Re: Memory controller merge (was Re: -mm merge plans for 2.6.24)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Hugh Dickins wrote:
> On Tue, 2 Oct 2007, Balbir Singh wrote:
>> Andrew Morton wrote:
>>> memory-controller-add-documentation.patch
>>> ...
>>> kswapd-should-only-wait-on-io-if-there-is-io.patch
>>>
>>>   Hold.  This needs a serious going-over by page reclaim people.
>> I mostly agree with your decision. I am a little concerned however
>> that as we develop and add more features (a.k.a better statistics/
>> forced reclaim), which are very important; the code base gets larger,
>> the review takes longer :)
> 
> I agree with putting the memory controller stuff on hold from 2.6.24.
> 
> Sorry, Balbir, I've failed to get back to you, still attending to
> priorities.  Let me briefly summarize my issue with the mem controller:
> you've not yet given enough attention to swap.
>

I am open to suggestions and ways and means of making swap control
complete and more usable.

> I accept that full swap control is something you're intending to add
> incrementally later; but the current state doesn't make sense to me.
> 
> The problems are swapoff and swapin readahead.  These pull pages into
> the swap cache, which are assigned to the cgroup (or the whatever-we-
> call-the-remainder-outside-all-the-cgroups) which is running swapoff
> or faulting in its own page; yet they very clearly don't (in general)
> belong to that cgroup, but to other cgroups which will be discovered
> later.
> 

I understand what your trying to say, but with several approaches that
we tried in the past, we found caches the hardest to most accurately
account. IIRC, with readahead, we don't even know if all the pages
readahead will be used, that's why we charge everything to the cgroup
that added the page to the cache.

> I did try removing the cgroup mods to mm/swap_state.c, so swap pages
> get assigned to a cgroup only once it's really known; but that's not
> enough by itself, because cgroup RSS reclaim doesn't touch those
> pages, so the cgroup can easily OOM much too soon.  I was thinking
> that you need a "limbo" cgroup for these pages, which can be attacked
> for reclaim along with any cgroup being reclaimed, but from which
> pages are readily migrated to their real cgroup once that's known.
> 

Is migrating the charge to the real cgroup really required?

> But I had to switch over to other work before trying that out:
> perhaps the idea doesn't really fly at all.  And it might well
> be no longer needed once full mem+swap control is there.
> 
> So in the current memory controller, that unuse_pte mem charge I was
> originally worried about failing (I hadn't at that point delved in
> to see how it tries to reclaim) actually never fails (and never
> does anything): the page is already assigned to some cgroup-or-
> whatever and is never charged to vma->vm_mm at that point.
> 

Excellent!

> And small point: once that is sorted out and the page is properly
> assigned in unuse_pte, you'll be needing to pte_unmap_unlock and
> pte_offset_map_lock around the mem_cgroup_charge call there -
> you're right to call it with GFP_KERNEL, but cannot do so while
> holding the page table locked and mapped.  (But because the page
> lock is held, there shouldn't be any raciness to dropping and
> retaking the ptl.)
> 

Good catch! I'll fix that.


> Hugh


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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