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

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Message-ID: <20170420175350.44mvhizyulyzb6oc@hirez.programming.kicks-ass.net>
Date:   Thu, 20 Apr 2017 19:53:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
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, 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.

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.