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From: Leonardo Bras <leonardo@linux.ibm.com>
To: Christophe Leroy <christophe.leroy@c-s.fr>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Nicholas Piggin <npiggin@gmail.com>,
	Steven Price <steven.price@arm.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>,
	Balbir Singh <bsingharora@gmail.com>,
	Reza Arbab <arbab@linux.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Allison Randal <allison@lohutok.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Michal Suchanek <msuchanek@suse.de>
Cc: linux-arch@vger.kernel.org, linux-mm@kvack.org,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	kvm-ppc@vger.kernel.org
Subject: Re: [PATCH v6 10/11] powerpc/mm: Adds counting method to track lockless pagetable walks
Date: Thu, 06 Feb 2020 22:56:40 -0300	[thread overview]
Message-ID: <ee81338952c474f2bb4c19055105e906ee89ed8f.camel@linux.ibm.com> (raw)
In-Reply-To: <d9bf6878-43d5-b45a-7abb-cdcb712a0d7a@c-s.fr>

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Hello Christophe, thanks for the feedback!

On Thu, 2020-02-06 at 07:23 +0100, Christophe Leroy wrote:
> > Due to not locking nor using atomic variables, the impact on the
> > lockless pagetable walk is intended to be minimum.
> 
> atomic variables have a lot less impact than preempt_enable/disable.
> 
> preemt_disable forces a re-scheduling, it really has impact. Why not use 
> atomic variables instead ?

In fact, v5 of this patch used atomic variables. But it seems to cause
contention on a single exclusive cacheline, which had no better
performance than locking.
(discussion here: http://patchwork.ozlabs.org/patch/1171012/)

When I try to understand the effect of preempt_disable(), all I can
see is a barrier() and possibly a preempt_count_inc(), which updates a
member of current thread struct if CONFIG_PREEMPT_COUNT is enabled.

If CONFIG_PREEMPTION is also enabled, preempt_enable() can run a
__preempt_schedule() on unlikely(__preempt_count_dec_and_test()).

On most configs available, CONFIG_PREEMPTION is not set, being replaced
either by CONFIG_PREEMPT_NONE (kernel defconfigs) or
CONFIG_PREEMPT_VOLUNTARY in most supported distros. With that, most
probably CONFIG_PREEMPT_COUNT will also not be set, and
preempt_{en,dis}able() are replaced by a barrier().

Using preempt_disable approach, I intent to get better performance for
most used cases.

What do you think of it?

I am still new on this subject, and I am still trying to better
understand how it works. If you notice something I am missing, please
let me know.

Best regards,
Leonardo Bras



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  reply	other threads:[~2020-02-07  1:59 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-06  3:08 [PATCH v6 00/11] Introduces new functions for tracking lockless pagetable walks Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 01/11] asm-generic/pgtable: Adds generic functions to track lockless pgtable walks Leonardo Bras
2020-02-06  5:54   ` Christophe Leroy
2020-02-07  2:19     ` Leonardo Bras
2020-02-07  5:39   ` kbuild test robot
2020-02-06  3:08 ` [PATCH v6 02/11] mm/gup: Use functions to track lockless pgtbl walks on gup_pgd_range Leonardo Bras
2020-02-06  3:25   ` Leonardo Bras
2020-02-07 22:54     ` John Hubbard
2020-02-17 20:55       ` Leonardo Bras
2020-10-15 14:46     ` Michal Suchánek
2020-10-16  3:27       ` Aneesh Kumar K.V
2020-02-07  1:19   ` kbuild test robot
2020-02-07  8:01   ` kbuild test robot
2020-02-06  3:08 ` [PATCH v6 03/11] powerpc/mm: Adds arch-specificic functions to track lockless pgtable walks Leonardo Bras
2020-02-06  5:46   ` Christophe Leroy
2020-02-07  4:38     ` Leonardo Bras
2020-02-17 20:32       ` Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 04/11] powerpc/mce_power: Use functions to track lockless pgtbl walks Leonardo Bras
2020-02-06  5:48   ` Christophe Leroy
2020-02-07  4:00     ` Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 05/11] powerpc/perf: " Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 06/11] powerpc/mm/book3s64/hash: " Leonardo Bras
2020-02-06  6:06   ` Christophe Leroy
2020-02-07  3:49     ` Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 07/11] powerpc/kvm/e500: " Leonardo Bras
2020-02-06  6:18   ` Christophe Leroy
2020-02-07  3:10     ` Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 08/11] powerpc/kvm/book3s_hv: " Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 09/11] powerpc/kvm/book3s_64: " Leonardo Bras
2020-02-06  3:08 ` [PATCH v6 10/11] powerpc/mm: Adds counting method to track lockless pagetable walks Leonardo Bras
2020-02-06  6:23   ` Christophe Leroy
2020-02-07  1:56     ` Leonardo Bras [this message]
2020-02-06  3:09 ` [PATCH v6 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing Leonardo Bras

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