
>>   translate-all.c    | 11 +++++++++--
>>   7 files changed, 58 insertions(+), 25 deletions(-)
>>
>> diff --git a/cpu-exec.c b/cpu-exec.c
>> index 3572256..76891fd 100644
>> --- a/cpu-exec.c
>> +++ b/cpu-exec.c
>> @@ -28,6 +28,7 @@
>>   #include "qemu/rcu.h"
>>   #include "exec/tb-hash.h"
>>   #include "exec/log.h"
>> +#include "qemu/main-loop.h"
>>   #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
>>   #include "hw/i386/apic.h"
>>   #endif
>> @@ -449,6 +450,13 @@ int cpu_exec(CPUState *cpu)
>>               for(;;) {
>>                   interrupt_request = cpu->interrupt_request;
>>                   if (unlikely(interrupt_request)) {
>> +                    /* FIXME: this needs to take the iothread lock.
>> +                     * For this we need to find all places in
>> +                     * cc->cpu_exec_interrupt that can call cpu_loop_exit,
>> +                     * and call qemu_unlock_iothread_mutex() there.  Else,
>> +                     * add a flag telling cpu_loop_exit() to unlock it.
>> +                     */
>> +
>>                       if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
>>                           /* Mask out external interrupts for this step. */
>>                           interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
>> @@ -501,6 +509,7 @@ int cpu_exec(CPUState *cpu)
>>                              the program flow was changed */
>>                           next_tb = 0;
>>                       }
>> +
>>                   }
>>                   if (unlikely(cpu->exit_request
>>                                || replay_has_interrupt())) {
>> @@ -618,6 +627,7 @@ int cpu_exec(CPUState *cpu)
>>               g_assert(env == &x86_cpu->env);
>>   #endif
>>   #endif /* buggy compiler */
>> +
>>               cpu->can_do_io = 1;
>>               tb_lock_reset();
>>           }
>> diff --git a/cpus.c b/cpus.c
>> index a87fbf9..0e3d5f9 100644
>> --- a/cpus.c
>> +++ b/cpus.c
>> @@ -911,8 +911,6 @@ static void qemu_kvm_init_cpu_signals(CPUState *cpu)
>>   #endif /* _WIN32 */
>>
>>   static QemuMutex qemu_global_mutex;
>> -static QemuCond qemu_io_proceeded_cond;
>> -static unsigned iothread_requesting_mutex;
>>
>>   static QemuThread io_thread;
>>
>> @@ -928,7 +926,6 @@ void qemu_init_cpu_loop(void)
>>       qemu_cond_init(&qemu_cpu_cond);
>>       qemu_cond_init(&qemu_pause_cond);
>>       qemu_cond_init(&qemu_work_cond);
>> -    qemu_cond_init(&qemu_io_proceeded_cond);
>>       qemu_mutex_init(&qemu_global_mutex);
>>
>>       qemu_thread_get_self(&io_thread);
>> @@ -1043,10 +1040,6 @@ static void qemu_tcg_wait_io_event(CPUState *cpu)
>>           qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
>>       }
>>
>> -    while (iothread_requesting_mutex) {
>> -        qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex);
>> -    }
>> -
>>       CPU_FOREACH(cpu) {
>>           qemu_wait_io_event_common(cpu);
>>       }
>> @@ -1314,22 +1307,7 @@ bool qemu_mutex_iothread_locked(void)
>>
>>   void qemu_mutex_lock_iothread(void)
>>   {
>> -    atomic_inc(&iothread_requesting_mutex);
>> -    /* In the simple case there is no need to bump the VCPU thread out of
>> -     * TCG code execution.
