linux-kernel - Re: [PATCH v2 -tip 0/6] locking: Introduce range reader/writer lock

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Message-ID: <20170420171326.GD20746@linux-80c1.suse>
Date:   Thu, 20 Apr 2017 10:13:26 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, akpm@...ux-foundation.org, jack@...e.cz,
        kirill.shutemov@...ux.intel.com, ldufour@...ux.vnet.ibm.com,
        mhocko@...e.com, mgorman@...hsingularity.net,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 -tip 0/6] locking: Introduce range reader/writer lock

On Wed, 19 Apr 2017, Peter Zijlstra wrote:

>  - explain why the loss of lock stealing makes sense. IIRC walken added
>    that specifically to address mmap_sem performance issues.

That's right, and the same applies to the writer spinning stuff; which
can makes a huge difference - more so than plain stealing. But as I've
mentioned, range locks can improve parallelism, which is/should be much
more welcomed than optimizations to the primitive. So yeah, we loose 
in comparing a full range to rwsem (not to mention the xadd stuff).
I have thought of some heuristics for avoiding sleeping under certain
constraints, which could mitigate the spinning step we loose, but 
I fear it will never be exactly as fast as rwsems -- just consider
we always take the tree->lock.

Thanks,
Davidlohr