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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
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 <aneesh.kumar@linux.ibm.com>, Nicholas Piggin <npiggin@gmail.com>, Steven
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 <bsingharora@gmail.com>, Reza Arbab <arbab@linux.ibm.com>, Thomas Gleixner
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 Kroah-Hartman <gregkh@linuxfoundation.org>, Mike Rapoport
 <rppt@linux.ibm.com>, Michal Suchanek <msuchanek@suse.de>
Date: Thu, 06 Feb 2020 22:56:40 -0300
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References: <20200206030900.147032-1-leonardo@linux.ibm.com>
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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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