linux-kernel - Re: [PATCH RFC v4 12/13] mm/vmscan: Export drop_slab() and drop_slab

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Message-ID: <9c21be0c-5eef-58c2-bfb9-ff787a5a2c08@redhat.com>
Date:   Tue, 25 Feb 2020 18:23:23 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        virtio-dev@...ts.oasis-open.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC v4 12/13] mm/vmscan: Export drop_slab() and
 drop_slab_node()

On 25.02.20 18:06, Michal Hocko wrote:
> On Tue 25-02-20 16:09:29, David Hildenbrand wrote:
>> On 25.02.20 15:58, Michal Hocko wrote:
>>> On Thu 12-12-19 18:11:36, David Hildenbrand wrote:
>>>> We already have a way to trigger reclaiming of all reclaimable slab objects
>>>> from user space (echo 2 > /proc/sys/vm/drop_caches). Let's allow drivers
>>>> to also trigger this when they really want to make progress and know what
>>>> they are doing.
>>>
>>> I cannot say I would be fan of this. This is a global action with user
>>> visible performance impact. I am worried that we will find out that all
>>> sorts of drivers have a very good idea that dropping slab caches is
>>> going to help their problem whatever it is. We have seen the same patter
>>> in the userspace already and that is the reason we are logging the usage
>>> to the log and count invocations in the counter.
>>
>> Yeah, I decided to hold back patch 11-13 for the v1 (which I am planning
>> to post in March after more testing). What we really want is to make
>> memory offlining an alloc_contig_range() work better with reclaimable
>> objects.
>>
>>>
>>>> virtio-mem wants to use these functions when it failed to unplug memory
>>>> for quite some time (e.g., after 30 minutes). It will then try to
>>>> free up reclaimable objects by dropping the slab caches every now and
>>>> then (e.g., every 30 minutes) as long as necessary. There will be a way to
>>>> disable this feature and info messages will be logged.
>>>>
>>>> In the future, we want to have a drop_slab_range() functionality
>>>> instead. Memory offlining code has similar demands and also other
>>>> alloc_contig_range() users (e.g., gigantic pages) could make good use of
>>>> this feature. Adding it, however, requires more work/thought.
>>>
>>> We already do have a memory_notify(MEM_GOING_OFFLINE) for that purpose
>>> and slab allocator implements a callback (slab_mem_going_offline_callback).
>>> The callback is quite dumb and it doesn't really try to free objects
>>> from the given memory range or even try to drop active objects which
>>> might turn out to be hard but this sounds like a more robust way to
>>> achieve what you want.
>>
>> Two things:
>>
>> 1. memory_notify(MEM_GOING_OFFLINE) is called after trying to isolate
>> the page range and checking if we only have movable pages. Won't help
>> much I guess.
> 
> You are right, I have missed that. Can we reorder those two calls?

AFAIK no (would have to look up the details, but there was a good reason
for the order, e.g., avoid races with other users of page isolation like
alloc_contig_range()).

Especially, "[PATCH RFC v4 06/13] mm: Allow to offline unmovable
PageOffline() pages via MEM_GOING_OFFLINE" (which is still impatiently
waiting for an ACK ;) ) also works around that ordering issue in a way
we discussed back then.

-- 
Thanks,

David / dhildenb