>> -     */
>> -    if (!tcg_enabled() || qemu_in_vcpu_thread() ||
>> -        !first_cpu || !first_cpu->created) {
>> -        qemu_mutex_lock(&qemu_global_mutex);
>> -        atomic_dec(&iothread_requesting_mutex);
>> -    } else {
>> -        if (qemu_mutex_trylock(&qemu_global_mutex)) {
>> -            qemu_cpu_kick_no_halt();
>> -            qemu_mutex_lock(&qemu_global_mutex);
>> -        }
>> -        atomic_dec(&iothread_requesting_mutex);
>> -        qemu_cond_broadcast(&qemu_io_proceeded_cond);
>> -    }
>> +    qemu_mutex_lock(&qemu_global_mutex);
>>       iothread_locked = true;
>>   }
>>
>> @@ -1575,7 +1553,9 @@ static int tcg_cpu_exec(CPUState *cpu)
>>           cpu->icount_decr.u16.low = decr;
>>           cpu->icount_extra = count;
>>       }
>> +    qemu_mutex_unlock_iothread();
>>       ret = cpu_exec(cpu);
>> +    qemu_mutex_lock_iothread();
>>   #ifdef CONFIG_PROFILER
>>       tcg_time += profile_getclock() - ti;
>>   #endif
>> diff --git a/cputlb.c b/cputlb.c
>> index 2f7a166..d607471 100644
>> --- a/cputlb.c
>> +++ b/cputlb.c
>> @@ -30,6 +30,10 @@
>>   #include "exec/ram_addr.h"
>>   #include "tcg/tcg.h"
>>
>> +#ifdef CONFIG_SOFTMMU
>> +#include "qemu/main-loop.h"
>> +#endif
>> +
>>   //#define DEBUG_TLB
>>   //#define DEBUG_TLB_CHECK
>>
>> diff --git a/exec.c b/exec.c
>> index 402b751..9986d59 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -2113,6 +2113,12 @@ static void check_watchpoint(int offset, int len, 
>> MemTxAttrs attrs, int flags)
>>                   }
>>                   cpu->watchpoint_hit = wp;
>>
>> +                /*
>> +                 * Unlock iothread mutex before calling cpu_loop_exit
>> +                 * or cpu_resume_from_signal.
>> +                 */
>> +                qemu_mutex_unlock_iothread();
>> +
>>                   /* Unlocked by cpu_loop_exit or cpu_resume_from_signal.  */
>>                   tb_lock();
>>                   tb_check_watchpoint(cpu);
>> @@ -2348,8 +2354,14 @@ static void io_mem_init(void)
>>       memory_region_init_io(&io_mem_rom, NULL, &unassigned_mem_ops, NULL, 
>> NULL, UINT64_MAX);
>>       memory_region_init_io(&io_mem_unassigned, NULL, &unassigned_mem_ops, 
>> NULL,
>>                             NULL, UINT64_MAX);
>> +
>> +    /* io_mem_notdirty calls tb_invalidate_phys_page_fast,
>> +     * which must be called without the iothread mutex.
>> +     */
>>       memory_region_init_io(&io_mem_notdirty, NULL, &notdirty_mem_ops, NULL,
>>                             NULL, UINT64_MAX);
>> +    memory_region_clear_global_locking(&io_mem_notdirty);
>> +
>>       memory_region_init_io(&io_mem_watch, NULL, &watch_mem_ops, NULL,
>>                             NULL, UINT64_MAX);
>>   }
>> diff --git a/hw/i386/kvmvapic.c b/hw/i386/kvmvapic.c
>> index 7c0d542..a0439a8 100644
>> --- a/hw/i386/kvmvapic.c
>> +++ b/hw/i386/kvmvapic.c
>> @@ -447,6 +447,9 @@ static void patch_instruction(VAPICROMState *s, X86CPU 
>> *cpu, target_ulong ip)
>>       resume_all_vcpus();
>>
>>       if (!kvm_enabled()) {
>> +        /* Unlock iothread mutex before calling cpu_resume_from_signal.  */
>> +        qemu_mutex_unlock_iothread();
>> +
>>           /* Unlocked by cpu_resume_from_signal.  */
>>           tb_lock();
>>           cs->current_tb = NULL;
>> diff --git a/softmmu_template.h b/softmmu_template.h
>> index 208f808..0b6c609 100644
>> --- a/softmmu_template.h
>> +++ b/softmmu_template.h
>> @@ -151,6 +151,7 @@ static inline DATA_TYPE glue(io_read, 
>> SUFFIX)(CPUArchState *env,
