From: Peter Zijlstra <peterz@infradead.org>
To: Leonardo Bras <leonardo@linux.ibm.com>
Cc: "Song Liu" <songliubraving@fb.com>,
"Michal Hocko" <mhocko@suse.com>,
"Dmitry V. Levin" <ldv@altlinux.org>,
"Keith Busch" <keith.busch@intel.com>,
linux-mm@kvack.org, "Paul Mackerras" <paulus@samba.org>,
"Christoph Lameter" <cl@linux.com>,
"Ira Weiny" <ira.weiny@intel.com>,
"Dan Williams" <dan.j.williams@intel.com>,
"Elena Reshetova" <elena.reshetova@intel.com>,
linux-arch@vger.kernel.org,
"Santosh Sivaraj" <santosh@fossix.org>,
"Davidlohr Bueso" <dave@stgolabs.net>,
"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
"Mike Rapoport" <rppt@linux.ibm.com>,
"Jason Gunthorpe" <jgg@ziepe.ca>,
"Vlastimil Babka" <vbabka@suse.cz>,
"Mahesh Salgaonkar" <mahesh@linux.vnet.ibm.com>,
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"Alexey Dobriyan" <adobriyan@gmail.com>,
"Ingo Molnar" <mingo@kernel.org>,
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"Jérôme Glisse" <jglisse@redhat.com>,
"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
kvm-ppc@vger.kernel.org, "Thomas Gleixner" <tglx@linutronix.de>,
"Reza Arbab" <arbab@linux.ibm.com>,
"Allison Randal" <allison@lohutok.net>,
"Christian Brauner" <christian.brauner@ubuntu.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org,
"Logan Gunthorpe" <logang@deltatee.com>,
"Souptick Joarder" <jrdr.linux@gmail.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
linuxppc-dev@lists.ozlabs.org, "Roman Gushchin" <guro@fb.com>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
"Al Viro" <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks
Date: Thu, 3 Oct 2019 09:29:52 +0200 [thread overview]
Message-ID: <20191003072952.GN4536@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191003013325.2614-1-leonardo@linux.ibm.com>
On Wed, Oct 02, 2019 at 10:33:14PM -0300, Leonardo Bras wrote:
> If a process (qemu) with a lot of CPUs (128) try to munmap() a large
> chunk of memory (496GB) mapped with THP, it takes an average of 275
> seconds, which can cause a lot of problems to the load (in qemu case,
> the guest will lock for this time).
>
> Trying to find the source of this bug, I found out most of this time is
> spent on serialize_against_pte_lookup(). This function will take a lot
> of time in smp_call_function_many() if there is more than a couple CPUs
> running the user process. Since it has to happen to all THP mapped, it
> will take a very long time for large amounts of memory.
>
> By the docs, serialize_against_pte_lookup() is needed in order to avoid
> pmd_t to pte_t casting inside find_current_mm_pte(), or any lockless
> pagetable walk, to happen concurrently with THP splitting/collapsing.
>
> It does so by calling a do_nothing() on each CPU in mm->cpu_bitmap[],
> after interrupts are re-enabled.
> Since, interrupts are (usually) disabled during lockless pagetable
> walk, and serialize_against_pte_lookup will only return after
> interrupts are enabled, it is protected.
This is something entirely specific to Power, you shouldn't be touching
generic code at all.
Also, I'm not sure I understand things properly.
So serialize_against_pte_lookup() wants to wait for all currently
out-standing __find_linux_pte() instances (which are very similar to
gup_fast).
It seems to want to do this before flushing the THP TLB for some reason;
why? Should not THP observe the normal page table freeing rules which
includes a RCU-like grace period like this already.
Why is THP special here? This doesn't seem adequately explained.
Also, specifically to munmap(), this seems entirely superfluous,
munmap() uses the normal page-table freeing code and should be entirely
fine without additional waiting.
Furthermore, Power never accurately tracks mm_cpumask(), so using that
makes the whole thing more expensive than it needs to be. Also, I
suppose that is buggered vs file backed THP.
next prev parent reply other threads:[~2019-10-03 7:33 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 1:33 [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks Leonardo Bras
2019-10-03 7:11 ` Peter Zijlstra
2019-10-03 11:51 ` Peter Zijlstra
2019-10-03 20:40 ` John Hubbard
2019-10-04 11:24 ` Peter Zijlstra
2019-10-03 21:24 ` Leonardo Bras
2019-10-04 11:28 ` Peter Zijlstra
2019-10-09 18:09 ` Leonardo Bras
2019-10-05 8:35 ` Aneesh Kumar K.V
2019-10-08 14:47 ` Kirill A. Shutemov
2019-10-03 1:33 ` [PATCH v5 02/11] powerpc/mm: Adds counting method " Leonardo Bras
2019-10-08 15:11 ` Christopher Lameter
2019-10-08 17:13 ` Leonardo Bras
2019-10-08 17:43 ` Christopher Lameter
2019-10-08 18:02 ` Leonardo Bras
2019-10-08 18:27 ` Christopher Lameter
2019-10-03 1:33 ` [PATCH v5 03/11] mm/gup: Applies counting method to monitor gup_pgd_range Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 04/11] powerpc/mce_power: Applies counting method to monitor lockless pgtbl walks Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 05/11] powerpc/perf: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 07/11] powerpc/kvm/e500: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 09/11] powerpc/kvm/book3s_64: " Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 10/11] mm/Kconfig: Adds config option to track lockless pagetable walks Leonardo Bras
2019-10-03 2:08 ` Qian Cai
2019-10-03 19:04 ` Leonardo Bras
2019-10-03 19:08 ` Leonardo Bras
2019-10-03 7:44 ` Peter Zijlstra
2019-10-03 20:40 ` Leonardo Bras
2019-10-03 1:33 ` [PATCH v5 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-10-03 7:29 ` Peter Zijlstra [this message]
2019-10-03 20:36 ` [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks Leonardo Bras
2019-10-03 20:49 ` John Hubbard
2019-10-03 21:38 ` Leonardo Bras
2019-10-04 11:42 ` Peter Zijlstra
2019-10-04 12:57 ` Peter Zijlstra
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