On Wed, Jun 16, 2021 at 10:32:15PM -0700, Andy Lutomirski wrote: > Here it is. Not even boot tested! It is now, it even builds a kernel.. so it must be perfect :-) > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=sched/lazymm&id=ecc3992c36cb88087df9c537e2326efb51c95e31 Since I had to turn it into a patch to post, so that I could comment on it, I've cleaned it up a little for you. I'll reply to self with some notes, but I think I like it. --- arch/x86/include/asm/mmu.h | 5 ++ include/linux/sched/mm.h | 3 + kernel/fork.c | 2 + kernel/sched/core.c | 138 ++++++++++++++++++++++++++++++++++++--------- kernel/sched/sched.h | 10 +++- 5 files changed, 130 insertions(+), 28 deletions(-) diff --git a/arch/x86/include/asm/mmu.h b/arch/x86/include/asm/mmu.h index 5d7494631ea9..ce94162168c2 100644 --- a/arch/x86/include/asm/mmu.h +++ b/arch/x86/include/asm/mmu.h @@ -66,4 +66,9 @@ typedef struct { void leave_mm(int cpu); #define leave_mm leave_mm +/* On x86, mm_cpumask(mm) contains all CPUs that might be lazily using mm */ +#define for_each_possible_lazymm_cpu(cpu, mm) \ + for_each_cpu((cpu), mm_cpumask((mm))) + + #endif /* _ASM_X86_MMU_H */ diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index e24b1fe348e3..5c7eafee6fea 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -77,6 +77,9 @@ static inline bool mmget_not_zero(struct mm_struct *mm) /* mmput gets rid of the mappings and all user-space */ extern void mmput(struct mm_struct *); + +extern void mm_unlazy_mm_count(struct mm_struct *mm); + #ifdef CONFIG_MMU /* same as above but performs the slow path from the async context. Can * be called from the atomic context as well diff --git a/kernel/fork.c b/kernel/fork.c index e595e77913eb..57415cca088c 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1104,6 +1104,8 @@ static inline void __mmput(struct mm_struct *mm) } if (mm->binfmt) module_put(mm->binfmt->module); + + mm_unlazy_mm_count(mm); mmdrop(mm); } diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8ac693d542f6..e102ec53c2f6 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -19,6 +19,7 @@ #include <asm/switch_to.h> #include <asm/tlb.h> +#include <asm/mmu.h> #include "../workqueue_internal.h" #include "../../fs/io-wq.h" @@ -4501,6 +4502,81 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev, prepare_arch_switch(next); } +static void mmdrop_lazy(struct rq *rq) +{ + struct mm_struct *old_mm; + + if (likely(!READ_ONCE(rq->drop_mm))) + return; + + /* + * Slow path. This only happens when we recently stopped using + * an mm that is exiting. + */ + old_mm = xchg(&rq->drop_mm, NULL); + if (old_mm) + mmdrop(old_mm); +} + +#ifndef for_each_possible_lazymm_cpu +#define for_each_possible_lazymm_cpu(cpu, mm) for_each_online_cpu((cpu)) +#endif + +/* + * This converts all lazy_mm references to mm to mm_count refcounts. Our + * caller holds an mm_count reference, so we don't need to worry about mm + * being freed out from under us. + */ +void mm_unlazy_mm_count(struct mm_struct *mm) +{ + unsigned int drop_count = num_possible_cpus(); + int cpu; + + /* + * mm_users is zero, so no cpu will set its rq->lazy_mm to mm. + */ + WARN_ON_ONCE(atomic_read(&mm->mm_users) != 0); + + /* Grab enough references for the rest of this function. */ + atomic_add(drop_count, &mm->mm_count); + + for_each_possible_lazymm_cpu(cpu, mm) { + struct rq *rq = cpu_rq(cpu); + struct mm_struct *old_mm; + + if (smp_load_acquire(&rq->lazy_mm) != mm) + continue; + + drop_count--; /* grab a reference; cpu will drop it later. */ + + old_mm = xchg(&rq->drop_mm, mm); + + /* + * We know that old_mm != mm: when we did the xchg(), we were + * the only cpu to be putting mm into any drop_mm variable. + */ + WARN_ON_ONCE(old_mm == mm); + if (unlikely(old_mm)) { + /* + * We just stole an mm reference from the target CPU. + * + * drop_mm was set to old by another call to + * mm_unlazy_mm_count(). After that call xchg'd old + * into drop_mm, the target CPU did: + * + * smp_store_release(&rq->lazy_mm, mm); + * + * which synchronized with our smp_load_acquire() + * above, so we know that the target CPU is done with + * old. Drop old on its behalf. + */ + mmdrop(old_mm); + } + } + + atomic_sub(drop_count, &mm->mm_count); +} + /** * finish_task_switch - clean up after a task-switch * @prev: the thread we just switched away from. @@ -4524,7 +4600,6 @@ static struct rq *finish_task_switch(struct task_struct *prev) __releases(rq->lock) { struct rq *rq = this_rq(); - struct mm_struct *mm = rq->prev_mm; long prev_state; /* @@ -4543,8 +4618,6 @@ static struct rq *finish_task_switch(struct task_struct *prev) current->comm, current->pid, preempt_count())) preempt_count_set(FORK_PREEMPT_COUNT); - rq->prev_mm = NULL; - /* * A task struct has one reference for the use as "current". * If a task dies, then it sets TASK_DEAD in tsk->state and calls @@ -4574,22 +4647,16 @@ static struct rq *finish_task_switch(struct task_struct *prev) kmap_local_sched_in(); fire_sched_in_preempt_notifiers(current); + /* - * When switching through a kernel thread, the loop in - * membarrier_{private,global}_expedited() may have observed that - * kernel thread and not issued an IPI. It is therefore possible to - * schedule between user->kernel->user threads without passing though - * switch_mm(). Membarrier requires a barrier after storing to - * rq->curr, before returning to userspace, so provide them here: - * - * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly - * provided by mmdrop(), - * - a sync_core for SYNC_CORE. + * Do this unconditionally. There's a race in which a remote CPU + * sees rq->lazy_mm != NULL and gives us an extra mm ref while we + * are executing this code and we don't notice. Instead of letting + * that ref sit around until the next time we unlazy, do it on every + * context switch. */ - if (mm) { - membarrier_mm_sync_core_before_usermode(mm); - mmdrop(mm); - } + mmdrop_lazy(rq); + if (unlikely(prev_state == TASK_DEAD)) { if (prev->sched_class->task_dead) prev->sched_class->task_dead(prev); @@ -4652,25 +4719,32 @@ context_switch(struct rq *rq, struct task_struct *prev, /* * kernel -> kernel lazy + transfer active - * user -> kernel lazy + mmgrab() active + * user -> kernel lazy + lazy_mm grab active * - * kernel -> user switch + mmdrop() active + * kernel -> user switch + lazy_mm release active * user -> user switch */ if (!next->mm) { // to kernel enter_lazy_tlb(prev->active_mm, next); next->active_mm = prev->active_mm; - if (prev->mm) // from user - mmgrab(prev->active_mm); - else + if (prev->mm) { // from user + SCHED_WARN_ON(rq->lazy_mm); + + /* + * Acqure a lazy_mm reference to the active + * (lazy) mm. No explicit barrier needed: we still + * hold an explicit (mm_users) reference. __mmput() + * can't be called until we call mmput() to drop + * our reference, and __mmput() is a release barrier. + */ + WRITE_ONCE(rq->lazy_mm, next->active_mm); + } else { prev->active_mm = NULL; + } } else { // to user membarrier_switch_mm(rq, prev->active_mm, next->mm); /* - * sys_membarrier() requires an smp_mb() between setting - * rq->curr / membarrier_switch_mm() and returning to userspace. - * * The below provides this either through switch_mm(), or in * case 'prev->active_mm == next->mm' through * finish_task_switch()'s mmdrop(). @@ -4678,9 +4752,19 @@ 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(). */ - rq->prev_mm = prev->active_mm; + /* + * Even though nothing should reference ->active_mm + * for a non-current task, don't leave a stale pointer + * to an mm that might be freed. + */ prev->active_mm = NULL; + + /* + * Drop our lazy_mm reference to the old lazy mm. + * After this, any CPU may free it if it is + * unreferenced. + */ + smp_store_release(&rq->lazy_mm, NULL); } } diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 8f0194cee0ba..703d95a4abd0 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -966,7 +966,15 @@ struct rq { struct task_struct *idle; struct task_struct *stop; unsigned long next_balance; - struct mm_struct *prev_mm; + + /* + * Fast refcounting scheme for lazy mm. lazy_mm is a hazard pointer: + * setting it to point to a lazily used mm keeps that mm from being + * freed. drop_mm points to am mm that needs an mmdrop() call + * after the CPU owning the rq is done with it. + */ + struct mm_struct *lazy_mm; + struct mm_struct *drop_mm; unsigned int clock_update_flags; u64 clock;
