linux-kernel - Re: [RFC PATCH-tip 2/6] locking/rwsem: Enable optional count-based spinning on reader

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Date:	Tue, 14 Jun 2016 11:27:57 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, linux-alpha@...r.kernel.org,
	linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
	linux-arch@...r.kernel.org, xfs@....sgi.com,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH-tip 2/6] locking/rwsem: Enable optional count-based
 spinning on reader

On Tue, 14 Jun 2016, Waiman Long wrote:

>This patch provides a way for the kernel code to designate specific
>rwsems to be more aggressive in term of optimistic spinning that the
>writers will continue to spin for some additional count-based time to
>see if it can get the lock before sleeping. This aggressive spinning
>mode should only be used on rwsems where the readers are unlikely to
>go to sleep.

Yikes, exposing this sort of thing makes me _very_ uneasy, not to mention
the ad-hoc nature and its easiness to mess up. I'm not really for this, even
if it shows extraordinary performance boosts on benchmarks.

Thanks,
Davidlohr