Re: [PATCH 25/40] autonuma: follow_page check for pte_numa/pmd_numa

[Andrea Arcangeli <aarcange@redhat.com>]

On Mon, Jul 02, 2012 at 12:14:11AM -0400, Rik van Riel wrote:
> On 06/28/2012 08:56 AM, Andrea Arcangeli wrote:
> > Without this, follow_page wouldn't trigger the NUMA hinting faults.
> >
> > Signed-off-by: Andrea Arcangeli<aarcange@redhat.com>
> 
> follow_page is called from many different places, not just
> the process itself. One example would be ksm.
> 
> Do you really want to trigger NUMA hinting faults when the
> mm != current->mm, or is that magically prevented somewhere?

The NUMA hinting page fault will update "current->task_autonuma"
according to the page_nid of the page triggering the numa hinting page
fault in follow_page. It doesn't matter if the page belongs to the
different mm, all we care is the page_nid that was accessed by the
"current" task through a numa hinting fault.

When I started thinking the benefit it could provide, I thought it
wouldn't be worth it because task_autonuma statistics are only used to
balance threads belonging to the same process, and mm_autonuma is used
to balance tasks belonging to different processes. And mm_autonuma
will never be able to take into account things like this.

So I converted the !current->mm check to a current->mm != mm check
here to save a bit of cpu and skip it in the autonuma branch.

void numa_hinting_fault(struct mm_struct *mm, struct page *page, int numpages)
{
	/*
	 * "current->mm" could be different from "mm" if
	 * get_user_pages() triggered the fault on some other process
	 * "mm". It wouldn't be a problem to account this NUMA hinting
	 * page fault on the current->task_autonuma statistics even if
	 * it was triggered on a page mapped on a different
	 * "mm". However task_autonuma isn't used to balance threads
	 * belonging to different processes so it wouldn't help and in
	 * turn it's not worth it.
	 */
	if (likely(current->mm == mm && !current->mempolicy && autonuma_enabled())) {

But I was thinking at the usual case of one ptracer task with a single
thread, however now I changed my mind and I think it can help when
there's just one process and a ton of threads spanning multiple nodes,
and one of the threads is ptracing an otherwise idle task and
accessing lot of ram through ptrace. So I think I'll roll it back to
autonuma21 status and allow the accounting of all page_nid even for
different mm again. But this is mostly a theoretical issue.

It can lead to a funny weighting where mm_autonuma shows 100% of the
weight in one node, and task_autonuma shows 95% of the weight to
another different node. But it should still work fine as we won't
allow the thread to go to that different node if a different process
run there. If a thread of the same process runs in the node where
task_autonuma shows 95% of the weight, then it's better to put the
thread there if it has higher weight than the other thread of the same
process so it'll be fine despite mm_autonuma and task_autonuma disagree.

Disagreement of task_autonuma and mm_autonuma happens all the time and
it's perfectly normal, just this will exacerbate a little more.

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