Re: [RFC 08/16] vfio/type1: Cache locked_vm to ease mmap_lock contention

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On Wed, Jan 05, 2022 at 08:53:39PM -0400, Jason Gunthorpe wrote:
> On Wed, Jan 05, 2022 at 07:46:48PM -0500, Daniel Jordan wrote:
> > padata threads hold mmap_lock as reader for the majority of their
> > runtime in order to call pin_user_pages_remote(), but they also
> > periodically take mmap_lock as writer for short periods to adjust
> > mm->locked_vm, hurting parallelism.
> > 
> > Alleviate the write-side contention with a per-thread cache of locked_vm
> > which allows taking mmap_lock as writer far less frequently.
> > 
> > Failure to refill the cache due to insufficient locked_vm will not cause
> > the entire pinning operation to error out.  This avoids spurious failure
> > in case some pinned pages aren't accounted to locked_vm.
> > 
> > Cache size is limited to provide some protection in the unlikely event
> > of a concurrent locked_vm accounting operation in the same address space
> > needlessly failing in case the cache takes more locked_vm than it needs.
> 
> Why not just do the pinned page accounting once at the start? Why does
> it have to be done incrementally?

Yeah, good question.  I tried doing it that way recently and it did
improve performance a bit, but I thought it wasn't enough of a gain to
justify how it overaccounted by the size of the entire pin.

If the concurrent accounting I worried about above isn't really a
concern, though, I can reconsider this.



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