linux-kernel - Re: [PATCH v6 10/11] powerpc/mm: Adds counting method to track lockless pagetable walks

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Date:   Thu, 06 Feb 2020 22:56:40 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Steven Price <steven.price@....com>,
        Robin Murphy <robin.murphy@....com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Suchanek <msuchanek@...e.de>
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org
Subject: Re: [PATCH v6 10/11] powerpc/mm: Adds counting method to track
 lockless pagetable walks

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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