linux-kernel - Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to refcount

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Date:   Wed, 19 Jul 2017 16:11:34 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        gregkh@...uxfoundation.org, mingo@...hat.com, adobriyan@...il.com,
        serge@...lyn.com, arozansk@...hat.com, keescook@...omium.org,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>
Subject: Re: [PATCH 1/3] ipc: convert ipc_namespace.count from atomic_t to
 refcount_t

On Wed, 19 Jul 2017, Andrew Morton wrote:

>On Wed, 19 Jul 2017 15:54:27 -0700 Davidlohr Bueso <dave@...olabs.net> wrote:
>
>> On Wed, 19 Jul 2017, Andrew Morton wrote:
>>
>> >I do rather dislike these conversions from the point of view of
>> >performance overhead and general code bloat.  But I seem to have lost
>> >that struggle and I don't think any of these are fastpath(?).
>>
>> Well, since we now have fd25d19 (locking/refcount: Create unchecked atomic_t
>> implementation), performance is supposed to be ok.
>
>Sure, things are OK for people who disable the feature.
>
>But for people who want to enable the feature we really should minimize
>the cost by avoiding blindly converting sites which simply don't need
>it: simple, safe, old, well-tested code.  Why go and slow down such
>code?  Need to apply some common sense here...

Fair points.

>
>> It would be lovely to have
>> some actual numbers nonetheless.
>
>Very much so.

May I suggest using mmtests with the following config file:

https://github.com/gormanm/mmtests/blob/7e070a810bc0af92e592e5121d0ea75fada51aeb/configs/config-global-dhp__workload-ipc-scale-short

It will run two of Manfred's ipcscale sem benchmarks.

Thanks,
Davidlohr