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Date: Thu, 8 Aug 2019 20:54:54 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: mst@...hat.com, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH V4 7/9] vhost: do not use RCU to synchronize MMU notifier
with worker
On 2019/8/7 下午10:02, Jason Wang wrote:
>
> On 2019/8/7 下午8:07, Jason Gunthorpe wrote:
>> On Wed, Aug 07, 2019 at 03:06:15AM -0400, Jason Wang wrote:
>>> We used to use RCU to synchronize MMU notifier with worker. This leads
>>> calling synchronize_rcu() in invalidate_range_start(). But on a busy
>>> system, there would be many factors that may slow down the
>>> synchronize_rcu() which makes it unsuitable to be called in MMU
>>> notifier.
>>>
>>> So this patch switches use seqlock counter to track whether or not the
>>> map was used. The counter was increased when vq try to start or finish
>>> uses the map. This means, when it was even, we're sure there's no
>>> readers and MMU notifier is synchronized. When it was odd, it means
>>> there's a reader we need to wait it to be even again then we are
>>> synchronized. Consider the read critical section is pretty small the
>>> synchronization should be done very fast.
>>>
>>> Reported-by: Michael S. Tsirkin <mst@...hat.com>
>>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel
>>> virtual address")
>>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>>> drivers/vhost/vhost.c | 141
>>> ++++++++++++++++++++++++++----------------
>>> drivers/vhost/vhost.h | 7 ++-
>>> 2 files changed, 90 insertions(+), 58 deletions(-)
>>>
>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>>> index cfc11f9ed9c9..57bfbb60d960 100644
>>> +++ b/drivers/vhost/vhost.c
>>> @@ -324,17 +324,16 @@ static void vhost_uninit_vq_maps(struct
>>> vhost_virtqueue *vq)
>>> spin_lock(&vq->mmu_lock);
>>> for (i = 0; i < VHOST_NUM_ADDRS; i++) {
>>> - map[i] = rcu_dereference_protected(vq->maps[i],
>>> - lockdep_is_held(&vq->mmu_lock));
>>> + map[i] = vq->maps[i];
>>> if (map[i]) {
>>> vhost_set_map_dirty(vq, map[i], i);
>>> - rcu_assign_pointer(vq->maps[i], NULL);
>>> + vq->maps[i] = NULL;
>>> }
>>> }
>>> spin_unlock(&vq->mmu_lock);
>>> - /* No need for synchronize_rcu() or kfree_rcu() since we are
>>> - * serialized with memory accessors (e.g vq mutex held).
>>> + /* No need for synchronization since we are serialized with
>>> + * memory accessors (e.g vq mutex held).
>>> */
>>> for (i = 0; i < VHOST_NUM_ADDRS; i++)
>>> @@ -362,6 +361,40 @@ static bool vhost_map_range_overlap(struct
>>> vhost_uaddr *uaddr,
>>> return !(end < uaddr->uaddr || start > uaddr->uaddr - 1 +
>>> uaddr->size);
>>> }
>>> +static void inline vhost_vq_access_map_begin(struct
>>> vhost_virtqueue *vq)
>>> +{
>>> + write_seqcount_begin(&vq->seq);
>>> +}
>>> +
>>> +static void inline vhost_vq_access_map_end(struct vhost_virtqueue *vq)
>>> +{
>>> + write_seqcount_end(&vq->seq);
>>> +}
>> The write side of a seqlock only provides write barriers. Access to
>>
>> map = vq->maps[VHOST_ADDR_USED];
>>
>> Still needs a read side barrier, and then I think this will be no
>> better than a normal spinlock.
>>
>> It also doesn't seem like this algorithm even needs a seqlock, as this
>> is just a one bit flag
>
>
> Right, so then I tend to use spinlock first for correctness.
>
>
>>
>> atomic_set_bit(using map)
>> smp_mb__after_atomic()
>> .. maps [...]
>> atomic_clear_bit(using map)
>>
>>
>> map = NULL;
>> smp_mb__before_atomic();
>> while (atomic_read_bit(using map))
>> relax()
>>
>> Again, not clear this could be faster than a spinlock when the
>> barriers are correct...
>
I've done some benchmark[1] on x86, and yes it looks even slower. It
looks to me the atomic stuffs is not necessary, so in order to compare
it better with spinlock. I tweak it a little bit through
smp_load_acquire()/store_releaes() + mb() like:
static struct vhost_map *vhost_vq_access_map_begin(struct
vhost_virtqueue
*vq,
unsigned int
type)
{
++vq->counter;
/* Ensure map was read after incresing the counter.
Paired
* with smp_mb() in
vhost_vq_sync_access().
*/
smp_mb();
return
vq->maps[type];
}
static void inline vhost_vq_access_map_end(struct vhost_virtqueue
*vq)
{
/* Ensure all memory access through map was done
before
* reducing the counter. Paired with smp_load_acquire()
in
* vhost_vq_sync_access()
*/
smp_store_release(&vq->counter,
--vq->counter);
}
static void inline vhost_vq_sync_access(struct vhost_virtqueue
*vq)
{
/* Ensure new map value is visible before checking counter.
*/
smp_mb();
/* Ensure map was freed after reading counter value,
paired
* with smp_store_release() in
vhost_vq_access_map_end().
*/
while (smp_load_acquire(&vq->counter))
{
if
(need_resched())
schedule();
}
}
And the result is something like:
base | direct + atomic bitops | direct + spinlock() | direct +
counter + smp_mb() | direct + RCU |
SMAP on | 5.0Mpps | 5.0Mpps (+0%) | 5.7Mpps (+14%) |
5.9Mpps (+18%) | 6.2Mpps (+24%) |
SMAP off | 7.0Mpps | 7.0Mpps (+0%) | 7.0Mpps (+0%) |
7.5Mpps (+7%) | 8.2Mpps (+17%) |
base: normal copy_to_user()/copy_from_user() path.
direct + atomic bitops: using direct mapping but synchronize through
atomic bitops like you suggested above
direct + spinlock(): using direct mapping but synchronize through spinlocks
direct + counter + smp_mb(): using direct mapping but synchronize
through counter + smp_mb()
direct + RCU: using direct mapping and synchronize through RCU (buggy
and need to be addressed by this series)
So smp_mb() + counter is fastest way. And spinlock can still show some
improvement (+14%) in the case of SMAP, but no the case when SMAP is off.
I don't have any objection to convert to spinlock() but just want to
know if any case that the above smp_mb() + counter looks good to you?
Thanks
>
> Yes, for next release we may want to use the idea from Michael like to
> mitigate the impact of mb.
>
> https://lwn.net/Articles/775871/
>
> Thanks
>
>
>>
>> Jason
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