| Message ID | 1594019787.286knc5cet.astroid@bobo.none (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [RFC] avoid refcounting the lazy tlb mm struct | expand |
Hi Nick, > On big systems, the mm refcount can become highly contented when doing > a lot of context switching with threaded applications (particularly > switching between the idle thread and an application thread). > > Not doing lazy tlb at all slows switching down quite a bit, so I > wonder if we can avoid the refcount for the lazy tlb, but have > __mmdrop() IPI all CPUs that might be using this mm lazily. > > This patch has only had light testing so far, but seems to work okay. I tested this patch on a large POWER8 system with 1536 hardware threads. I can create a worst case situation for mm refcounting by using the threaded context switch test in will-it-scale set to half the number of available CPUs (768). With that workload the patch improves the context switch rate by 118x! Tested-by: Anton Blanchard <anton@ozlabs.org> Thanks, Anton > diff --git a/arch/Kconfig b/arch/Kconfig > index 8cc35dc556c7..69ea7172db3d 100644 > --- a/arch/Kconfig > +++ b/arch/Kconfig > @@ -411,6 +411,16 @@ config MMU_GATHER_NO_GATHER > bool > depends on MMU_GATHER_TABLE_FREE > > +config MMU_LAZY_TLB_SHOOTDOWN > + bool > + help > + Instead of refcounting the "lazy tlb" mm struct, which can > cause > + contention with multi-threaded apps on large > multiprocessor systems, > + this option causes __mmdrop to IPI all CPUs in the > mm_cpumask and > + switch to init_mm if they were using the to-be-freed mm as > the lazy > + tlb. Architectures which do not track all possible lazy > tlb CPUs in > + mm_cpumask can not use this (without modification). > + > config ARCH_HAVE_NMI_SAFE_CMPXCHG > bool > > diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig > index 920c4e3ca4ef..24ac85c868db 100644 > --- a/arch/powerpc/Kconfig > +++ b/arch/powerpc/Kconfig > @@ -225,6 +225,7 @@ config PPC > select HAVE_PERF_USER_STACK_DUMP > select MMU_GATHER_RCU_TABLE_FREE > select MMU_GATHER_PAGE_SIZE > + select MMU_LAZY_TLB_SHOOTDOWN > select HAVE_REGS_AND_STACK_ACCESS_API > select HAVE_RELIABLE_STACKTRACE if > PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN select HAVE_SYSCALL_TRACEPOINTS > diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c > b/arch/powerpc/mm/book3s64/radix_tlb.c index > b5cc9b23cf02..52730629b3eb 100644 --- > a/arch/powerpc/mm/book3s64/radix_tlb.c +++ > b/arch/powerpc/mm/book3s64/radix_tlb.c @@ -652,10 +652,10 @@ static > void do_exit_flush_lazy_tlb(void *arg) > * Must be a kernel thread because sender is > single-threaded. */ > BUG_ON(current->mm); > - mmgrab(&init_mm); > + mmgrab_lazy_tlb(&init_mm); > switch_mm(mm, &init_mm, current); > current->active_mm = &init_mm; > - mmdrop(mm); > + mmdrop_lazy_tlb(mm); > } > _tlbiel_pid(pid, RIC_FLUSH_ALL); > } > diff --git a/fs/exec.c b/fs/exec.c > index e6e8a9a70327..6c96c8feba1f 100644 > --- a/fs/exec.c > +++ b/fs/exec.c > @@ -1119,7 +1119,7 @@ static int exec_mmap(struct mm_struct *mm) > mmput(old_mm); > return 0; > } > - mmdrop(active_mm); > + mmdrop_lazy_tlb(active_mm); > return 0; > } > > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 480a4d1b7dd8..ef28059086a1 100644 > --- a/include/linux/sched/mm.h > +++ b/include/linux/sched/mm.h > @@ -51,6 +51,25 @@ static inline void mmdrop(struct mm_struct *mm) > > void mmdrop(struct mm_struct *mm); > > +static inline void mmgrab_lazy_tlb(struct mm_struct *mm) > +{ > + if (!IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) > + mmgrab(mm); > +} > + > +static inline void mmdrop_lazy_tlb(struct mm_struct *mm) > +{ > + if (!IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) > + mmdrop(mm); > +} > + > +static inline void mmdrop_lazy_tlb_smp_mb(struct mm_struct *mm) > +{ > + mmdrop_lazy_tlb(mm); > + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) > + smp_mb(); > +} > + > /* > * This has to be called after a get_task_mm()/mmget_not_zero() > * followed by taking the mmap_lock for writing before modifying the > diff --git