Re: [PATCH 04/33] autonuma: define _PAGE_NUMA

[Mel Gorman]

On Thu, Oct 11, 2012 at 06:43:00PM +0200, Andrea Arcangeli wrote:
> On Thu, Oct 11, 2012 at 12:01:37PM +0100, Mel Gorman wrote:
> > On Thu, Oct 04, 2012 at 01:50:46AM +0200, Andrea Arcangeli wrote:
> > > The objective of _PAGE_NUMA is to be able to trigger NUMA hinting page
> > > faults to identify the per NUMA node working set of the thread at
> > > runtime.
> > > 
> > > Arming the NUMA hinting page fault mechanism works similarly to
> > > setting up a mprotect(PROT_NONE) virtual range: the present bit is
> > > cleared at the same time that _PAGE_NUMA is set, so when the fault
> > > triggers we can identify it as a NUMA hinting page fault.
> > > 
> > 
> > That implies that there is an atomic update requirement or at least
> > an ordering requirement -- present bit must be cleared before setting
> > NUMA bit. No doubt it'll be clear later in the series how this is
> > accomplished. What you propose seems ok but it all depends how it's
> > implemented so I'm leaving my ack off this particular patch for now.
> 
> Correct. The switch is done atomically (clear _PAGE_PRESENT at the
> same time _PAGE_NUMA is set). The tlb flush is deferred (it's batched
> to avoid firing an IPI for every pte/pmd_numa we establish).
> 

Good. I think you might still be flushing more than you need to but
commented on the patch itself.

> It's still similar to setting a range PROT_NONE (except the way
> _PAGE_PROTNONE and _PAGE_NUMA works is the opposite, and they are
> mutually exclusive, so they can easily share the same pte/pmd
> bitflag). Except PROT_NONE must be synchronous, _PAGE_NUMA is set lazily.
> 
> The NUMA hinting page fault also won't require any TLB flush ever.
> 

It sortof can. The fault itself is still a heavy operation that can do
things like this

numa_hinting_fault
 -> numa_hinting_fault_memory_follow_cpu
    -> autonuma_migrate_page
      -> sync_isolate_migratepages
	 (lru lock for single page)
      -> migrate_pages

and buried down there where it unmaps the page and makes a migration PTE
is a TLB flush due to calling ptep_clear_flush_notify(). That's a bad case
obviously and the expectation is that as the threads converage to a node that
it's not a problem. While it's converging though it will be a heavy cost.

Tracking how often a numa_hinting_fault results in a migration should be
enough to keep an eye on it.

> So the whole process (establish/teardown) has an incredibly low TLB
> flushing cost.
> 
> The only fixed cost is in knuma_scand and the enter/exit kernel for
> every not-shared page every 10 sec (or whatever you set the duration
> of a knuma_scand pass in sysfs).
> 

10 seconds should be sufficiently low. It itself might need to adapt in
the future but at least 10 seconds now by default will not stomp too heavily.

> Furthermore, if the pmd_scan mode is activated, I guarantee there's at
> max 1 NUMA hinting page fault every 2m virtual region (even if some
> accuracy is lost). You can try to set scan_pmd = 0 in sysfs and also
> to disable THP (echo never >enabled) to measure the exact cost per 4k
> page. It's hardly measurable here. With THP the fault is also 1 every
> 2m virtual region but no accuracy is lost in that case (or more
> precisely, there's no way to get more accuracy than that as we deal
> with a pmd).
> 

-- 
Mel Gorman
SUSE Labs
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