On 13/08/14 16:01, Daniel Vetter wrote:
On Wed, Aug 13, 2014 at 02:35:52PM +0200, Thomas Hellstrom wrote:
On 08/13/2014 12:42 PM, Daniel Vetter wrote:
On Wed, Aug 13, 2014 at 11:06:25AM +0200, Thomas Hellstrom wrote:
On 08/13/2014 05:52 AM, Jérôme Glisse wrote:
From: Jérôme Glisse <jglisse@xxxxxxxxxx>
When experiencing memory pressure we want to minimize pool size so that
memory we just shrinked is not added back again just as the next thing.
This will divide by 2 the maximum pool size for each device each time
the pool have to shrink. The limit is bumped again is next allocation
happen after one second since the last shrink. The one second delay is
obviously an arbitrary choice.
Jérôme,
I don't like this patch. It adds extra complexity and its usefulness is
highly questionable.
There are a number of caches in the system, and if all of them added
some sort of voluntary shrink heuristics like this, we'd end up with
impossible-to-debug unpredictable performance issues.
We should let the memory subsystem decide when to reclaim pages from
caches and what caches to reclaim them from.
Yeah, artificially limiting your cache from growing when your shrinker
gets called will just break the equal-memory pressure the core mm uses to
rebalance between all caches when workload changes. In i915 we let
everything grow without artificial bounds and only rely upon the shrinker
callbacks to ensure we don't consume more than our fair share of available
memory overall.
-Daniel
Now when you bring i915 memory usage up, Daniel,
I can't refrain from bringing up the old user-space unreclaimable kernel
memory issue, for which gem open is a good example ;) Each time
user-space opens a gem handle, some un-reclaimable kernel memory is
allocated, for which there is no accounting, so theoretically I think a
user can bring a system to unusability this way.
Typically there are various limits on unreclaimable objects like this,
like open file descriptors, and IIRC the kernel even has an internal
limit on the number of struct files you initialize, based on the
available system memory, so dma-buf / prime should already have some
sort of protection.
Oh yeah, we have zero cgroups limits or similar stuff for gem allocations,
so there's not really a way to isolate gpu memory usage in a sane way for
specific processes. But there's also zero limits on actual gpu usage
itself (timeslices or whatever) so I guess no one asked for this yet.
My comment really was about balancing mm users under the assumption that
they're all unlimited.
-Daniel
I think the point you brought up becomes very important for compute (HSA)
processes. I still don't know how to distinguish between legitimate use of GPU
local memory and misbehaving/malicious processes.
We have a requirement that HSA processes will be allowed to allocate and pin GPU
local memory. They do it through an ioctl.
In the kernel driver, we have an accounting of those memory allocations, meaning
that I can print a list of all the objects that were allocated by a certain
process, per device.
Therefore, in theory, I can reclaim any object, but that will probably break the
userspace app. If the app is misbehaving/malicious than that's ok, I guess. But
how do I know that ? And what prevents that malicious app to re-spawn and do the
same allocation again ?
Oded
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