Re: [RFC PATCH 0/6] TLB flush multiple pages with a single IPI v3

[Vlastimil Babka <vbabka@suse.cz>]

On 04/21/2015 12:41 PM, Mel Gorman wrote:
> Changelog since V2
> o Ensure TLBs are flushed before pages are freed		(mel)

I admit not reading all the patches thoroughly, but doesn't this change 
of ordering mean that you no longer need the architectural guarantee 
discussed in patch 2? What's the harm if some other CPU (because the CPU 
didn't receive an IPI yet) manages to write to a page that you have 
unmapped in the page tables *but not yet freed*?

Vlastimil

> Changelog since V1
> o Structure and variable renaming				(hughd)
> o Defer flushes even if the unmapping process is sleeping	(huged)
> o Alternative sizing of structure				(peterz)
> o Use GFP_KERNEL instead of GFP_ATOMIC, PF_MEMALLOC protects	(andi)
> o Immediately flush dirty PTEs to avoid corruption		(mel)
> o Further clarify docs on the required arch guarantees		(mel)
>
> When unmapping pages it is necessary to flush the TLB. If that page was
> accessed by another CPU then an IPI is used to flush the remote CPU. That
> is a lot of IPIs if kswapd is scanning and unmapping >100K pages per second.
>
> There already is a window between when a page is unmapped and when it is
> TLB flushed. This series simply increases the window so multiple pages can
> be flushed using a single IPI.
>
> Patch 1 simply made the rest of the series easier to write as ftrace
> 	could identify all the senders of TLB flush IPIS.
>
> Patch 2 collects a list of PFNs and sends one IPI to flush them all
>
> Patch 3 uses more memory so further defer when the IPI gets sent
>
> Patch 4 uses the same infrastructure as patch 2 to batch IPIs sent during
> 	page migration.
>
> The performance impact is documented in the changelogs but in the optimistic
> case on a 4-socket machine the full series reduces interrupts from 900K
> interrupts/second to 60K interrupts/second.
>
>   arch/x86/Kconfig                |   1 +
>   arch/x86/include/asm/tlbflush.h |   2 +
>   arch/x86/mm/tlb.c               |   1 +
>   include/linux/init_task.h       |   8 +++
>   include/linux/mm_types.h        |   1 +
>   include/linux/rmap.h            |  13 ++--
>   include/linux/sched.h           |  15 ++++
>   include/trace/events/tlb.h      |   3 +-
>   init/Kconfig                    |   8 +++
>   kernel/fork.c                   |   7 ++
>   kernel/sched/core.c             |   3 +
>   mm/internal.h                   |  16 +++++
>   mm/migrate.c                    |  27 +++++--
>   mm/rmap.c                       | 151 ++++++++++++++++++++++++++++++++++++----
>   mm/vmscan.c                     |  35 +++++++++-
>   15 files changed, 267 insertions(+), 24 deletions(-)
>