From: Nicholas Piggin <npiggin@xxxxxxxxx> Subject: lazy tlb: introduce lazy mm refcount helper functions Patch series "shoot lazy tlbs", v4. On a 16-socket 192-core POWER8 system, a context switching benchmark with as many software threads as CPUs (so each switch will go in and out of idle), upstream can achieve a rate of about 1 million context switches per second. After this series it goes up to 118 million. This patch (of 4): Add explicit _lazy_tlb annotated functions for lazy mm refcounting. This makes lazy mm references more obvious, and allows explicit refcounting to be removed if it is not used. If a kernel thread's current lazy tlb mm happens to be the one it wants to use, then kthread_use_mm() cleverly transfers the mm refcount from the lazy tlb mm reference to the returned reference. If the lazy tlb mm reference is no longer identical to a normal reference, this trick does not work, so that is changed to be explicit about the two references. [npiggin@xxxxxxxxx: fix a refcounting bug in kthread_use_mm] Link: https://lkml.kernel.org/r/1623125298.bx63h3mopj.astroid@xxxxxxxxx Link: https://lkml.kernel.org/r/20210605014216.446867-1-npiggin@xxxxxxxxx Link: https://lkml.kernel.org/r/20210605014216.446867-2-npiggin@xxxxxxxxx Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> Cc: Paul Mackerras <paulus@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Anton Blanchard <anton@xxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- arch/arm/mach-rpc/ecard.c | 2 +- arch/powerpc/kernel/smp.c | 2 +- arch/powerpc/mm/book3s64/radix_tlb.c | 4 ++-- fs/exec.c | 4 ++-- include/linux/sched/mm.h | 11 +++++++++++ kernel/cpu.c | 2 +- kernel/exit.c | 2 +- kernel/kthread.c | 21 +++++++++++++-------- kernel/sched/core.c | 15 ++++++++------- 9 files changed, 40 insertions(+), 23 deletions(-) --- a/arch/arm/mach-rpc/ecard.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/arch/arm/mach-rpc/ecard.c @@ -253,7 +253,7 @@ static int ecard_init_mm(void) current->mm = mm; current->active_mm = mm; activate_mm(active_mm, mm); - mmdrop(active_mm); + mmdrop_lazy_tlb(active_mm); ecard_init_pgtables(mm); return 0; } --- a/arch/powerpc/kernel/smp.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/arch/powerpc/kernel/smp.c @@ -1582,7 +1582,7 @@ void start_secondary(void *unused) if (IS_ENABLED(CONFIG_PPC32)) setup_kup(); - mmgrab(&init_mm); + mmgrab_lazy_tlb(&init_mm); current->active_mm = &init_mm; smp_store_cpu_info(cpu); --- a/arch/powerpc/mm/book3s64/radix_tlb.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/arch/powerpc/mm/book3s64/radix_tlb.c @@ -786,10 +786,10 @@ void exit_lazy_flush_tlb(struct mm_struc if (current->active_mm == mm) { WARN_ON_ONCE(current->mm != NULL); /* Is a kernel thread and is using mm as the lazy tlb */ - mmgrab(&init_mm); + mmgrab_lazy_tlb(&init_mm); current->active_mm = &init_mm; switch_mm_irqs_off(mm, &init_mm, current); - mmdrop(mm); + mmdrop_lazy_tlb(mm); } /* --- a/fs/exec.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/fs/exec.c @@ -1028,9 +1028,9 @@ static int exec_mmap(struct mm_struct *m setmax_mm_hiwater_rss(&tsk->signal->maxrss, old_mm); mm_update_next_owner(old_mm); mmput(old_mm); - return 0; + } else { + mmdrop_lazy_tlb(active_mm); } - mmdrop(active_mm); return 0; } --- a/include/linux/sched/mm.h~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/include/linux/sched/mm.h @@ -49,6 +49,17 @@ static inline void mmdrop(struct mm_stru __mmdrop(mm); } +/* Helpers for lazy TLB mm refcounting */ +static inline void mmgrab_lazy_tlb(struct mm_struct *mm) +{ + mmgrab(mm); +} + +static inline void mmdrop_lazy_tlb(struct mm_struct *mm) +{ + mmdrop(mm); +} + /** * mmget() - Pin the address space associated with a &struct mm_struct. * @mm: The address space to pin. --- a/kernel/cpu.