Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms

From: Andy Lutomirski
Date: Fri Jun 18 2021 - 01:05:26 EST

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In reply to: Paul E. McKenney: "Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms"
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On Thu, Jun 17, 2021, at 8:29 PM, Paul E. McKenney wrote:
> 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

Whoops, good catch! The interaction between idle_thread_exit() and finish_cpu() is utter nonsense with my patch applied. I need to figure out why it works the way it does in the first place and rework it to do the right thing.

> [ 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;
>

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Previous message: Aneesh Kumar K.V: "Re: [next] [clang] x86_64-linux-gnu-ld: mm/mremap.o: in function `move_pgt_entry': mremap.c:(.text+0x763): undefined reference to `__compiletime_assert_342'"
In reply to: Paul E. McKenney: "Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms"
Next in thread: Paul E. McKenney: "Re: [PATCH 4/8] membarrier: Make the post-switch-mm barrier explicit"
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