linux-kernel - Re: [RFC 4/4] mm: Ignore cpuset enforcement when allocation flag has __GFP

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Date:   Wed, 30 Nov 2016 11:43:39 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, srikar@...ux.vnet.ibm.com,
        haren@...ux.vnet.ibm.com, jglisse@...hat.com,
        Li Zefan <lizefan@...wei.com>
Subject: Re: [RFC 4/4] mm: Ignore cpuset enforcement when allocation flag has
 __GFP_THISNODE

On 11/30/2016 03:17 AM, Anshuman Khandual wrote:
> Right but what is the rationale behind this ? This what is in the in-code
> documentation for this function __cpuset_node_allowed().
> 
>  *	GFP_KERNEL   - any node in enclosing hardwalled cpuset ok
>  
> If the allocation has requested GFP_KERNEL, should not it look for the
> entire system for memory ? Does cpuset still has to be enforced ?

Documentation/cgroup-v1/cpusets.txt explains it quite a bit.

>> What exactly are the kernel-internal places that need to allocate from
>> the coherent device node?  When would this be done out of the context of
>> an application *asking* for memory in the new node?
> 
> The primary user right now is a driver who wants to move around mapped
> pages of an application from system RAM to CDM nodes and back. If the
> application has requested for it though an ioctl(), during migration
> the destination pages will be allocated on the CDM *in* the task context.

Side note: uhh, so you're doing migrate_pages() through some kind of new
ioctl()?  Why?

I think you're actually pointing out a hole in how cpusets currently
works, especially about the workqueue.  I'm not quite sure if this is by
design for migrate_pages() (a task doing migrate_pages() can pages for a
task from a cpuset even though that task isn't able to allocate itself).

> The driver could also have scheduled migration chunks in the work queue
> which can execute later on. IIUC those execution and corresponding
> allocation into CDM node will be *out* of context of the task.

Yeah, the current->mems_allowed in __cpuset_node_allowed() does seem
rather wrong for something happening in another task's context.