From: Laurent Dufour <ldufour@linux.ibm.com>
To: Michel Lespinasse <walken@google.com>
Cc: Jan Kara <jack@suse.cz>,
sergey.senozhatsky.work@gmail.com,
Peter Zijlstra <peterz@infradead.org>,
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haren@linux.vnet.ibm.com
Subject: Re: [PATCH v12 00/31] Speculative page faults
Date: Wed, 24 Apr 2019 20:01:20 +0200 [thread overview]
Message-ID: <aadc7b03-9121-6800-341b-6f2849088a09@linux.ibm.com> (raw)
In-Reply-To: <CANN689F1h9XoHPzr_FQY2WfN5bb2TTd6M3HLqoJ-DQuHkNbA7g@mail.gmail.com>
Le 22/04/2019 à 23:29, Michel Lespinasse a écrit :
> Hi Laurent,
>
> Thanks a lot for copying me on this patchset. It took me a few days to
> go through it - I had not been following the previous iterations of
> this series so I had to catch up. I will be sending comments for
> individual commits, but before tat I would like to discuss the series
> as a whole.
Hi Michel,
Thanks for reviewing this series.
> I think these changes are a big step in the right direction. My main
> reservation about them is that they are additive - adding some complexity
> for speculative page faults - and I wonder if it'd be possible, over the
> long term, to replace the existing complexity we have in mmap_sem retry
> mechanisms instead of adding to it. This is not something that should
> block your progress, but I think it would be good, as we introduce spf,
> to evaluate whether we could eventually get all the way to removing the
> mmap_sem retry mechanism, or if we will actually have to keep both.
Until we get rid of the mmap_sem which seems to be a very long story, I
can't see how we could get rid of the retry mechanism.
> The proposed spf mechanism only handles anon vmas. Is there a
> fundamental reason why it couldn't handle mapped files too ?
> My understanding is that the mechanism of verifying the vma after
> taking back the ptl at the end of the fault would work there too ?
> The file has to stay referenced during the fault, but holding the vma's
> refcount could be made to cover that ? the vm_file refcount would have
> to be released in __free_vma() instead of remove_vma; I'm not quite sure
> if that has more implications than I realize ?
The only concern is the flow of operation done in the vm_ops->fault()
processing. Most of the file system relie on the generic filemap_fault()
which should be safe to use. But we need a clever way to identify fault
processing which are compatible with the SPF handler. This could be done
using a tag/flag in the vm_ops structure or in the vma's flags.
This would be the next step.
> The proposed spf mechanism only works at the pte level after the page
> tables have already been created. The non-spf page fault path takes the
> mm->page_table_lock to protect against concurrent page table allocation
> by multiple page faults; I think unmapping/freeing page tables could
> be done under mm->page_table_lock too so that spf could implement
> allocating new page tables by verifying the vma after taking the
> mm->page_table_lock ?
I've to admit that I didn't dig further here.
Do you have a patch? ;)
>
> The proposed spf mechanism depends on ARCH_HAS_PTE_SPECIAL.
> I am not sure what is the issue there - is this due to the vma->vm_start
> and vma->vm_pgoff reads in *__vm_normal_page() ?
Yes that's the reason, no way to guarantee the value of these fields in
the SPF path.
>
> My last potential concern is about performance. The numbers you have
> look great, but I worry about potential regressions in PF performance
> for threaded processes that don't currently encounter contention
> (i.e. there may be just one thread actually doing all the work while
> the others are blocked). I think one good proxy for measuring that
> would be to measure a single threaded workload - kernbench would be
> fine - without the special-case optimization in patch 22 where
> handle_speculative_fault() immediately aborts in the single-threaded case.
I'll have to give it a try.
> Reviewed-by: Michel Lespinasse <walken@google.com>
> This is for the series as a whole; I expect to do another review pass on
> individual commits in the series when we have agreement on the toplevel
> stuff (I noticed a few things like out-of-date commit messages but that's
> really minor stuff).
Thanks a lot for reviewing this long series.
>
> I want to add a note about mmap_sem. In the past there has been
> discussions about replacing it with an interval lock, but these never
> went anywhere because, mostly, of the fact that such mechanisms were
> too expensive to use in the page fault path. I think adding the spf
> mechanism would invite us to revisit this issue - interval locks may
> be a great way to avoid blocking between unrelated mmap_sem writers
> (for example, do not delay stack creation for new threads while a
> large mmap or munmap may be going on), and probably also to handle
> mmap_sem readers that can't easily use the spf mechanism (for example,
> gup callers which make use of the returned vmas). But again that is a
> separate topic to explore which doesn't have to get resolved before
> spf goes in.