>>       CPUState *cpu = ENV_GET_CPU(env);
>>       hwaddr physaddr = iotlbentry->addr;
>>       MemoryRegion *mr = iotlb_to_region(cpu, physaddr, iotlbentry->attrs);
>> +    bool locked = false;
>>
>>       physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
>>       cpu->mem_io_pc = retaddr;
>> @@ -159,8 +160,15 @@ static inline DATA_TYPE glue(io_read, 
>> SUFFIX)(CPUArchState *env,
>>       }
>>
>>       cpu->mem_io_vaddr = addr;
>> +    if (mr->global_locking) {
>> +        qemu_mutex_lock_iothread();
>> +        locked = true;
>> +    }
>>       memory_region_dispatch_read(mr, physaddr, &val, 1 << SHIFT,
>>                                   iotlbentry->attrs);
>> +    if (locked) {
>> +        qemu_mutex_unlock_iothread();
>> +    }
>>       return val;
>>   }
>>   #endif
>> @@ -358,6 +366,7 @@ static inline void glue(io_write, SUFFIX)(CPUArchState 
>> *env,
>>       CPUState *cpu = ENV_GET_CPU(env);
>>       hwaddr physaddr = iotlbentry->addr;
>>       MemoryRegion *mr = iotlb_to_region(cpu, physaddr, iotlbentry->attrs);
>> +    bool locked = false;
>>
>>       physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
>>       if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu->can_do_io) {
>> @@ -366,8 +375,16 @@ static inline void glue(io_write, SUFFIX)(CPUArchState 
>> *env,
>>
>>       cpu->mem_io_vaddr = addr;
>>       cpu->mem_io_pc = retaddr;
>> +
>> +    if (mr->global_locking) {
>> +        qemu_mutex_lock_iothread();
>> +        locked = true;
>> +    }
>>       memory_region_dispatch_write(mr, physaddr, val, 1 << SHIFT,
>>                                    iotlbentry->attrs);
>> +    if (locked) {
>> +        qemu_mutex_unlock_iothread();
>> +    }
>>   }
>>
>>   void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
>> diff --git a/translate-all.c b/translate-all.c
>> index 1aab243..fe1392a 100644
>> --- a/translate-all.c
>> +++ b/translate-all.c
>> @@ -1221,6 +1221,7 @@ void tb_invalidate_phys_range(tb_page_addr_t start, 
>> tb_page_addr_t end)
>>    * this TB.
>>    *
>>    * Called with mmap_lock held for user-mode emulation
>> + * If called from generated code, iothread mutex must not be held.
>>    */
>>   void tb_invalidate_phys_page_range(tb_page_addr_t start, tb_page_addr_t 
>> end,
>>                                      int is_cpu_write_access)
>> @@ -1333,7 +1334,9 @@ void tb_invalidate_phys_page_range(tb_page_addr_t 
>> start, tb_page_addr_t end,
>>   }
>>
>>   #ifdef CONFIG_SOFTMMU
>> -/* len must be <= 8 and start must be a multiple of len */
>> +/* len must be <= 8 and start must be a multiple of len.
>> + * Called via softmmu_template.h, with iothread mutex not held.
>> + */
>>   void tb_invalidate_phys_page_fast(tb_page_addr_t start, int len)
>>   {
>>       PageDesc *p;
>> @@ -1591,6 +1594,7 @@ static TranslationBlock *tb_find_pc(uintptr_t tc_ptr)
>>   }
>>
>>   #if !defined(CONFIG_USER_ONLY)
>> +/* If called from generated code, iothread mutex must not be held.  */
>>   void tb_invalidate_phys_addr(AddressSpace *as, hwaddr addr)
>>   {
>>       ram_addr_t ram_addr;
>> @@ -1637,7 +1641,10 @@ void tb_check_watchpoint(CPUState *cpu)
>>
>>   #ifndef CONFIG_USER_ONLY
>>   /* in deterministic execution mode, instructions doing device I/Os
>> -   must be at the end of the TB */
>> + * must be at the end of the TB.
>> + *
>> + * Called by softmmu_template.h, with iothread mutex not held.
>> + */
>>   void cpu_io_recompile(CPUState *cpu, uintptr_t retaddr)
>>   {
>>   #if defined(TARGET_MIPS) || defined(TARGET_SH4)


--
Alex Bennée