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From: Leonardo Bras <leonardo@linux.ibm.com>
To: John Hubbard <jhubbard@nvidia.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Linux-MM <linux-mm@kvack.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>,
	Thomas Gleixner <tglx@linutronix.de>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	YueHaibing <yuehaibing@huawei.com>,
	Keith Busch <keith.busch@intel.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	Richard Fontana <rfontana@redhat.com>,
	Paul Mackerras <paulus@samba.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Ganesh Goudar <ganeshgr@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ira Weiny <ira.weiny@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Allison Randal <allison@lohutok.net>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
Date: Mon, 23 Sep 2019 14:25:51 -0300	[thread overview]
Message-ID: <18c5c378db98f223a0663034baa9fd6ce42f1ec7.camel@linux.ibm.com> (raw)
In-Reply-To: <4ea26ffb-ad03-bdff-7893-95332b22a5fd@nvidia.com>

[-- Attachment #1: Type: text/plain, Size: 1538 bytes --]

On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
> 
[...]
> So it seems that full memory barriers (not just compiler barriers) are required.
> If the irq enable/disable somehow provides that, then your new code just goes
> along for the ride and Just Works. (You don't have any memory barriers in
> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
> barriers provided by the atomic inc/dec.)
> 
> So it's really a pre-existing question about the correctness of the gup_fast()
> irq disabling approach.

I am not experienced in other archs, and I am still pretty new to
Power, but by what I could understand, this behavior is better
explained in serialize_against_pte_lookup. 

What happens here is that, before doing a THP split/collapse, the
function does a update of the pmd and a serialize_against_pte_lookup,
in order do avoid a invalid output on a lockless pagetable walk.

Serialize basically runs a do_nothing in every cpu related to the
process, and wait for it to return. 

This running depends on interrupt being enabled, so disabling it before
gup_pgd_range() and re-enabling after the end, makes the THP
split/collapse wait for gup_pgd_range() completion in every cpu before
continuing. (here happens the lock)

(As told before, every gup_pgd_range() that occurs after it uses a
updated pmd, so no problem.)

I am sure other archs may have a similar mechanism using
local_irq_{disable,enable}.

Did it answer your questions?

Best regards,

Leonardo Bras

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WARNING: multiple messages have this Message-ID (diff)
From: Leonardo Bras <leonardo@linux.ibm.com>
To: John Hubbard <jhubbard@nvidia.com>,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	Linux-MM <linux-mm@kvack.org>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Richard Fontana <rfontana@redhat.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	YueHaibing <yuehaibing@huawei.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Keith Busch <keith.busch@intel.com>,
	Jason Gunthorpe <jgg@ziepe.ca>, Paul Mackerras <paulus@samba.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Allison Randal <allison@lohutok.net>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	Ganesh Goudar <ganeshgr@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ira Weiny <ira.weiny@intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing
Date: Mon, 23 Sep 2019 14:25:51 -0300	[thread overview]
Message-ID: <18c5c378db98f223a0663034baa9fd6ce42f1ec7.camel@linux.ibm.com> (raw)
In-Reply-To: <4ea26ffb-ad03-bdff-7893-95332b22a5fd@nvidia.com>

[-- Attachment #1: Type: text/plain, Size: 1538 bytes --]

On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
> 
[...]
> So it seems that full memory barriers (not just compiler barriers) are required.
> If the irq enable/disable somehow provides that, then your new code just goes
> along for the ride and Just Works. (You don't have any memory barriers in
> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
> barriers provided by the atomic inc/dec.)
> 
> So it's really a pre-existing question about the correctness of the gup_fast()
> irq disabling approach.

I am not experienced in other archs, and I am still pretty new to
Power, but by what I could understand, this behavior is better
explained in serialize_against_pte_lookup. 

What happens here is that, before doing a THP split/collapse, the
function does a update of the pmd and a serialize_against_pte_lookup,
in order do avoid a invalid output on a lockless pagetable walk.

Serialize basically runs a do_nothing in every cpu related to the
process, and wait for it to return. 