>
next prev parent reply other threads:[~2019-04-24 18:50 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-16 13:44 [PATCH v12 00/31] Speculative page faults Laurent Dufour
2019-04-16 13:44 ` [PATCH v12 01/31] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT Laurent Dufour
2019-04-18 21:47 ` Jerome Glisse
2019-04-23 15:21 ` Laurent Dufour
2019-04-16 13:44 ` [PATCH v12 02/31] x86/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Laurent Dufour
2019-04-18 21:48 ` Jerome Glisse
2019-04-16 13:44 ` [PATCH v12 03/31] powerpc/mm: set ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Laurent Dufour
2019-04-18 21:49 ` Jerome Glisse
2019-04-16 13:44 ` [PATCH v12 04/31] arm64/mm: define ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT Laurent Dufour
2019-04-16 14:27 ` Mark Rutland
2019-04-16 14:31 ` Laurent Dufour
2019-04-16 14:41 ` Mark Rutland
2019-04-18 21:51 ` Jerome Glisse
2019-04-23 15:36 ` Laurent Dufour
2019-04-23 16:19 ` Mark Rutland
2019-04-24 10:34 ` Laurent Dufour
2019-04-16 13:44 ` [PATCH v12 05/31] mm: prepare for FAULT_FLAG_SPECULATIVE Laurent Dufour
2019-04-18 22:04 ` Jerome Glisse
2019-04-23 15:45 ` Laurent Dufour
2019-04-16 13:44 ` [PATCH v12 06/31] mm: introduce pte_spinlock " Laurent Dufour
2019-04-18 22:05 ` Jerome Glisse
2019-04-16 13:44 ` [PATCH v12 07/31] mm: make pte_unmap_same compatible with SPF Laurent Dufour
2019-04-18 22:10 ` Jerome Glisse
2019-04-23 15:43 ` Matthew Wilcox
2019-04-23 15:47 ` Laurent Dufour
2019-04-16 13:44 ` [PATCH v12 08/31] mm: introduce INIT_VMA() Laurent Dufour
2019-04-18 22:22 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 09/31] mm: VMA sequence count Laurent Dufour
2019-04-18 22:48 ` Jerome Glisse
2019-04-19 15:45 ` Laurent Dufour
2019-04-22 15:51 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 10/31] mm: protect VMA modifications using " Laurent Dufour
2019-04-22 19:43 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 11/31] mm: protect mremap() against SPF hanlder Laurent Dufour
2019-04-22 19:51 ` Jerome Glisse
2019-04-23 15:51 ` Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 12/31] mm: protect SPF handler against anon_vma changes Laurent Dufour
2019-04-22 19:53 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 13/31] mm: cache some VMA fields in the vm_fault structure Laurent Dufour
2019-04-22 20:06 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 14/31] mm/migrate: Pass vm_fault pointer to migrate_misplaced_page() Laurent Dufour
2019-04-22 20:09 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 15/31] mm: introduce __lru_cache_add_active_or_unevictable Laurent Dufour
2019-04-22 20:11 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 16/31] mm: introduce __vm_normal_page() Laurent Dufour
2019-04-22 20:15 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 17/31] mm: introduce __page_add_new_anon_rmap() Laurent Dufour
2019-04-22 20:18 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 18/31] mm: protect against PTE changes done by dup_mmap() Laurent Dufour
2019-04-22 20:32 ` Jerome Glisse
2019-04-24 10:33 ` Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 19/31] mm: protect the RB tree with a sequence lock Laurent Dufour
2019-04-22 20:33 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 20/31] mm: introduce vma reference counter Laurent Dufour
2019-04-22 20:36 ` Jerome Glisse
2019-04-24 14:26 ` Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 21/31] mm: Introduce find_vma_rcu() Laurent Dufour
2019-04-22 20:57 ` Jerome Glisse
2019-04-24 14:39 ` Laurent Dufour
2019-04-23 9:27 ` Peter Zijlstra
2019-04-23 18:13 ` Davidlohr Bueso
2019-04-24 7:57 ` Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 22/31] mm: provide speculative fault infrastructure Laurent Dufour
2019-04-22 21:26 ` Jerome Glisse
2019-04-24 14:56 ` Laurent Dufour
2019-04-24 15:13 ` Jerome Glisse
2019-04-16 13:45 ` [PATCH v12 23/31] mm: don't do swap readahead during speculative page fault Laurent Dufour
2019-04-22 21:36 ` Jerome Glisse
2019-04-24 14:57 ` Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 24/31] mm: adding speculative page fault failure trace events Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 25/31] perf: add a speculative page fault sw event Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 26/31] perf tools: add support for the SPF perf event Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 27/31] mm: add speculative page fault vmstats Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 28/31] x86/mm: add speculative pagefault handling Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 29/31] powerpc/mm: add speculative page fault Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 30/31] arm64/mm: " Laurent Dufour
2019-04-16 13:45 ` [PATCH v12 31/31] mm: Add a speculative page fault switch in sysctl Laurent Dufour
2019-04-22 21:29 ` [PATCH v12 00/31] Speculative page faults Michel Lespinasse
2019-04-23 9:38 ` Peter Zijlstra
2019-04-24 7:33 ` Laurent Dufour
2019-04-27 1:53 ` Michel Lespinasse
2019-04-23 10:47 ` Michal Hocko
2019-04-23 12:41 ` Matthew Wilcox
2019-04-23 12:48 ` Peter Zijlstra
2019-04-23 13:42 ` Michal Hocko
2019-04-24 18:01 ` Laurent Dufour [this message]
2019-04-27 6:00 ` Michel Lespinasse
2019-04-23 11:35 ` Anshuman Khandual
2019-06-06 6:51 ` Haiyan Song
2019-06-14 8:37 ` Laurent Dufour
2019-06-14 8:44 ` Laurent Dufour
2019-06-20 8:19 ` Haiyan Song
2020-07-06 9:25 ` Chinwen Chang
2020-07-06 12:27 ` Laurent Dufour
2020-07-07 5:31 ` Chinwen Chang
2020-12-14 2:03 ` Joel Fernandes
2020-12-14 9:36 ` Laurent Dufour
2020-12-14 18:10 ` Joel Fernandes
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