Re: [rfc] lru_add_drain_all() vs isolation

[Minchan Kim]

On Wed, Sep 9, 2009 at 1:27 PM, KOSAKI Motohiro
<kosaki.motohiro@xxxxxxxxxxxxxx> wrote:
>> The usefulness of a scheme like this requires:
>>
>> 1. There are cpus that continually execute user space code
>>    without system interaction.
>>
>> 2. There are repeated VM activities that require page isolation /
>>    migration.
>>
>> The first page isolation activity will then clear the lru caches of the
>> processes doing number crunching in user space (and therefore the first
>> isolation will still interrupt). The second and following isolation will
>> then no longer interrupt the processes.
>>
>> 2. is rare. So the question is if the additional code in the LRU handling
>> can be justified. If lru handling is not time sensitive then yes.
>
> Christoph, I'd like to discuss a bit related (and almost unrelated) thing.
> I think page migration don't need lru_add_drain_all() as synchronous, because
> page migration have 10 times retry.
>
> Then asynchronous lru_add_drain_all() cause
>
>  - if system isn't under heavy pressure, retry succussfull.
>  - if system is under heavy pressure or RT-thread work busy busy loop, retry failure.
>
> I don't think this is problematic bahavior. Also, mlock can use asynchrounous lru drain.

I think, more exactly, we don't have to drain lru pages for mlocking.
Mlocked pages will go into unevictable lru due to
try_to_unmap when shrink of lru happens.
How about removing draining in case of mlock?

>
> What do you think?
>
>
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-- 
Kind regards,
Minchan Kim
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