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Message-ID: <20210618032931.GD4397@paulmck-ThinkPad-P17-Gen-1>
Date: Thu, 17 Jun 2021 20:29:31 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andy Lutomirski <luto@...nel.org>,
Nicholas Piggin <npiggin@...il.com>,
Rik van Riel <riel@...riel.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Dave Hansen <dave.hansen@...el.com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-mm@...ck.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy
mms
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;
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