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/kernel/cpu.c @@ -613,7 +613,7 @@ static int finish_cpu(unsigned int cpu) */ if (mm != &init_mm) idle->active_mm = &init_mm; - mmdrop(mm); + mmdrop_lazy_tlb(mm); return 0; } --- a/kernel/exit.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/kernel/exit.c @@ -475,7 +475,7 @@ static void exit_mm(void) __set_current_state(TASK_RUNNING); mmap_read_lock(mm); } - mmgrab(mm); + mmgrab_lazy_tlb(mm); BUG_ON(mm != current->active_mm); /* more a memory barrier than a real lock */ task_lock(current); --- a/kernel/kthread.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/kernel/kthread.c @@ -1350,14 +1350,19 @@ void kthread_use_mm(struct mm_struct *mm WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD)); WARN_ON_ONCE(tsk->mm); + /* + * It's possible that tsk->active_mm == mm here, but we must + * still mmgrab(mm) and mmdrop_lazy_tlb(active_mm), because lazy + * mm may not have its own refcount (see mmgrab/drop_lazy_tlb()). + */ + mmgrab(mm); + task_lock(tsk); /* Hold off tlb flush IPIs while switching mm's */ local_irq_disable(); active_mm = tsk->active_mm; - if (active_mm != mm) { - mmgrab(mm); + if (active_mm != mm) tsk->active_mm = mm; - } tsk->mm = mm; membarrier_update_current_mm(mm); switch_mm_irqs_off(active_mm, mm, tsk); @@ -1374,12 +1379,9 @@ void kthread_use_mm(struct mm_struct *mm * memory barrier after storing to tsk->mm, before accessing * user-space memory. A full memory barrier for membarrier * {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by - * mmdrop(), or explicitly with smp_mb(). + * mmdrop_lazy_tlb(). */ - if (active_mm != mm) - mmdrop(active_mm); - else - smp_mb(); + mmdrop_lazy_tlb(active_mm); to_kthread(tsk)->oldfs = force_uaccess_begin(); } @@ -1411,10 +1413,13 @@ void kthread_unuse_mm(struct mm_struct * local_irq_disable(); tsk->mm = NULL; membarrier_update_current_mm(NULL); + mmgrab_lazy_tlb(mm); /* active_mm is still 'mm' */ enter_lazy_tlb(mm, tsk); local_irq_enable(); task_unlock(tsk); + + mmdrop(mm); } EXPORT_SYMBOL_GPL(kthread_unuse_mm); --- a/kernel/sched/core.c~lazy-tlb-introduce-lazy-mm-refcount-helper-functions +++ a/kernel/sched/core.c @@ -4831,13 +4831,14 @@ static struct rq *finish_task_switch(str * rq->curr, before returning to userspace, so provide them here: * * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly - * provided by mmdrop(), + * provided by mmdrop_lazy_tlb(), * - a sync_core for SYNC_CORE. */ if (mm) { membarrier_mm_sync_core_before_usermode(mm); - mmdrop(mm); + mmdrop_lazy_tlb(mm); } + if (unlikely(prev_state == TASK_DEAD)) { if (prev->sched_class->task_dead) prev->sched_class->task_dead(prev); @@ -4900,9 +4901,9 @@ context_switch(struct rq *rq, struct tas /* * 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 @@ -4910,7 +4911,7 @@ context_switch(struct rq *rq, struct tas 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 @@ -4926,7 +4927,7 @@ context_switch(struct rq *rq, struct tas 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; } @@ -9442,7 +9443,7 @@ void __init sched_init(void) /* * The boot idle thread does lazy MMU switching as well: */ - mmgrab(&init_mm); + mmgrab_lazy_tlb(&init_mm); enter_lazy_tlb(&init_mm, current); /* _
