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 <[email protected]> >>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel >>> virtual address") >>> Signed-off-by: Jason Wang <[email protected]> >>> 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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