linux-kernel - Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock

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Date:   Thu, 30 Nov 2017 15:55:01 -0500
From:   Waiman Long <longman@...hat.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before
 acquiring lock

On 11/30/2017 03:38 PM, Dave Chinner wrote:
> On Thu, Nov 30, 2017 at 08:54:04AM -0500, Waiman Long wrote:
>>
>> For the record, I add one more list_empty() check at the beginning of
>> list_lru_del() in the patch for 2 purpose:
>> 1. it allows the code to bail out early.
> Which is what I said was wrong. You haven't addressed why you think
> it's safe to add racy specualtive checks to this code in your quest
> for speed.
>
> Also, I'm curious about is how much of the gain is from the
> prefetching, and how much of the gain is from avoiding the lock
> altogether by the early bailout...

The early bailout doesn't improve the test at all. In the case of
dentries, there is a flag that indicates that the dentry is in the LRU
list. So list_lru_del is only called when it is in the LRU list.

>> 2. It make sure the cacheline of the list_head entry itself is loaded.
>>
>> Other than that, I only add a likely() qualifier to the existing
>> list_empty() check within the lock critical region.
> Yup, but in many cases programmers get the static branch prediction
> hints are wrong. In this case, you are supposing that nobody ever
> calls list_lru_del() on objects that aren't on the lru. That's not
> true - inodes that are being evicted may never have been on the LRU
> at all, but we still call through list_lru_del() so it can determine
> the LRU state correctly (e.g. cache cold rm -rf workloads)....
>
> IOWs, I'm pretty sure even just adding static branch prediction
> hints here is wrong....

In the case of dentries, the static branch is right. However it may not
be true for other users of list_lru, so I am OK to take them out. Thanks
for the explanation.

Cheers,
Longman