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From: Leonardo Bras <leonardo@linux.ibm.com>
To: Christopher Lameter <cl@linux.com>
Cc: "Song Liu" <songliubraving@fb.com>,
	"Michal Hocko" <mhocko@suse.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	"Mahesh Salgaonkar" <mahesh@linux.vnet.ibm.com>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	"Keith Busch" <keith.busch@intel.com>,
	linux-mm@kvack.org, "Paul Mackerras" <paulus@samba.org>,
	"Christian Brauner" <christian.brauner@ubuntu.com>,
	"Ira Weiny" <ira.weiny@intel.com>,
	"Ingo Molnar" <mingo@kernel.org>,
	"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>,
	"Jann Horn" <jannh@google.com>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Jason Gunthorpe" <jgg@ziepe.ca>,
	"Allison Randal" <allison@lohutok.net>,
	"Jesper Dangaard Brouer" <brouer@redhat.com>,
	"Andrey Ryabinin" <aryabinin@virtuozzo.com>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"Andrea Arcangeli" <aarcange@redhat.com>,
	"Ralph Campbell" <rcampbell@nvidia.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	"Bartlomiej Zolnierkiewicz" <b.zolnierkie@samsung.com>,
	"John Hubbard" <jhubbard@nvidia.com>,
	linuxppc-dev@lists.ozlabs.org,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Jérôme Glisse" <jglisse@redhat.com>,
	"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
	kvm-ppc@vger.kernel.org,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Reza Arbab" <arbab@linux.ibm.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Souptick Joarder" <jrdr.linux@gmail.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Logan Gunthorpe" <logang@deltatee.com>,
	"Roman Gushchin" <guro@fb.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	"Al Viro" <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v5 02/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks
Date: Tue, 08 Oct 2019 15:02:27 -0300	[thread overview]
Message-ID: <c1676fb14313a8378ec6bb72d1a7b24f48b898b5.camel@linux.ibm.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1910081741460.2923@www.lameter.com>

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On Tue, 2019-10-08 at 17:43 +0000, Christopher Lameter wrote:
> On Tue, 8 Oct 2019, Leonardo Bras wrote:
> 
> > > You are creating contention on a single exclusive cacheline. Doesnt this
> > > defeat the whole purpose of the lockless page table walk? Use mmap_sem or
> > > so should cause the same performance regression?
> > 
> > Sorry, I did not understand that question.
> > I mean, this is just a refcount and never causes a lock.
> 
> Locks also use atomic operations like a refcount increment. Both require
> the cacheline to be in exclusive state. So the impact is very similar.

Thanks for explaining. :)

So you say that the performance impact of using my approach is the same
as using locks? (supposing that lock never waits)

So, there are 'lockless pagetable walks' only for the sake of better
performance? 

I thought they existed to enable doing pagetable walks in states where
locking was not safe.

Is that right?

Thanks!
Leonardo Brás,


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  reply	other threads:[~2019-10-08 18:06 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 [this message]
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 ` [PATCH v5 00/11] Introduces new count-based method for tracking lockless pagetable walks Peter Zijlstra
2019-10-03 20:36   ` 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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