[RFC PATCH v2 4/5] drm, cgroup: Add total GEM buffer allocation limit

Christian K├Ânig ckoenig.leichtzumerken at gmail.com
Thu May 16 07:25:31 UTC 2019

Am 16.05.19 um 09:16 schrieb Koenig, Christian:
> Am 16.05.19 um 04:29 schrieb Kenny Ho:
>> [CAUTION: External Email]
>> On Wed, May 15, 2019 at 5:26 PM Welty, Brian <brian.welty at intel.com> wrote:
>>> On 5/9/2019 2:04 PM, Kenny Ho wrote:
>>>> There are four control file types,
>>>> stats (ro) - display current measured values for a resource
>>>> max (rw) - limits for a resource
>>>> default (ro, root cgroup only) - default values for a resource
>>>> help (ro, root cgroup only) - help string for a resource
>>>> Each file is multi-lined with one entry/line per drm device.
>>> Multi-line is correct for multiple devices, but I believe you need
>>> to use a KEY to denote device for both your set and get routines.
>>> I didn't see your set functions reading a key, or the get functions
>>> printing the key in output.
>>> cgroups-v2 conventions mention using KEY of major:minor, but I think
>>> you can use drm_minor as key?
>> Given this controller is specific to the drm kernel subsystem which
>> uses minor to identify drm device,
> Wait a second, using the DRM minor is a good idea in the first place.

Well that should have read "is not a good idea"..


> I have a test system with a Vega10 and a Vega20. Which device gets which
> minor is not stable, but rather defined by the scan order of the PCIe bus.
> Normally the scan order is always the same, but adding or removing
> devices or delaying things just a little bit during init is enough to
> change this.
> We need something like the Linux sysfs location or similar to have a
> stable implementation.
> Regards,
> Christian.
>>    I don't see a need to complicate
>> the interfaces more by having major and a key.  As you can see in the
>> examples below, the drm device minor corresponds to the line number.
>> I am not sure how strict cgroup upstream is about the convention but I
>> am hoping there are flexibility here to allow for what I have
>> implemented.  There are a couple of other things I have done that is
>> not described in the convention: 1) inclusion of read-only *.help file
>> at the root cgroup, 2) use read-only (which I can potentially make rw)
>> *.default file instead of having a default entries (since the default
>> can be different for different devices) inside the control files (this
>> way, the resetting of cgroup values for all the drm devices, can be
>> done by a simple 'cp'.)
>>>> Usage examples:
>>>> // set limit for card1 to 1GB
>>>> sed -i '2s/.*/1073741824/' /sys/fs/cgroup/<cgroup>/drm.buffer.total.max
>>>> // set limit for card0 to 512MB
>>>> sed -i '1s/.*/536870912/' /sys/fs/cgroup/<cgroup>/drm.buffer.total.max
>>>>    /** @file drm_gem.c
>>>> @@ -154,6 +156,9 @@ void drm_gem_private_object_init(struct drm_device *dev,
>>>>         obj->handle_count = 0;
>>>>         obj->size = size;
>>>>         drm_vma_node_reset(&obj->vma_node);
>>>> +
>>>> +     obj->drmcgrp = get_drmcgrp(current);
>>>> +     drmcgrp_chg_bo_alloc(obj->drmcgrp, dev, size);
>>> Why do the charging here?
>>> There is no backing store yet for the buffer, so this is really tracking something akin to allowed virtual memory for GEM objects?
>>> Is this really useful for an administrator to control?
>>> Isn't the resource we want to control actually the physical backing store?
>> That's correct.  This is just the first level of control since the
>> backing store can be backed by different type of memory.  I am in the
>> process of adding at least two more resources.  Stay tuned.  I am
>> doing the charge here to enforce the idea of "creator is deemed owner"
>> at a place where the code is shared by all (the init function.)
>>>> +     while (i <= max_minor && limits != NULL) {
>>>> +             sval =  strsep(&limits, "\n");
>>>> +             rc = kstrtoll(sval, 0, &val);
>>> Input should be "KEY VALUE", so KEY will determine device to apply this to.
>>> Also, per cgroups-v2 documentation of limits, I believe need to parse and handle the special "max" input value.
>>> parse_resources() in rdma controller is example for both of above.
>> Please see my previous reply for the rationale of my hope to not need
>> a key.  I can certainly add handling of "max" and "default".
>>>> +void drmcgrp_chg_bo_alloc(struct drmcgrp *drmcgrp, struct drm_device *dev,
>>>> +             size_t size)
>>> Shouldn't this return an error and be implemented with same semantics as the
>>> try_charge() functions of other controllers?
>>> Below will allow stats_total_allocated to overrun limits_total_allocated.
>> This is because I am charging the buffer at the init of the buffer
>> which does not fail so the "try" (drmcgrp_bo_can_allocate) is separate
>> and placed earlier and nearer other condition where gem object
>> allocation may fail.  In other words, there are multiple possibilities
>> for which gem allocation may fail (cgroup limit being one of them) and
>> satisfying cgroup limit does not mean a charge is needed.  I can
>> certainly combine the two functions to have an additional try_charge
>> semantic as well if that is really needed.
>> Regards,
>> Kenny
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