Re: [PATCH 2/2] KVM: Convert read-only users of vm_list to RCU

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-02-10 13:57, Avi Kivity wrote:
> On 02/10/2011 02:56 PM, Avi Kivity wrote:
>>> What's the benefit? The downside is a bit more complexity as you need an
>>> additional callback handler.
>>
>>
>> synchronize_rcu() can be very slow (its a systemwide operation), and 
>> mmu_shrink() can be called often on a loaded system.
>>
> 
> In fact this just shows that vm_list is not a good candidate for rcu; 
> rcu is useful where most operations are reads, but if we discount stats, 
> most operations on vm_list are going to be writes.

Accept for mmu_shrink, which is write but not delete, thus works without
that slow synchronize_rcu. And I don't see the need for call_rcu in the
vm deletion path.

Jan

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