On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote: > 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. But rcutorture isn't too happy with it when applied to current mainline: ------------------------------------------------------------------------ [ 32.559192] ------------[ cut here ]------------ [ 32.559528] WARNING: CPU: 0 PID: 175 at kernel/fork.c:686 __mmdrop+0x9f/0xb0 [ 32.560197] Modules linked in: [ 32.560470] CPU: 0 PID: 175 Comm: torture_onoff Not tainted 5.13.0-rc6+ #23 [ 32.561077] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-1ubuntu1.1 04/01/2014 [ 32.561809] RIP: 0010:__mmdrop+0x9f/0xb0 [ 32.562179] Code: fb 20 75 e6 48 8b 45 68 48 85 c0 0f 85 1e 48 ad 00 48 8b 3d 93 e0 c3 01 5b 48 89 ee 5d 41 5c e9 97 45 18 00 0f 0b 0f 0b eb 87 <0f> 0b eb 95 48 89 ef e8 a5 f1 17 00 eb a9 0f 1f 00 48 81 ef c0 03 [ 32.563822] RSP: 0018:ffff944c40623d68 EFLAGS: 00010246 [ 32.564331] RAX: ffff8e84c2339c00 RBX: ffff8e84df5572e0 RCX: 00000000fffffffa [ 32.564978] RDX: 0000000000000000 RSI: 0000000000000033 RDI: ffff8e84c29a0000 [ 32.565648] RBP: ffff8e84c29a0000 R08: ffff8e84c11c774a R09: 0000000000000001 [ 32.566256] R10: ffff8e85411c773f R11: ffff8e84c11c774a R12: 0000000000000057 [ 32.566909] R13: 0000000000000000 R14: ffffffffb0e487f8 R15: 000000000000000d [ 32.567584] FS: 0000000000000000(0000) GS:ffff8e84df200000(0000) knlGS:0000000000000000 [ 32.568321] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 32.568860] CR2: 0000000000000000 CR3: 00000000029ec000 CR4: 00000000000006f0 [ 32.569559] Call Trace: [ 32.569804] ? takedown_cpu+0xd0/0xd0 [ 32.570123] finish_cpu+0x2e/0x40 [ 32.570449] cpuhp_invoke_callback+0xf6/0x3f0 [ 32.570755] cpuhp_invoke_callback_range+0x3b/0x80 [ 32.571137] _cpu_down+0xdf/0x2a0 [ 32.571467] cpu_down+0x2a/0x50 [ 32.571771] device_offline+0x80/0xb0 [ 32.572101] remove_cpu+0x1a/0x30 [ 32.572393] torture_offline+0x80/0x140 [ 32.572730] torture_onoff+0x147/0x260 [ 32.573068] ? torture_kthread_stopping+0xa0/0xa0 [ 32.573488] kthread+0xf9/0x130 [ 32.573777] ? kthread_park+0x80/0x80 [ 32.574119] ret_from_fork+0x22/0x30 [ 32.574418] ---[ end trace b77effd8aab7f902 ]--- [ 32.574819] BUG: Bad rss-counter state mm:00000000bccc5a55 type:MM_ANONPAGES val:1 [ 32.575450] BUG: non-zero pgtables_bytes on freeing mm: 24576 ------------------------------------------------------------------------ Are we absolutely sure that the mmdrop()s are balanced in all cases? Thanx, Paul > --- > 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;
