lru_add_drain query

lru_add_drain query

[Ray Bryant]

The deferred lru_add operations (e. g. lru_cache_add_active()) defer the
actual addition of a page to the lru list until a batch of such additions
are available.  lru_cache_add_active() uses a per cpu variable 
(lru_add_active_pvecs) to hold the deferred pages.

So, to get the deferred adds to complete (so that the lru list is in a
consistent state and we can scan lru list to do some processing) one calls
lru_add_drain().  But AFAI can tell, this just drains the local cpu's
deferred add queue.  Right?

So, here's my question:  Why is it that I don't need to call lru_add_drain()
on each CPU in the system before I go scan/manipulate the lru list?  (i. e.
what about deferred adds in other CPU's lru_add_active_pvecs?)

What peice of magic am I missing here?

Thanks,

-- 
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Ray Bryant
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The box said: "Requires Windows 98 or better",
	 so I installed Linux.
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Re: lru_add_drain query

[Nick Piggin]

Ray Bryant wrote:

> The deferred lru_add operations (e. g. lru_cache_add_active()) defer the
> actual addition of a page to the lru list until a batch of such additions
> are available.  lru_cache_add_active() uses a per cpu variable 
> (lru_add_active_pvecs) to hold the deferred pages.
>
> So, to get the deferred adds to complete (so that the lru list is in a
> consistent state and we can scan lru list to do some processing) one 
> calls
> lru_add_drain().  But AFAI can tell, this just drains the local cpu's
> deferred add queue.  Right?
>
> So, here's my question:  Why is it that I don't need to call 
> lru_add_drain()
> on each CPU in the system before I go scan/manipulate the lru list?  
> (i. e.
> what about deferred adds in other CPU's lru_add_active_pvecs?)
>
> What peice of magic am I missing here?
>

I don't think you're missing anything. See lru_drain_cache for when you
really need to drain another CPU's cache (ie cpu hotplug).

I think the idea is that it doesn't _really_ matter to have a few pages
not visible to the LRU lists at any point in time.

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