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

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Message-ID: <20170420183646.GF20746@linux-80c1.suse>
Date:   Thu, 20 Apr 2017 11:36:46 -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 Thu, 20 Apr 2017, Peter Zijlstra wrote:

>On Thu, Apr 20, 2017 at 10:13:26AM -0700, Davidlohr Bueso wrote:
>
>> 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.
>
>But tree->lock is a spinlock, so while this gets us out of rwsem-xadd
>territory for the fast paths, the whole lock-stealing and optimistic
>spinning stuff is on a different scale.

Oh, absolutely. I was merely pointing out the differences at a hair
splitting level.

>Those are about avoiding actually going to sleep and having to be woken
>up (and waiting to become running) again, which is a long time.

Yes, which is why I was thinking of ways to mitigate this. Ie: for
blocked writers with low counts of 'blocking_ranges'.

Thanks,
Davidlohr