On 02/23/2017 12:22 PM, Anshuman Khandual wrote: > On 02/22/2017 03:20 PM, Michal Hocko wrote: >> On Tue 21-02-17 19:09:18, Anshuman Khandual wrote: >>> On 02/21/2017 04:41 PM, Michal Hocko wrote: >>>> On Fri 17-02-17 17:11:57, Anshuman Khandual wrote: >>>> [...] >>>>> * User space using mbind() to get CDM memory is an additional benefit >>>>> we get by making the CDM plug in as a node and be part of the buddy >>>>> allocator. But the over all idea from the user space point of view >>>>> is that the application can allocate any generic buffer and try to >>>>> use the buffer either from the CPU side or from the device without >>>>> knowing about where the buffer is really mapped physically. That >>>>> gives a seamless and transparent view to the user space where CPU >>>>> compute and possible device based compute can work together. This >>>>> is not possible through a driver allocated buffer. >>>> >>>> But how are you going to define any policy around that. Who is allowed >>> >>> The user space VMA can define the policy with a mbind(MPOL_BIND) call >>> with CDM/CDMs in the nodemask. >>> >>>> to allocate and how much of this "special memory". Is it possible that >>> >>> Any user space application with mbind(MPOL_BIND) call with CDM/CDMs in >>> the nodemask can allocate from the CDM memory. "How much" gets controlled >>> by how we fault from CPU and the default behavior of the buddy allocator. >> >> In other words the policy is implemented by the kernel. Why is this a >> good thing? > > Its controlled by the kernel only during page fault paths of either CPU > or device. But the device driver will actually do the placements after > wards after taking into consideration access patterns and relative > performance. We dont want the driver to be involved during page fault > path memory allocations which should naturally go through the buddy > allocator. > >> >>>> we will eventually need some access control mechanism? If yes then mbind >>> >>> No access control mechanism is needed. If an application wants to use >>> CDM memory by specifying in the mbind() it can. Nothing prevents it >>> from using the CDM memory. >> >> What if we find out that an access control _is_ really needed? I can >> easily imagine that some devices will come up with really fast and expensive >> memory. You do not want some random user to steal it from you when you >> want to use it for your workload. > > Hmm, it makes sense but I think its not something we have to deal with > right away. Later we may have to think about some generic access control > mechanism for mbind() and then accommodate CDM with it. > >> >>>> is really not suitable interface to (ab)use. Also what should happen if >>>> the mbind mentions only CDM memory and that is depleted? >>> >>> IIUC *only CDM* cannot be requested from user space as there are no user >>> visible interface which can translate to __GFP_THISNODE. >> >> I do not understand what __GFP_THISNODE has to do with this. This is an >> internal flag. > > Right. My bad. I was just referring to the fact that there is nothing in > user space which can make buddy allocator pick NOFALLBACK list instead of > FALLBACK list. > >> >>> MPOL_BIND with >>> CDM in the nodemask will eventually pick a FALLBACK zonelist which will >>> have zones of the system including CDM ones. If the resultant CDM zones >>> run out of memory, we fail the allocation request as usual. >> >> OK, so let's say you mbind to a single node which is CDM. You seem to be >> saying that we will simply break the NUMA affinity in this special case? > > Why ? It should simply follow what happens when we pick a single NUMA node > in previous situations. > >> Currently we invoke the OOM killer if nodes which the application binds >> to are depleted and cannot be reclaimed. > > Right, the same should happen here for CDM as well. > >> >>>> Could you also explain why the transparent view is really better than >>>> using a device specific mmap (aka CDM awareness)? >>> >>> Okay with a transparent view, we can achieve a control flow of application >>> like the following. >>> >>> (1) Allocate a buffer: alloc_buffer(buf, size) >>> (2) CPU compute on buffer: cpu_compute(buf, size) >>> (3) Device compute on buffer: device_compute(buf, size) >>> (4) CPU compute on buffer: cpu_compute(buf, size) >>> (5) Release the buffer: release_buffer(buf, size) >>> >>> With assistance from a device specific driver, the actual page mapping of >>> the buffer can change between system RAM and device memory depending on >>> which side is accessing at a given point. This will be achieved through >>> driver initiated migrations. >> >> But then you do not need any NUMA affinity, right? The driver can do >> all this automagically. How does the numa policy comes into the game in >> your above example. Sorry for being dense, I might be really missing >> something important here, but I really fail to see why the NUMA is the >> proper interface here. > > You are right. Driver can migrate any mapping in the userspace to any > where on the system as long as cpuset does not prohibit it. But we still > want the driver to conform to the applicable VMA memory policy set from > the userspace. Hence a VMA policy needs to be set from the user space. > NUMA VMA memory policy also restricts the allocations inside the > applicable nodemask during page fault paths (CPU and device) as well.
Hello Michal, Does that answer your question ?
