linux-kernel - Re: [PATCH v5 7/7] locking/lockdep: Add a fast path for chain

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Message-ID: <20200204124759.GP14914@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Feb 2020 13:47:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v5 7/7] locking/lockdep: Add a fast path for chain_hlocks
 allocation

On Mon, Feb 03, 2020 at 11:41:47AM -0500, Waiman Long wrote:
> When alloc_chain_hlocks() is called, the most likely scenario is
> to allocate from the primordial chain block which holds the whole
> chain_hlocks[] array initially. It is the primordial chain block if its
> size is bigger than MAX_LOCK_DEPTH. As long as the number of entries left
> after splitting is still bigger than MAX_CHAIN_BUCKETS it will remain
> in bucket 0. By splitting out a sub-block at the end, we only need to
> adjust the size without changing any of the existing linkage information.
> This optimized fast path can reduce the latency of allocation requests.
> 
> This patch does change the order by which chain_hlocks entries are
> allocated. The original code allocates entries from the beginning of
> the array. Now it will be allocated from the end of the array backward.

Cute; but why do we care? Is there any measurable performance indicator?