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Message-ID: <ee81338952c474f2bb4c19055105e906ee89ed8f.camel@linux.ibm.com>
Subject: Re: [PATCH v6 10/11] powerpc/mm: Adds counting method to track
 lockless pagetable walks
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
Date:   Thu, 06 Feb 2020 22:56:40 -0300
In-Reply-To: <d9bf6878-43d5-b45a-7abb-cdcb712a0d7a@c-s.fr>
References: <20200206030900.147032-1-leonardo@linux.ibm.com>
         <20200206030900.147032-11-leonardo@linux.ibm.com>
         <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.
>=20
> atomic variables have a lot less impact than preempt_enable/disable.
>=20
> preemt_disable forces a re-scheduling, it really has impact. Why not use=
=20
> 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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