a/kernel/fork.c b/kernel/fork.c > index 142b23645d82..e3f1039cee9f 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -685,6 +685,34 @@ static void check_mm(struct mm_struct *mm) > #define allocate_mm() (kmem_cache_alloc(mm_cachep, > GFP_KERNEL)) #define free_mm(mm) (kmem_cache_free(mm_cachep, > (mm))) > +static void do_shoot_lazy_tlb(void *arg) > +{ > + struct mm_struct *mm = arg; > + > + if (current->active_mm == mm) { > + BUG_ON(current->mm); > + switch_mm(mm, &init_mm, current); > + current->active_mm = &init_mm; > + } > +} > + > +static void do_check_lazy_tlb(void *arg) > +{ > + struct mm_struct *mm = arg; > + > + BUG_ON(current->active_mm == mm); > +} > + > +void shoot_lazy_tlbs(struct mm_struct *mm) > +{ > + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) { > + smp_call_function_many(mm_cpumask(mm), > do_shoot_lazy_tlb, (void *)mm, 1); > + do_shoot_lazy_tlb(mm); > + } > + smp_call_function(do_check_lazy_tlb, (void *)mm, 1); > + do_check_lazy_tlb(mm); > +} > + > /* > * Called when the last reference to the mm > * is dropped: either by a lazy thread or by > @@ -692,6 +720,7 @@ static void check_mm(struct mm_struct *mm) > */ > void __mmdrop(struct mm_struct *mm) > { > + shoot_lazy_tlbs(mm); > BUG_ON(mm == &init_mm); > WARN_ON_ONCE(mm == current->mm); > WARN_ON_ONCE(mm == current->active_mm); > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index ca5db40392d4..4d615e0be9e0 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -3308,7 +3308,7 @@ static struct rq *finish_task_switch(struct > task_struct *prev) */ > if (mm) { > membarrier_mm_sync_core_before_usermode(mm); > - mmdrop(mm); > + mmdrop_lazy_tlb_smp_mb(mm); > } > if (unlikely(prev_state == TASK_DEAD)) { > if (prev->sched_class->task_dead) > @@ -3413,9 +3413,9 @@ context_switch(struct rq *rq, struct > task_struct *prev, > /* > * kernel -> kernel lazy + transfer active > - * user -> kernel lazy + mmgrab() active > + * user -> kernel lazy + mmgrab_lazy_tlb() active > * > - * kernel -> user switch + mmdrop() active > + * kernel -> user switch + mmdrop_lazy_tlb() active > * user -> user switch > */ > if (!next->mm) { // to kernel > @@ -3423,7 +3423,7 @@ context_switch(struct rq *rq, struct > task_struct *prev, > next->active_mm = prev->active_mm; > if (prev->mm) // from user > - mmgrab(prev->active_mm); > + mmgrab_lazy_tlb(prev->active_mm); > else > prev->active_mm = NULL; > } else { // to user > @@ -3439,7 +3439,7 @@ context_switch(struct rq *rq, struct > task_struct *prev, switch_mm_irqs_off(prev->active_mm, next->mm, > next); > if (!prev->mm) { // from > kernel > - /* will mmdrop() in finish_task_switch(). */ > + /* will mmdrop_lazy_tlb() in > finish_task_switch(). */ rq->prev_mm = prev->active_mm; > prev->active_mm = NULL; > } >
diff --git a/arch/Kconfig b/arch/Kconfig index 8cc35dc556c7..69ea7172db3d 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -411,6 +411,16 @@ config MMU_GATHER_NO_GATHER bool depends on MMU_GATHER_TABLE_FREE +config MMU_LAZY_TLB_SHOOTDOWN + bool + help + Instead of refcounting the "lazy tlb" mm struct, which can cause + contention with multi-threaded apps on large multiprocessor systems, + this option causes __mmdrop to IPI all CPUs in the mm_cpumask and + switch to init_mm if they were using the to-be-freed mm as the lazy + tlb. Architectures which do not track all possible lazy tlb CPUs in + mm_cpumask can not use this (without modification). + config ARCH_HAVE_NMI_SAFE_CMPXCHG bool diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig index 920c4e3ca4ef..24ac85c868db 100644 --- a/arch/powerpc/Kconfig +++ b/arch/powerpc/Kconfig @@ -225,6 +225,7 @@ config PPC select HAVE_PERF_USER_STACK_DUMP select MMU_GATHER_RCU_TABLE_FREE select MMU_GATHER_PAGE_SIZE + select MMU_LAZY_TLB_SHOOTDOWN select HAVE_REGS_AND_STACK_ACCESS_API select HAVE_RELIABLE_STACKTRACE if PPC_BOOK3S_64 && CPU_LITTLE_ENDIAN select HAVE_SYSCALL_TRACEPOINTS diff --git a/arch/powerpc/mm/book3s64/radix_tlb.c b/arch/powerpc/mm/book3s64/radix_tlb.c index b5cc9b23cf02..52730629b3eb 100644 --- a/arch/powerpc/mm/book3s64/radix_tlb.c +++ b/arch/powerpc/mm/book3s64/radix_tlb.c @@ -652,10 +652,10 @@ static void do_exit_flush_lazy_tlb(void *arg) * Must be a kernel thread because sender is single-threaded. */ BUG_ON(current->mm); - mmgrab(&init_mm); + mmgrab_lazy_tlb(&init_mm); switch_mm(mm, &init_mm, current); current->active_mm = &init_mm; - mmdrop(mm); + mmdrop_lazy_tlb(mm); } _tlbiel_pid(pid, RIC_FLUSH_ALL); } diff --git a/fs/exec.c b/fs/exec.c index e6e8a9a70327..6c96c8feba1f 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1119,7 +1119,7 @@ static int exec_mmap(struct mm_struct *mm) mmput(old_mm); return 0; } - mmdrop(active_mm); + mmdrop_lazy_tlb(active_mm); return 0; } diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 480a4d1b7dd8..ef28059086a1 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -51,6 +51,25 @@ static inline void mmdrop(struct mm_struct *mm) void mmdrop(struct mm_struct *mm); +static inline void mmgrab_lazy_tlb(struct mm_struct *mm) +{ + if (!IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) + mmgrab(mm); +} + +static inline void mmdrop_lazy_tlb(struct mm_struct *mm) +{ + if (!IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) + mmdrop(mm); +} + +static inline void mmdrop_lazy_tlb_smp_mb(struct mm_struct *mm) +{ + mmdrop_lazy_tlb(mm); + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) + smp_mb(); +} + /* * This has to be called after a get_task_mm()/mmget_not_zero() * followed by taking the mmap_lock for writing before modifying the diff --git a/kernel/fork.c b/kernel/fork.c index 142b23645d82..e3f1039cee9f 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -685,6 +685,34 @@ static void check_mm(struct mm_struct *mm) #define allocate_mm() (kmem_cache_alloc(mm_cachep, GFP_KERNEL)) #define free_mm(mm) (kmem_cache_free(mm_cachep, (mm))) +static void do_shoot_lazy_tlb(void *arg) +{ + struct mm_struct *mm = arg; + + if (current->active_mm == mm) { + BUG_ON(current->mm); + switch_mm(mm, &init_mm, current); + current->active_mm = &init_mm; + } +} + +static void do_check_lazy_tlb(void *arg) +{ + struct mm_struct *mm = arg; + + BUG_ON(current->active_mm == mm); +} + +void shoot_lazy_tlbs(struct mm_struct *mm) +{ + if (IS_ENABLED(CONFIG_MMU_LAZY_TLB_SHOOTDOWN)) { + smp_call_function_many(mm_cpumask(mm), do_shoot_lazy_tlb, (void *)mm, 1); + do_shoot_lazy_tlb(mm); + } + smp_call_function(do_check_lazy_tlb, (void *)mm, 1); + do_check_lazy_tlb(mm); +} + /* * Called when the last reference to the mm * is dropped: either by a lazy thread or by @@ -692,6 +720,7 @@ static void check_mm(struct mm_struct *mm) */ void __mmdrop(struct mm_struct *mm) { + shoot_lazy_tlbs(mm); BUG_ON(mm == &init_mm); WARN_ON_ONCE(mm == current->mm); WARN_ON_ONCE(mm == current->active_mm); diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ca5db40392d4..4d615e0be9e0 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3308,7 +3308,7 @@ static struct rq *finish_task_switch(struct task_struct *prev) */ if (mm) { membarrier_mm_sync_core_before_usermode(mm); - mmdrop(mm); + mmdrop_lazy_tlb_smp_mb(mm); } if (unlikely(prev_state == TASK_DEAD)) { if (prev->sched_class->task_dead) @@ -3413,9 +3413,9 @@ context_switch(struct rq *rq, struct task_struct *prev, /* * kernel -> kernel lazy + transfer active - * user -> kernel lazy + mmgrab() active + * user -> kernel lazy + mmgrab_lazy_tlb() active * - * kernel -> user switch + mmdrop() active + * kernel -> user switch + mmdrop_lazy_tlb() active * user -> user switch */ if (!next->mm) { // to kernel @@ -3423,7 +3423,7 @@ context_switch(struct rq *rq, struct task_struct *prev, next->active_mm = prev->active_mm; if (prev->mm) // from user - mmgrab(prev->active_mm); + mmgrab_lazy_tlb(prev->active_mm); else prev->active_mm = NULL; } else { // to user @@ -3439,7 +3439,7 @@ context_switch(struct rq *rq, struct task_struct *prev, switch_mm_irqs_off(prev->active_mm, next->mm, next); if (!prev->mm) { // from kernel - /* will mmdrop() in finish_task_switch(). */ + /* will mmdrop_lazy_tlb() in finish_task_switch(). */ rq->prev_mm = prev->active_mm; prev->active_mm = NULL; }