This running depends on interrupt being enabled, so disabling it before
gup_pgd_range() and re-enabling after the end, makes the THP
split/collapse wait for gup_pgd_range() completion in every cpu before
continuing. (here happens the lock)

(As told before, every gup_pgd_range() that occurs after it uses a
updated pmd, so no problem.)

I am sure other archs may have a similar mechanism using
local_irq_{disable,enable}.

Did it answer your questions?

Best regards,

Leonardo Bras

[-- Attachment #2: This is a digitally signed message part --]
[-- Type: application/pgp-signature, Size: 833 bytes --]

  reply	other threads:[~2019-09-23 17:27 UTC|newest]

Thread overview: 72+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-20 19:50 [PATCH v2 00/11] Introduces new count-based method for monitoring lockless pagetable wakls Leonardo Bras
2019-09-20 19:50 ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 01/11] powerpc/mm: Adds counting method to monitor lockless pgtable walks Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:42   ` John Hubbard
2019-09-23 20:42     ` John Hubbard
2019-09-23 20:50     ` Leonardo Bras
2019-09-23 20:50       ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 02/11] asm-generic/pgtable: Adds dummy functions " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:39   ` John Hubbard
2019-09-23 20:39     ` John Hubbard
2019-09-23 20:48     ` Leonardo Bras
2019-09-23 20:48       ` Leonardo Bras
2019-09-23 20:53       ` John Hubbard
2019-09-23 20:53         ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 03/11] mm/gup: Applies counting method to monitor gup_pgd_range Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:27   ` John Hubbard
2019-09-23 20:27     ` John Hubbard
2019-09-23 21:01     ` Leonardo Bras
2019-09-23 21:01       ` Leonardo Bras
2019-09-23 21:09       ` John Hubbard
2019-09-23 21:09         ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 04/11] powerpc/mce_power: Applies counting method to monitor lockless pgtbl walks Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 05/11] powerpc/perf: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 07/11] powerpc/kvm/e500: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-23 20:47   ` John Hubbard
2019-09-23 20:47     ` John Hubbard
2019-09-20 19:50 ` [PATCH v2 09/11] powerpc/kvm/book3s_64: " Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 10/11] powerpc/book3s_64: Enables counting method to monitor lockless pgtbl walk Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 19:50 ` [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-09-20 19:50   ` Leonardo Bras
2019-09-20 20:11   ` John Hubbard
2019-09-20 20:28     ` Leonardo Bras
2019-09-20 20:28       ` Leonardo Bras
2019-09-20 21:15       ` John Hubbard
2019-09-21  0:48       ` John Hubbard
2019-09-21  0:48         ` John Hubbard
2019-09-23 17:25         ` Leonardo Bras [this message]
2019-09-23 17:25           ` Leonardo Bras
2019-09-23 18:14           ` John Hubbard
2019-09-23 18:14             ` John Hubbard
2019-09-23 19:40             ` Leonardo Bras
2019-09-23 19:40               ` Leonardo Bras
2019-09-23 19:58               ` John Hubbard
2019-09-23 19:58                 ` John Hubbard
2019-09-23 20:23                 ` Leonardo Bras
2019-09-23 20:23                   ` Leonardo Bras
2019-09-23 20:26                   ` John Hubbard
2019-09-23 20:26                     ` John Hubbard
2019-09-20 19:56 ` [PATCH v2 00/11] Introduces new count-based method for monitoring lockless pagetable wakls Leonardo Bras
2019-09-20 19:56   ` Leonardo Bras
2019-09-20 20:12 ` Leonardo Bras
2019-09-20 20:12   ` Leonardo Bras
2019-09-20 21:24   ` John Hubbard
2019-09-20 21:24     ` John Hubbard
2019-09-23 20:51   ` John Hubbard
2019-09-23 20:51     ` John Hubbard
2019-09-23 20:58     ` Leonardo Bras
2019-09-23 20:58       ` Leonardo Bras
2019-09-24 21:23 [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras
2019-09-24 21:23 ` Leonardo Bras

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