Re: [PATCH] iommu/iova: Bettering utilizing cpu_rcaches in no-strict mode

[zhangzekun (A)]

在 2024/6/26 2:03, Robin Murphy 写道:
> On 2024-06-25 2:29 am, zhangzekun (A) wrote:
> > 在 2024/6/24 21:32, Robin Murphy 写道:
> >
> > > > This patch is firstly intent to minimize the chance of softlock 
> > > > issue in fq_flush_timeout(), which is already dicribed erarlier in 
> > > > [1], which has beed applied in a commercial kernel[2] for years.
> > > >
> > > > However, the later tests show that this single patch is not enough 
> > > > to fix the softlockup issue, since the root cause of softlockup is 
> > > > the underlying iova_rbtree_lock. In our softlockup scenarios, the 
> > > > average
> > > > time cost to get this spinlock is about 6ms.
> > >
> > > That should already be fixed, though. The only reason for 
> > > fq_flush_timeout() to interact with the rbtree at all was due to the 
> > > notion of a fixed-size depot which could become full. That no longer 
> > > exists since 911aa1245da8 ("iommu/iova: Make the rcache depot scale 
> > > better").
> > >
> > > Thanks,
> > > Robin.
> > Hi, Robin,
> >
> > The commit 911aa1245da8 ("iommu/iova: Make the rcache depot scale 
> > better") can reduce the risks of softlockup, but can not fix it 
> > entirely. We do solve a softlockup issue[1] with that patch, and that is
> > why it has aleady been backported in our branch. The softlockup issue 
> > which we met recently is a 5.10-based kernel with that patch already 
> > backported, which can be found in [2].
>
> Sorry, I was implying some context that I should have made clear - yes, 
> the softlockup can still happen in general if the flush queues are full 
> of IOVAs which are too large for the rcache mechanism at all, so are 
> always freed directly to the rbtree, but then there's no way *this* 
> patch could make any difference to that case either.
>
> Thanks,
> Robin.

Yes, this patch can't fix softlockup issue in this case. In such a case, 
it would be better to put the free iova logic in fq_flush_timeout() to a 
kthread and add a cond_resched() in it.

Thanks,
Zekun