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Subject[PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> > And if CONFIG_DEBUG_PAGEALLOC is set, we don't care about
> > performance *at*all*. We will have worse performance problems than
> > doing some RCU read-locking inside the loop.
> >
> > And if CONFIG_DEBUG_PAGEALLOC isn't set, we don't really care
> > about locking, since at worst we just access stale memory for one
> > iteration.
>
> But if we are running on a hypervisor, mightn't our VCPU be
> preempted just before accessing ->on_cpu, the task exit and its
> structures be freed and unmapped? Or is the task structure in
> memory that is never unmapped? (If the latter, clearly not a
> problem.)

kmalloc()able kernel memory is never unmapped in that fashion [*].
Even hotplug memory is based on limiting what gets allocated in that
area and never putting critical kernel data structures there.

Personally I'd be more comfortable with having a special primitive for
this that is DEBUG_PAGEALLOC aware (Linus's first suggestion), so that
we don't use different RCU primitives in the rare case someone tests
CONFIG_DEBUG_PAGEALLOC=y ...

We even have such a primitive: __copy_from_user_inatomic(). It
compiles to a single instruction for integer types on x86. I've
attached a patch that implements it for the regular mutexes (xadd can
be done too), and it all compiles to a rather sweet, compact routine:

0000000000000030 <mutex_spin_on_owner.isra.4>:
30: 48 3b 37 cmp (%rdi),%rsi
33: 48 8d 4e 28 lea 0x28(%rsi),%rcx
37: 75 4e jne 87 <mutex_spin_on_owner.isra.4+0x57>
39: 55 push %rbp
3a: 45 31 c0 xor %r8d,%r8d
3d: 65 4c 8b 0c 25 00 00 mov %gs:0x0,%r9
44: 00 00
46: 48 89 e5 mov %rsp,%rbp
49: 48 83 ec 10 sub $0x10,%rsp
4d: eb 08 jmp 57 <mutex_spin_on_owner.isra.4+0x27>
4f: 90 nop
50: f3 90 pause
52: 48 3b 37 cmp (%rdi),%rsi
55: 75 29 jne 80 <mutex_spin_on_owner.isra.4+0x50>
57: 44 89 c0 mov %r8d,%eax
5a: 90 nop
5b: 90 nop
5c: 90 nop
5d: 8b 11 mov (%rcx),%edx
5f: 90 nop
60: 90 nop
61: 90 nop
62: 85 d2 test %edx,%edx
64: 89 55 fc mov %edx,-0x4(%rbp)
67: 74 0b je 74 <mutex_spin_on_owner.isra.4+0x44>
69: 49 8b 81 10 c0 ff ff mov -0x3ff0(%r9),%rax
70: a8 08 test $0x8,%al
72: 74 dc je 50 <mutex_spin_on_owner.isra.4+0x20>
74: 31 c0 xor %eax,%eax
76: c9 leaveq
77: c3 retq
78: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
7f: 00
80: b8 01 00 00 00 mov $0x1,%eax
85: c9 leaveq
86: c3 retq
87: b8 01 00 00 00 mov $0x1,%eax
8c: c3 retq
8d: 0f 1f 00 nopl (%rax)

No RCU overhead, and this is the access to owner->on_cpu:

69: 49 8b 81 10 c0 ff ff mov -0x3ff0(%r9),%rax

Totally untested and all that, I only built the mutex.o.

What do you think? Am I missing anything?

Thanks,

Ingo

[*] with the exception of CONFIG_DEBUG_PAGEALLOC and other debug
mechanisms like CONFIG_KMEMCHECK (which is on the way out) that
are based on provoking page faults and fixing up page tables to
catch unexpected memory accesses.

=================================>
From ef3e5e763747d47a43a32f846ee94706089222cf Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@kernel.org>
Date: Fri, 10 Apr 2015 10:49:11 +0200
Subject: [PATCH] mutex: Speed up mutex_spin_on_owner() by not taking the RCU lock

Linus suggested to get rid of the held RCU read-lock in
mutex_spin_on_owner(). The only real complication is that the 'owner'
task might be freed from under us and we might dereference into
possibly freed kernel memory.

As long as the kernel pointer itself is valid this approach is fine in
this case (see the analysis below) - with the exception of
CONFIG_DEBUG_PAGEALLOC=y and similarly instrumented kernels which
might fault on referencing freed kernel memory.

Use the non-faulting copy-from-user primitive to get the owner->on_cpu
value that we use in NMI handlers and which works even on
CONFIG_DEBUG_PAGEALLOC=y instrumented kernels. This compiles to a
single instruction on most platforms.

This approach might briefly copy in junk data from an already freed
(previous) owner task, which might trigger two scenarios:

1) The junk data causes us to loop once more. This is not
a problem as we'll check the owner on the next loop and
break out of the loop.

2) If the junk value causes us to break out of the loop
that's fine too: it's what we'd have done anyway on
the next iteration, as the lock owner changed.

The inatomic context copy primitives are compiler barriers
too - this matters to make sure the above owner check is
emitted to before the copy attempt.

We also ignore failed copies, as the next iteration will clean
up after us. This saves an extra branch in the common case.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Not-Yet-Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
kernel/locking/mutex.c | 40 +++++++++++++++++++++++++++-------------
1 file changed, 27 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 4cccea6b8934..fcc7db45d62e 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -224,28 +224,42 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
static noinline
bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
{
- bool ret = true;
+ int on_cpu;
+ int ret;

- rcu_read_lock();
while (lock->owner == owner) {
/*
- * Ensure we emit the owner->on_cpu, dereference _after_
- * checking lock->owner still matches owner. If that fails,
- * owner might point to freed memory. If it still matches,
- * the rcu_read_lock() ensures the memory stays valid.
+ * Use the non-faulting copy-user primitive to get the owner->on_cpu
+ * value that works even on CONFIG_DEBUG_PAGEALLOC=y instrumented
+ * kernels. This compiles to a single instruction on most platforms.
+ *
+ * This might briefly copy in junk data from an already freed
+ * (previous) owner task, which might trigger two scenarios:
+ *
+ * 1) The junk data causes us to loop once more. This is not
+ * a problem as we'll check the owner on the next loop and
+ * break out of the loop.
+ *
+ * 2) If the junk value causes us to break out of the loop
+ * that's fine too: it's what we'd have done anyway on
+ * the next iteration, as the lock owner changed.
+ *
+ * NOTE: the inatomic context copy primitives are compiler barriers
+ * too - this matters to make sure the above owner check is
+ * emitted to before the copy attempt.
+ *
+ * NOTE2: We ignore failed copies, as the next iteration will clean
+ * up after us. This saves an extra branch in the common case.
*/
- barrier();
+ ret = __copy_from_user_inatomic(&on_cpu, &owner->on_cpu, sizeof(on_cpu));

- if (!owner->on_cpu || need_resched()) {
- ret = false;
- break;
- }
+ if (!on_cpu || need_resched())
+ return false;

cpu_relax_lowlatency();
}
- rcu_read_unlock();

- return ret;
+ return true;
}

/*

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